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  TRANSCRIBER'S NOTE

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  PRACTICAL TALKS BY AN ASTRONOMER

[Illustration: The Moon. First Quarter.

Photographed by Loewy and Puiseux, February 13, 1894.]




  PRACTICAL TALKS BY AN ASTRONOMER

  BY
  HAROLD JACOBY

  ADJUNCT PROFESSOR OF ASTRONOMY IN
  COLUMBIA UNIVERSITY

  ILLUSTRATED

  NEW YORK
  CHARLES SCRIBNER'S SONS
  1902




  COPYRIGHT, 1902, BY
  CHARLES SCRIBNER'S SONS

  Published, April, 1902

  TROW DIRECTORY
  PRINTING AND BOOKBINDING COMPANY
  NEW YORK




PREFACE


The present volume has not been designed as a systematic treatise
on astronomy. There are many excellent books of that kind, suitable
for serious students as well as the general reader; but they are
necessarily somewhat dry and unattractive, because they must aim at
completeness. Completeness means detail, and detail means dryness.

But the science of astronomy contains subjects that admit of
detached treatment; and as many of these are precisely the ones of
greatest general interest, it has seemed well to select several,
and describe them in language free from technicalities. It is hoped
that the book will thus prove useful to persons who do not wish to
give the time required for a study of astronomy as a whole, but
who may take pleasure in devoting a half-hour now and then to a
detached essay on some special topic.

Preparation of the book in this form has made it suitable for prior
publication in periodicals; and the several essays have in fact
all been printed before. But the intention of collecting them into
a book was kept in mind from the first; and while no attempt has
been made at consecutiveness, it is hoped that nothing of merely
ephemeral value has been included.




CONTENTS


        PAGE

  NAVIGATION AT SEA                   1

  THE PLEIADES                       10

  THE POLE-STAR                      18

  NEBULÆ                             27

  TEMPORARY STARS                    37

  GALILEO                            47

  THE PLANET OF 1898                 58

  HOW TO MAKE A SUN-DIAL             69

  PHOTOGRAPHY IN ASTRONOMY           81

  TIME STANDARDS OF THE WORLD       111

  MOTIONS OF THE EARTH'S POLE       131

  SATURN'S RINGS                    140

  THE HELIOMETER                    152

  OCCULTATIONS                      161

  MOUNTING GREAT TELESCOPES         170

  THE ASTRONOMER'S POLE             184

  THE MOON HOAX                     199

  THE SUN'S DESTINATION             210




ILLUSTRATIONS


  THE MOON. FIRST QUARTER             _Frontispiece_
      _Photographed by Loewy and Puiseux,
       February 13, 1894._

                                             FACING
                                              PAGE

  SPIRAL NEBULA IN CONSTELLATION LEO            26
      _Photographed by Keeler,
       February 24, 1900._

  NEBULA IN ANDROMEDA                           28
      _Photographed by Barnard,
       November 21, 1892._

  THE "DUMB-BELL" NEBULA                        34
      _Photographed by Keeler,
       July 31, 1899._

  STAR-FIELD IN CONSTELLATION MONOCEROS         84
      _Photographed by Barnard,
       February 1, 1894._

  SOLAR CORONA. TOTAL ECLIPSE                  108
      _Photographed by Campbell,
       January 22, 1898; Jeur, India._

  FORTY-INCH TELESCOPE, YERKES OBSERVATORY     170

  YERKES OBSERVATORY, UNIVERSITY OF CHICAGO    176




PRACTICAL TALKS BY AN ASTRONOMER




NAVIGATION AT SEA


A short time ago the writer had occasion to rummage among the
archives of the Royal Astronomical Society in London, to consult,
if possible, the original manuscripts left by one Stephen
Groombridge, an English astronomer of the good old days, who
died in 1832. It was known that they had been filed away about a
generation ago, by the late Sir George Airy, who was Astronomer
Royal of England between the years 1835 and 1881. After a long
search, a large and dusty box was found and opened. It was filled
with documents, of which the topmost was in Sir George's own
handwriting, and began substantially as follows:

  "List of articles within this box.
      "No. 1, This list,
      "No. 2, etc., etc."

Astronomical precision can no further go: he had listed even the
list itself. Truly, Airy was rightly styled "prince of precisians."
A worthy Astronomer Royal was he, to act under the royal warrant
of Charles II., who established the office in 1675. Down to this
present day that warrant still makes it the duty of His Majesty's
Astronomer "to apply himself with the most exact care and diligence
to the rectifying of the tables of the motions of the heavens and
the places of the fixed stars, in order to find out the so much
desired longitude at sea, for the perfecting the art of navigation."

The "so much desired longitude at sea" is, indeed, a vastly
important thing to a maritime nation like England. And only in
comparatively recent years has it become possible and easy for
vessels to be navigated with safety and convenience upon long
voyages. The writer was well acquainted with an old sea-captain
of New York, who had commanded one of the earliest transatlantic
steamers, and who died only a few years ago. He had a goodly store
of ocean yarn, fit and ready for the spinning, if he could but
find someone who, like himself, had known and loved the ocean. In
his early sea-going days, only the wealthiest of captains owned
chronometers. This instrument is now considered indispensable in
navigation, but at that time it was a new invention, very rare and
costly. Upon a certain voyage from England to Rio Janeiro, in South
America, the old captain could remember the following odd method of
navigation: The ship was steered by compass to the southward and
westward, more or less, until the skipper's antique quadrant showed
that they had about reached the latitude of Rio. Then they swung
her on a course due west by compass, and away she went for Rio,
relying on the lookout man forward to keep the ship from running
ashore. For after a certain lapse of time, being ignorant of the
longitude, they could not know whether they would "raise" the land
within an hour or in six weeks. We are glad of an opportunity to
put this story on record, for the time is not far distant when
there will be no man left among the living who can remember how
ships were taken across the seas in the good old days before
chronometers.

Anyone who has ever been a passenger on a great transatlantic
liner of to-day knows what an important, imposing personage is
the brass-bound skipper. A very different creature is he on the
deck of his ship from the modest seafaring man we meet on land,
clad for the time being in his shore-going togs. But the captain's
dignity is not all brass buttons and gold braid. He has behind
him the powerful support of a deep, delightful mystery. He it is
who "takes the sun" at noon, and finds out the ship's path at
sea. And in truth, regarded merely as a scientific experiment,
the guiding of a vessel across the unmarked trackless ocean has
few equals within the whole range of human knowledge. It is our
purpose here to explain quite briefly the manner in which this
seeming impossibility is accomplished. We shall not be able to
go sufficiently into details to enable him who reads to run and
navigate a magnificent steamer. But we hope to diminish somewhat
that small part of the captain's vast dignity which depends upon
his mysterious operations with the sextant.

To begin, then, with the sextant itself. It is nothing but an
instrument with which we can measure how high up the sun is in the
sky. Now, everyone knows that the sun slowly climbs the sky in
the morning, reaches its greatest height at noon, and then slowly
sinks again in the afternoon. The captain simply begins to watch
the sun through the sextant shortly before noon, and keeps at it
until he discovers that the sun is just beginning to descend. That
is the instant of noon on the ship. The captain quickly glances
at the chronometer, or calls out "noon" to an officer who is near
that instrument. And so the error of the chronometer becomes known
then and there without any further astronomical calculations
whatever. Navigators can also find the chronometer error by sextant
observations when the sun is a long way from noon. The methods of
doing this are somewhat less simple than for the noon observation;
they belong to the details of navigation, into which we cannot
enter here.

Incidentally, the captain also notes with the sextant how high
the sun was in the sky at the noon observation. He has in his
mysterious "chart-room" some printed astronomical tables,
which tell him in what terrestrial latitude the sun will have
precisely that height on that particular day of the year. Thus the
terrestrial latitude becomes known easily enough, and if only the
captain could get his longitude too, he would know just where his
ship was that day at noon.

We have seen that the sextant observations furnish the error of
the chronometer according to ship's time. In other words, the
captain is in possession of the correct local time in the place
where the ship actually is. Now, if he also had the correct time
at that moment of some well-known place on shore, he would know
the difference in time between that place on shore and the ship.
But every traveller by land or sea is aware that there are always
differences of time between different places on the earth. If a
watch be right on leaving New York, for instance, it will be much
too fast on arriving at Chicago or San Francisco; the farther you
go the larger becomes the error of your watch. In fact, if you
could find out how much your watch had gone into error, you would
in a sense know how far east or west you had travelled.

Now the captain's chronometer is set to correct "Greenwich time"
on shore before the ship leaves port. His observations having then
told him how much this is wrong on that particular day, and in
that particular spot where the ship is, he knows at once just how
far he has travelled east or west from Greenwich. In other words,
he knows his "longitude from Greenwich," for longitude is nothing
more than distance from Greenwich in an east-and-west direction,
just as latitude is only distance from the equator measured in
a north-and-south direction. Greenwich observatory is usually
selected as the beginning of things for measuring longitudes,
because it is almost the oldest of existing astronomical
establishments, and belongs to the most prominent maritime nation,
England.

One of the most interesting bits of astronomical history was
enacted in connection with this matter of longitude. From what has
been said, it is clear that the ship's longitude will be obtained
correctly only if the chronometer has kept exact time since the
departure of the ship from port. Even a very small error of the
chronometer will throw out the longitude a good many miles, and we
can understand readily that it must be difficult in the extreme to
construct a mechanical contrivance capable of keeping exact time
when subjected to the rolling and pitching of a vessel at sea.

It was as recently as the year 1736 that the first instrument
capable of keeping anything like accurate time at sea was
successfully completed. It was the work of an English watchmaker
named John Harrison, and is one of the few great improvements in
matters scientific which the world owes to a desire for winning a
money prize. It appears that in 1714 a committee was appointed by
the House of Commons, with no less a person than Sir Isaac Newton
himself as one of its members, to consider the desirability of
offering governmental encouragement for the invention of some means
of finding the longitude at sea. Finally, the British Government
offered a reward of $50,000 for an instrument which would find
the longitude within sixty miles; $75,000, if within forty miles,
and $100,000, if within thirty miles. Harrison's chronometer was
finished in 1736, but he did not receive the final payment of his
prize until 1764.

We shall not enter into a detailed account of the vexatious delays
and official procedures to which he was forced to submit during
those twenty-eight long years. It is a matter of satisfaction that
Harrison lived to receive the money which he had earned. He had the
genius to plan and master intricate mechanical details, but perhaps
he lacked in some degree the ability of tongue and pen to bring
them home to others. This may be the reason he is so little known,
though it was his fortune to contribute so large and essential
a part to the perfection of modern navigation. Let us hope this
brief mention may serve to recall his memory from oblivion even for
a fleeting moment; that we may not have written in vain of that
longitude to which his life was given.




THE PLEIADES


Famed in legend; sung by early minstrels of Persia and Hindustan;

    "--like a swarm of fire-flies tangled in a silver braid";

yonder distant misty little cloud of Pleiades has always won
and held the imagination of men. But it was not only for the
inspiration of poets, for quickening fancy into song, that the
seven daughters of Atlas were fixed upon the firmament. The
problems presented by this group of stars to the unobtrusive
scientific investigator are among the most interesting known to
astronomy. Their solution is still very incomplete, but what we
have already learned may be counted justly among the richest spoils
brought back by science from the stored treasure-house of Nature's
secrets.

The true student of astronomy is animated by no mere vulgar
curiosity to pry into things hidden. If he seeks the concealed
springs that move the complex visible mechanism of the heavens, he
does so because his imagination is roused by the grandeur of what
he sees; and deep down within him stirs the true love of the artist
for his art. For it is indeed a fine art, that science of astronomy.

It can have been no mere chance that has massed the Pleiades
from among their fellow stars. Men of ordinary eyesight see but
a half-dozen distinct objects in the cluster; those of acuter
vision can count fourteen; but it is not until we apply the
space-penetrating power of the telescope that we realize the
extraordinary scale upon which the system of the Pleiades is
constructed. With the Paris instrument Wolf in 1876 catalogued 625
stars in the group; and the searching photographic survey of Henry
in 1887 revealed no less than 2,326 distinct stars within and near
the filmy gauze of nebulous matter always so conspicuous a feature
of the Pleiades.

The means at our disposal for the study of stellar distances are
but feeble. Only in the case of a very small number of stars have
we been able to obtain even so much as an approximate estimate
of distance. The most powerful observational machinery, though
directed by the tried skill of experience, has not sufficed to
sound the profounder depths of space. The Pleiad stars are among
those for which no measurement of distance has yet been made, so
that we do not know whether they are all equally far away from
us. We see them projected on the dark background of the celestial
vault; but we cannot tell from actual measurement whether they are
all situated near the same point in space. It may be that some
are immeasurably closer to us than are the great mass of their
companions; possibly we look through the cluster at others far
behind it, clinging, as it were, to the very fringe of the visible
universe.

Farther on we shall find evidence that something like this really
is the case. But under no circumstances is it reasonable to suppose
that the whole body of stars can be strung out at all sorts of
distances near a straight line pointing in the direction of the
visible cluster. Such a distribution may perhaps remain among the
possibilities, so long as we cannot measure directly the actual
distances of the individual stars. But science never accepts a mere
possibility against which we can marshal strong circumstantial
evidence. We may conclude on general principles that the gathering
of these many objects into a single close assemblage denotes
community of origin and interests.

The Pleiades then really belong to one another. What is the nature
of their mutual tie? What is their mystery, and can we solve it?
The most obvious theory is, of course, suggested by what we know
to be true within our own solar system. We owe to Newton the
beautiful conception of gravitation, that unique law by means of
which astronomers have been enabled to reduce to perfect order the
seeming tangle of planetary evolutions. The law really amounts, in
effect, to this: All objects suspended within the vacancy of space
attract or pull one another. How they can do this without a visible
connecting link between them is a mystery which may always remain
unsolved. But mystery as it is, we must accept it as an ascertained
fact. It is this pull of gravitation that holds together the sun
and planets, forcing them all to follow out their due and proper
paths, and so to continue throughout an unbroken cycle until the
great survivor, Time, shall be no more.

This same gravitational attraction must be at work among the
Pleiades. They, too, like ourselves, must have bounds and orbits
set and interwoven, revolutions and gyrations far more complex
than the solar system knows. The visual discovery of such motion
of rotation among the Pleiades may be called one of the pressing
problems of astronomy to-day. We feel sure that the time is ripe,
and that the discovery is actually being made at the present
moment: for a generation of men is not too great a period to call a
moment, when we have to deal with cosmic time.

It is indeed the lack of observations extending through sufficient
centuries that stays our hand from grasping the coveted result. The
Pleiades are so far from us that we cannot be sure of changes among
them. Magnitudes are always relative. It matters not how large the
actual movements may be; if they are extremely small in comparison
with our distance, they must shrink to nothingness in our eyes.
Trembling on the verge of invisibility, elusive, they are in that
borderland where science as yet but feels her way, though certain
that the way is there.

The foundations of exact modern knowledge of the group were laid
by Bessel about 1840. With the modesty characteristic of the
great, he says quite simply that he has made a number of measures
of the Pleiades, thinking that the time may come when astronomers
will be able to find some evidence of motion. In this unassuming
way he prefaces what is still the classic model of precision and
thoroughness in work of this kind. Bessel cleared the ground for a
study of inter-stellar motion within the close star-clusters; and
it is probable that only by such study may we hope to demonstrate
the universality of the law of gravitation in cosmic space.

Bessel's acuteness in forecasting the direction of coming research
was amply verified by the work of Elkin in 1885 at Yale College.
Provided with a more modern instrument, but similar to Bessel's,
Elkin was able to repeat his observations with a slight increase
of precision. Motions in the interval of forty-five years,
sufficiently great to hint at coming possibilities, were shown
conclusively to exist. Six stars at all events have been fairly
excluded from the group on account of their peculiar motions shown
by Elkin's research. It is possible that they are merely seen in
the background through the interstices of the cluster itself,
or they may be suspended between us and the Pleiades, in either
case having no real connection with the group. Finally, these
observations make it reasonably certain that many of the remaining
mass of stars really constitute a unit aggregation in space.
Astronomers of a coming generation will again repeat the Besselian
work. At present we have been able to use his method only for the
separation from the true Pleiades of chance stars that happen to
lie in the same direction. Let us hope that man shall exist long
enough upon this earth to see the clustered stars themselves begin
and carry out such gyrations as gravitation imposes.

These will doubtless be of a kind not even suggested by the
lesser complexities of our solar system. For the most wonderful
thing of all about the Pleiades seems to point to an intricacy of
structure whose details may be destined to shake the confidence of
the profoundest mathematician. There is an extraordinary nebulous
condensation that seems to pervade the entire space occupied by
the stellar constituents of the group. The stars are swimming in a
veritable sea of luminous cloud. There are filmy tenuous places,
and again condensing whirls of material doubtless still in the
gaseous or plastic stage. Most noticeable of all are certain almost
straight lines of nebula that connect series of stars. In one case,
shown upon a photograph made by Henry at Paris, six stars are
strung out upon such a hazy line. We might give play to fancy, and
see in this the result of some vast eruption of gaseous matter that
has already begun to solidify here and there into stellar nuclei.
But sound science gives not too great freedom to mere speculative
theories. Her duty has been found in quiet research, and her
greatest rewards have flowed from imaginative speculation, only
when tempered by pure reason.




THE POLE-STAR


One of the most brilliant observations of the last few years
is Campbell's recent discovery of the triple character of this
star. Centuries and centuries ago, when astronomy, that venerable
ancient among the sciences, was but an infant, the pole-star must
have been considered the very oldest of observed heavenly bodies.
In the beginning it was the only sure guide of the navigator at
night, just as to this day it is the foundation-stone for all
observational stellar astronomy of precision. There has never been
a time in the history of astronomy when the pole-star might not
have been called the most frequently measured object in the sky of
night. So it is indeed strange that we should find out something
altogether new about it after all these ages of study.

But the importance of the discovery rests upon a surer foundation
than this. The method by which it has been made is almost a new
one in the science. A generation ago, men thought the "perfect
science," for so we love to call astronomy, could advance only by
increasing a little the exact precision of observation. The citadel
of perfect truth might be more closely invested; the forces of
science might push forward step by step; the machinery of research
might be strengthened, but that a new engine of investigation
would be discovered capable of penetrating where no telescope can
ever reach, this, indeed, seemed far beyond the liveliest hope
of science. Even the discoverer of the spectroscope could never
have dreamed of its possibilities, could never have foreseen its
successes, its triumphs.

The very name of this instrument suggests mystery to the popular
mind. It is set down at once among the things too difficult, too
intricate, too abstruse to understand. Yet in its essentials there
is nothing about the spectroscope that cannot be made clear in a
few words. Even the modern "undulatory theory" of light itself is
terrible only in the length of its name. Anyone who has seen the
waves of ocean roll, roll, and ever again roll in upon the shore,
can form a very good notion of how light moves. 'Tis just such
a series of rolling waves; started perhaps from some brilliant
constellation far out upon the confining bounds of the visible
universe, or perhaps coming from a humble light upon the student's
table; yet it is never anything but a succession of rolling waves.
Only, unlike the waves of the sea, light waves are all excessively
small. We should call one whose length was a twenty-thousandth of
an inch a big one!

Now the human eye possesses the property of receiving and
understanding these little waves. The process is an unconscious
one. Let but a set of these tiny waves roll up, as it were, out
of the vast ocean of space and impinge upon the eye, and all the
phenomena of light and color become what we call "visible." We see
the light.

And how does all this find an application in astronomy? Not
to enter too much into technical details, we may say that the
spectroscope is an instrument which enables us to measure the
length of these light waves, though their length is so exceedingly
small. The day has indeed gone by when that which poets love to
call the Book of Nature was printed in type that could be read
by the eye unaided. Telescope, microscope, and spectroscope are
essential now to him who would penetrate any of Nature's secrets.
But measurements with a telescope, like eye observations, are
limited strictly to determining the directions in which we see
the heavenly bodies. Ever since the beginning of things, when
old Hipparchus and Ulugh Beg made the first rude but successful
attempts to catalogue the stars, the eye and telescope have been
able to measure only such directions. We aim the telescope at a
star, and record the direction in which it was pointed. Distances
in astronomy can never be measured directly. All that we know of
them has been obtained by calculations based upon the Newtonian law
of gravitation and observations of directions.

Now the spectroscope seems to offer a sort of exception to this
rule. Suppose we can measure the wave-lengths of the light sent us
from a star. Suppose again that the star is itself moving swiftly
toward us through space, while continually setting in motion the
waves of light that are ultimately to reach the waiting astronomer.
Evidently the light waves will be crowded together somewhat on
account of the star's motion. More waves per second will reach us
than would be received from a star at rest. It is as though the
light waves were compressed or shortened a little. And if the star
is leaving us, instead of coming nearer, opposite effects will
occur. We have then but to compare spectroscopically starlight with
some artificial source of light in the observatory in order to find
out whether the star is approaching us or receding from us. And by
a simple process of calculation this stellar motion can be obtained
in miles per second. Thus we can now actually measure directly, in
a certain sense, linear speed in stellar space, though we are still
without the means of getting directly at stellar distances.

But the most wonderful thing of all about these spectroscopic
measures is the fact that it makes no difference whatever how far
away is the star under observation. What we learn through the
spectroscope comes from a study of the waves themselves, and it
is of no consequence how far they have travelled, or how long
they have been a-coming. For it must not be supposed that these
waves consume no time in passing from a distant star to our own
solar system. It is true that they move exceeding fast; certainly
180,000 miles per second may be called rapid motion. But if
this cosmic velocity of light is tremendous, so also are cosmic
distances correspondingly vast. Light needs to move quickly coming
from a star, for even at the rate of motion we have mentioned it
requires many years to reach us from some of the more distant
constellations. It has been well said that an observer on some
far-away star, if endowed with the power to see at any distance,
however great, might at this moment be looking on the Crusaders
proceeding from Europe against the Saracen at Jerusalem. For it is
quite possible that not until now has the light which would make
the earth visible had time to reach him. Yet distant as such an
observer might be, light from the star on which he stood could be
measured in the spectroscope, and would infallibly tell us whether
the earth and star are approaching in space or gradually drawing
farther asunder.

The pole-star is not one of the more distant stellar systems. We do
not know how far it is from us very exactly, but certainly not less
than forty or fifty years are necessary for its light to reach us.
The star might have gone out of existence twenty years ago, and we
not yet know of it, for we would still be receiving the light which
began its long journey to us about 1850 or 1860. But no matter
what may be its distance, Campbell found by careful observations,
made in the latter part of 1896, that the pole-star was then
approaching the earth at the rate of about twelve miles per second.
So far there was nothing especially remarkable. But in August and
September of the present year twenty-six careful determinations
were made, and these showed that now the rate of approach varied
between about five and nine miles per second. More astonishing
still, there was a uniform period in the changes of velocity. In
about four days the rate of motion changed from about five to
nine miles and back again. And this variation kept on with great
regularity. Every successive period of four days saw a complete
cycle of velocity change forward and back between the same limits.
There can be but one reasonable explanation. This star must be a
double, or "binary" star. The two components, under the influence
of powerful mutual gravitational attraction, must be revolving
in a mighty orbit. Yet this vast orbit, as a whole, with the two
great stars in it, must be approaching our part of the universe
all the time. For the spectroscope shows the velocity of approach
to increase and diminish, indeed, but it is always present. Here,
then, is this great stellar system, having a four-day revolution of
its own, and yet swinging rapidly through space in our direction.
Nor is this all. One of the component stars must be nearly or
quite dark; else its presence would infallibly be detected by our
instruments.

And now we come to the most astonishing thing of all. How comes it
that the average rate of approach of the "four-day system," as a
whole, changed between 1896 and 1899? In 1896 only this velocity
of the whole system was determined, the four-day period remaining
undiscovered until the more numerous observations of 1899.
But even without considering the four-day period, the changing
velocity of the entire system offers one of those problems that
exact science can treat only by the help of the imagination. There
must be some other great centre of attraction, some cosmic giant,
holding the visible double pole-star under its control. Thus, that
which we see, and call the pole-star, is in reality threading its
path about the third and greatest member of the system, itself
situated in space, we know not where.

[Illustration: Spiral Nebula in Constellation Leo.

Photographed by Keeler, February 24, 1900.

Exposure, three hours, fifty minutes.]




NEBULÆ


Scattered about here and there among the stars are certain patches
of faint luminosity called by astronomers Nebulæ. These "little
clouds" of filmy light are among the most fascinating of all the
kaleidoscopic phenomena of the heavens; for it needs but a glance
at one of them to give the impression that here before us is the
stuff of which worlds are made. All our knowledge of Nature leads
us to expect in her finished work the result of a series of gradual
processes of development. Highly organized phenomena such as those
existing in our solar system did not spring into perfection in
an instant. Influential forces, easy to imagine, but difficult
to define, must have directed the slow, sure transformation of
elemental matter into sun and planets, things and men. Therefore
a study of those forces and of their probable action upon nebular
material has always exerted a strong attraction upon the acutest
thinkers among men of exact science.

Our knowledge of the nebulæ is of two kinds--that which has
been ascertained from observation as to their appearance, size,
distribution, and distance; and that which is based upon hypotheses
and theoretical reasoning about the condensation of stellar systems
out of nebular masses. It so happens that our observational
material has received a very important addition quite recently
through the application of photography to the delineation of
nebulæ, and this we shall describe farther on.

Two nebulæ only are visible to the unaided eye. The brighter
of these is in the constellation Andromeda; it is of oval or
elliptical shape, and has a distinct central condensation or
nucleus. Upon a photograph by Roberts it appears to have several
concentric rings surrounding the nebula proper, and gives the
general impression of a flat round disk foreshortened into an oval
shape on account of the observer's position not being square to
the surface of the disk. Very recent photographs of this nebula,
made with the three-foot reflecting telescope of the Lick
Observatory, bring out the fact that it is really spiral in form,
and that the outlying nebulous rings are only parts of the spires
in a great cosmic whorl.

[Illustration: Nebula in Andromeda.

Lower object in the photograph is a Comet.

Photographed by Barnard, November 21, 1892.]

This Andromeda nebula is the one in which the temporary star of
1885 appeared. It blazed up quite suddenly near the apparent
centre of the nebula, and continued in view for six months, fading
finally beyond the reach of our most powerful telescopes. There
can be little doubt that the star was actually in the nebula,
and not merely seen through it, though in reality situated in
the extreme outlying part of space at a distance immeasurably
greater than that separating us from the nebula itself. Such an
accidental superposition of nebula and star might even be due to
sudden incandescence of a new star between us and the nebula. In
such a case we should see the star projected upon the surface of
the nebula, so that the superposition would be identical with that
actually observed. Therefore, while it is, indeed, possible that
the star may have been either far behind the nebula or in front of
it, we must accept as more probable the supposition that there was
a real connection between the two. In that case there is little
doubt that we have actually observed one of those cataclysms that
mark successive steps of cosmic evolution. We have no thoroughly
satisfactory theory to account for such an explosive catastrophe
within the body of the nebula itself.

The other naked-eye nebula is in the constellation Orion. In
the telescope it is a more striking object, perhaps, than the
Andromeda nebula; for it has no well-defined geometrical form,
but consists of an immense odd-shaped mass of light enclosing and
surrounding a number of stars. It is unquestionably of a very
complicated structure, and is, therefore, less easily studied and
explained than the nebulæ of simpler form. There is no doubt that
the Orion nebula is composed of luminous gas, and is not merely a
cluster of small stars too numerous and too near together to be
separated from each other, even in our most powerful telescopes.
It was, indeed, supposed, until about forty years ago, that all
the nebulæ are simply irresolvable star-clusters; but we now have
indisputable evidence, derived from the spectroscope, that many
nebulæ are composed of true gases, similar to those with which
we experiment in chemical laboratories. This spectroscopic proof
of the gaseous character of nebulæ is one of the most important
discoveries contributed by that instrument to our small stock of
facts concerning the structure of the sidereal universe.

Coming now to the smaller nebulæ, we find a great diversity of
form and appearance. Some are ring-shaped, perhaps having a
less brilliant nebulosity within the ring. Many show a central
condensation of disk-like appearance (planetary nebulæ), or have
simply a star at the centre (nebulous stars). Altogether about
ten thousand such objects have been catalogued by successive
generations of astronomers since the invention of the telescope,
and most of these have been reported as oval in form. Now we have
already referred to the important addition to our knowledge of
the nebulæ obtained by recent photographic observations; and this
addition consists in the discovery that most of these oval nebulæ
are in reality spirals. Indeed, it appears that the spiral type is
the normal type, and that nebulæ of irregular or other forms are
exceptions to the general rule. Even the great Andromeda nebula, as
we have seen, is now recognized as a spiral.

The instrument with which its convolute structure was discovered
is a three-foot reflecting telescope, made by Common of England,
and now mounted at the Lick Observatory, in California. The late
Professor Keeler devoted much of his time to photographing nebulæ
during the last year or two. He was able to establish the important
fact just mentioned, that most nebulæ formerly thought to be mere
ovals, turn out to be spiral when brought under the more searching
scrutiny of the photographic plate applied at the focus of a
telescope of great size, and with an exposure to the feeble nebular
light extending through three or four consecutive hours.

Many of the spirals have more than a single volute. It is as though
one were to attach a number of very flexible rods to an axle,
like spokes of a wheel without a rim and then revolve the axle
rapidly. The flexible rods would bend under the rapid rotation, and
form a series of spiral curves not unlike many of these nebulæ.
Indeed, it is impossible to escape the conviction that these great
celestial whorls are whirling around an axis. And it is most
important in the study of the growth of worlds, to recognize that
the type specimen is a revolving spiral. Therefore, the rotating
flattened globe of incandescent matter postulated by Laplace's
nebular hypothesis would make of our solar system an exceptional
world, and not a type of stellar evolution in general.

Keeler's photographs have taught us one thing more. Scarcely is
there a single one of his negatives that does not show nebulæ
previously uncatalogued. It is estimated that if this process of
photography could be extended so as to cover the entire sky, the
whole number of nebulæ would add up to the stupendous total of
120,000; and of these the great majority would be spiral.

When we approach the question of the distribution of nebulæ in
different parts of the sky, as shown by their catalogued positions,
we are met by a curious fact. It appears that the region in the
neighborhood of the Milky Way is especially poor in nebulæ,
whereas these objects seem to cluster in much larger numbers about
those points in the sky that are farthest from the Milky Way.
But we know that the Milky Way is richer in stars than any other
part of the sky, since it is, in fact, made up of stellar bodies
clustered so closely that it is wellnigh impossible to see between
them in the denser portions. Now, it cannot be the result of chance
that the stars should tend to congregate in the Milky Way, while
the nebulæ tend to seek a position as far from it as possible.
Whatever may be the cause, we must conclude that the sidereal
system, as we see it, is in general constructed upon a single plan,
and does not consist of a series of universes scattered at random
throughout space. If we are to suppose that nebulæ turn into stars
as a result of condensation or any other change, then it is not
astonishing to find a minimum of nebulæ where there is a maximum of
stars, since the nebulæ will have been consumed, as it were, in the
formation of the stars.

[Illustration: The "Dumb-Bell" Nebula.

Photographed by Keeler, July 31, 1899.

Exposure, three hours.]

It is never advisable to push philosophical speculation very
far when supported by too slender a basis of fact. But if we
are to regard the visible universe as made up on the whole of a
single system of bodies, we may well ask one or two questions to
be answered by speculative theory. We have said the stars are not
uniformly distributed in space. Their concentration in the Milky
Way, forming a narrow band dividing the sky into two very nearly
equal parts, must be due to their being actually massed in a
thin disk or ring of space within which our solar system is also
situated. This thin disk projected upon the sky would then appear
as the narrow star-band of the Milky Way. Now, suppose this disk
has an axis perpendicular to itself, and let us imagine a rotation
of the whole sidereal system about that axis. Then the fact that
the visible nebulæ are congregated far from the Milky Way means
that they are actually near the imaginary axis.

Possibly the diminished velocity of motion near the axis may have
something to do with the presence of the nebulæ there. Possibly
the nebulæ themselves have axes perpendicular to the plane of
the Milky Way. If so, we should see the spiral nebulæ near the
Milky Way edgewise, and those far from it without foreshortening.
Thus, the paucity of nebulæ near the Milky Way may be due in
part to the increased difficulty of seeing them when looked at
edgewise. Indeed, there is no limit to the possibilities of
hypothetical reasoning about the nebular structure of our universe;
unfortunately, the whole question must be placed for the present
among those intensely interesting cosmic problems awaiting
elucidation, let us hope, in this new century.




TEMPORARY STARS


Nothing can be more erroneous than to suppose that the stellar
multitude has continued unchanged throughout all generations of
men. "Eternal fires" poets have called the stars; yet they burn
like any little conflagration on the earth; now flashing with
energy, brilliant, incandescent, and again sinking into the dull
glow of smouldering half-burned ashes. It is even probable that
space contains many darkened orbs, stars that may have risen in
constellations to adorn the skies of prehistoric time--now cold,
unseen, unknown. So far from dealing with an unvarying universe,
it is safe to say that sidereal astronomy can advance only by the
discovery of change. Observational science watches with untiring
industry, and night hides few celestial events from the ardent
scrutiny of astronomers. Old theories are tested and newer ones
often perfected by the detection of some slight and previously
unsuspected alteration upon the face of the sky. The interpretation
of such changes is the most difficult task of science; it has taxed
the acutest intellects among men throughout all time.

If, then, changes can be seen among the stars, what are we to think
of the most important change of all, the blazing into life of a
new stellar system? Fifteen times since men began to write their
records of the skies has the birth of a star been seen. Surely
we may use this term when we speak of the sudden appearance of a
brilliant luminary where nothing visible existed before. But we
shall see further on that scientific considerations make it highly
probable that the phenomenon in question does not really involve
the creation of new matter. It is old material becoming suddenly
luminous for some hidden reason. In fact, whenever a new object of
great brilliancy has been discovered, it has been found to lose its
light again quite soon, ending either in total extinction or at
least in comparative darkness. It is for this reason that the name
"temporary star" has been applied to cases of this kind.

The first authenticated instance dates from the year 134 B.C.,
when a new star appeared in the constellation Scorpio. It was this
star that led Hipparchus to construct his stellar catalogue, the
first ever made. It occurred to him, of course, that there could
be but one way to make sure in the future that any given object
discovered in the sky was new; it was necessary to make a complete
list of everything visible in his day. Later astronomers need then
only compare Hipparchus's catalogue with the heavens from time to
time in order to find out whether anything unknown had appeared.
This work of Hipparchus became the foundation of sidereal study,
and led to most important discoveries of various kinds.

But no records remain concerning his new star except the bare fact
of its appearance in Scorpio. Hipparchus's published works are all
lost. We do not even know the exact place of his birth, and as for
those two dates of entry and exit that history attaches to great
names--we have them not. Yet he was easily the first astronomer of
antiquity, one of the first of all time; and we know of him only
from the writings of Ptolemy, who lived three hundred years after
him.

More than five centuries elapsed before another temporary star was
entered in the records of astronomy. This happened in the year 389
A.D., when a star appeared in Aquila; and of this one also we know
nothing further. But about twelve centuries later, in November,
1572, a new and brilliant object was found in the constellation
Cassiopeia. It is known as Tycho's star, since it was the means
of winning for astronomy a man who will always take high rank in
her annals, Tycho Brahe, of Denmark. When he first saw this star,
it was already very bright, equalling even Venus at her best; and
he continued a careful series of observations for sixteen months,
when it faded finally from his view. The position of the new star
was measured with reference to other stars in the constellation
Cassiopeia, and the results of Tycho's observations were finally
published by him in the year 1573. It appears that much urging on
the part of friends was necessary to induce him to consent to this
publication, not because of a modest reluctance to rush into print,
but for the reason that he considered it undignified for a nobleman
of Denmark to be the author of a book!

An important question in cosmic astronomy is opened by Tycho's
star. Did it really disappear from the heavens when he saw it
no more, or had its lustre simply been reduced below the visual
power of the unaided eye? Unfortunately, Tycho's observations of
the star's position in the constellation were necessarily crude.
He possessed no instruments of precision such as we now have
at our disposal, and so his work gives us only a rather rough
approximation of the true place of the star. A small circle might
be imagined on the sky of a size comparable with the possible
errors of Tycho's observations. We could then say with certainty
that his star must have been situated somewhere within that little
circle, but it is impossible to know exactly where.

It happens that our modern telescopes reveal the existence of
several faint stars within the space covered by such a circle.
Any one of these would have been too small for Tycho to see, and,
therefore, any one of them may be his once brilliant luminary
reduced to a state of permanent or temporary semi-darkness. These
considerations are, indeed, of great importance in explaining the
phenomena of temporary stars. If Tycho had been able to leave us a
more exact determination of his star's place in the sky, and even
if our most powerful instruments could not show anything in that
place to-day, we might nevertheless theorize on the supposition
that the object still exists, but has reached a condition almost
entirely dark.

Indeed, the latest theory classes temporary stars among those known
as variable. For many stars are known to undergo quite decided
changes in brilliancy; possibly inconstancy of light is the rule
rather than the exception. But while such changes, when they
exist, are too small to be perceptible in most cases, there is
certainly a large number of observable variables, subject to easily
measurable alterations of light. Astronomers prefer to see in the
phenomena of temporary stars simple cases of variation in which the
increase of light is sudden, and followed by a gradual diminution.
Possibly there is then a long period of comparative or even
complete darkness, to be followed as before by a sudden blazing up
and extinction. No temporary star, however, has been observed to
reappear in the same celestial place where once had glowed its
sudden outburst. But cases are not wanting where incandescence has
been both preceded and followed by a continued existence, visible
though not brilliant.

For such cases as these it is necessary to come down to modern
records. We cannot be sure that some faint star has been
temporarily brilliant, unless we actually see the conflagration
itself, or are able to make the identity of the object's precise
location in the sky before and after the event perfectly certain
by the aid of modern instruments of precision. But no one has
ever seen the smouldering fires break out. Temporary stars have
always been first noticed only after having been active for hours
if not for days. So we must perforce fall back on instrumental
identification by determinations of the star's exact position upon
the celestial vault.

Some time between May 10th and 12th in the year 1866 the ninth star
in the list of known "temporaries" appeared. It possessed very
great light-giving power, being surpassed in brilliancy by only
about a score of stars in all the heavens. It retained a maximum
luminosity only three or four days, and in less than two months
had diminished to a point somewhere between the ninth and tenth
"magnitudes." In other words, from a conspicuous star, visible to
the naked eye, it had passed beyond the power of anything less than
a good telescope. Fortunately, we had excellent star-catalogues
before 1866. These were at once searched, and it was possible to
settle quite definitely that a star of about the ninth or tenth
magnitude had really existed before 1866 at precisely the same
point occupied by the new one. Needless to say, observations were
made of the new star itself, and afterward compared with later
observations of the faint one that still occupies its place. These
render quite certain the identity of the temporary bright star with
the faint ones that preceded and followed it.

Such results, on the one hand, offer an excellent vindication
of the painstaking labor expended on the construction of
star-catalogues, and, on the other, serve to elucidate the mystery
of temporary stars. Nothing can be more plausible than to explain
by analogy those cases in which no previous or subsequent existence
has been observed. It is merely necessary to suppose that, instead
of varying from the ninth or tenth magnitude, other temporary
objects have begun and ended with the twentieth; for the twentieth
magnitude would be beyond the power of our best instruments.

Nor is the star of 1866 an isolated instance. Ten years later, in
1876, a temporary star blazed up to about the second magnitude, and
returned to invisibility, so far as the naked eye is concerned,
within a month, having retained its greatest brilliancy only one
or two days. This star is still visible as a tiny point of light,
estimated to be of the fifteenth magnitude. Whether it existed
prior to its sudden outburst can never be known, because we do not
possess catalogues including the generality of stars as faint as
this one must have been. But at all events, the continued existence
of the object helps to place the temporary stars in the class of
variables.

The next star, already mentioned under "nebula," was first seen
in 1885. It was in one respect the most remarkable of all, for
it appeared almost in the centre of the great nebula in the
constellation Andromeda. It was never very bright, reaching only
the sixth magnitude or thereabouts, was observed during a period of
only six months, and at the end of that time had faded beyond the
reach of our most powerful glasses. It is a most impressive fact
that this event occurred within the nebula. Whatever may be the
nature of the explosive catastrophe to which the temporary stars
owe their origin, we can now say with certainty that not even those
vast elemental luminous clouds men call nebulæ are free from danger.

The last outburst on our records was first noticed February 22,
1901. The star appeared in the constellation Perseus, and soon
reached the first magnitude, surpassing almost every other star
in the sky. It has been especially remarkable in that it has
become surrounded by a nebulous mass in which are several bright
condensations or nuclei; and these seem to be in very rapid motion.
The star is still under observation (January, 1902).




GALILEO


Among the figures that stand out sharply upon the dim background
of old-time science, there is none that excites a keener interest
than Galileo. Most people know him only as a distinguished man
of learning; one who carried on a vigorous controversy with the
Church on matters scientific. It requires some little study, some
careful reading between the lines of astronomical history, to gain
acquaintance with the man himself. He had a brilliant, incisive
wit; was a genuine humorist; knew well and loved the amusing side
of things; and could not often forego a sarcastic pleasantry, or
deny himself the pleasure of argument. Yet it is more than doubtful
if he ever intended impertinence, or gave willingly any cause of
quarrel to the Church.

His acute understanding must have seen that there exists no real
conflict between science and religion; for time, in passing, has
made common knowledge of this truth, as it has of many things once
hidden. When we consider events that occurred three centuries
ago, it is easy to replace excited argument with cool judgment;
to remember that those were days of violence and cruelty; that
public ignorance was of a density difficult to imagine to-day;
and that it was universally considered the duty of the Church to
assume an authoritative attitude upon many questions with which she
is not now required to concern herself in the least. Charlatans,
unbalanced theorists, purveyors of scientific marvels, were all
liable to be passed upon definitely by the Church, not in a spirit
of impertinent interference, but simply as part of her regular
duties.

If the Church's judgment in such matters was sometimes erroneous;
if her interference now and again was cruel, the cause must be
sought in the manners and customs of the time, when persecution
rioted in company with ignorance, and violence was the law. Perhaps
even to-day it would not be amiss to have a modern scientific board
pass authoritatively upon novel discoveries and inventions, so as
to protect the public against impostors as the Church tried to do
of old.

Galileo was born at Pisa in 1564, and his long life lasted
until 1642, the very year of Newton's birth. His most important
scientific discoveries may be summed up in a few words; he was the
first to use a telescope for examining the heavenly bodies; he
discovered mountains on the moon; the satellites of Jupiter; the
peculiar appearance of Saturn which Huygens afterward explained as
a ring surrounding the ball of the planet; and, finally, he found
black spots on the sun's disk. These discoveries, together with his
remarkable researches in mechanical science, constitute Galileo's
claim to immortality as an investigator. But, as we have said, it
is not our intention to consider his work as a series of scientific
discoveries. We shall take a more interesting point of view, and
deal with him rather as a human being who had contracted the habit
of making scientific researches.

What must have been his feelings when he first found with his "new"
telescope the satellites of Jupiter? They were seen on the night
of January 7, 1610. He had already viewed the planet through his
earlier and less powerful glass, and was aware that it possessed
a round disk like the moon, only smaller. Now he saw also three
objects that he took to be little stars near the planet. But on the
following night, as he says, "drawn by what fate I know not," the
tube was again turned upon the planet. The three small stars had
changed their positions, and were now all situated to the west of
Jupiter, whereas on the previous night two had been on the eastern
side. He could not explain this phenomenon, but he recognized that
there was something peculiar at work. Long afterward, in one of
his later works, translated into quaint old English by Salusbury,
he declared that "one sole experiment sufficeth to batter to the
ground a thousand probable Arguments." This was already the guiding
principle of his scientific activity, a principle of incomparable
importance, and generally credited to Bacon. Needless to say,
Jupiter was now examined every night.

The 9th was cloudy, but on the 10th he again saw his little stars,
their number now reduced to two. He guessed that the third was
behind the planet's disk. The position of the two visible ones was
altogether different from either of the previous observations. On
the 11th he became sure that what he saw was really a series of
satellites accompanying Jupiter on his journey through space, and
at the same time revolving around him. On the 12th, at 3 A.M.,
he actually saw one of the small objects emerge from behind the
planet; and on the 13th he finally saw four satellites. Two hundred
and eighty-two years were destined to pass away before any human
eye should see a fifth. It was Barnard in 1892 who followed Galileo.

To understand the effect of this discovery upon Galileo requires
a person who has himself watched the stars, not, as a dilettante,
seeking recreation or amusement, but with that deep reverence
that comes only to him who feels--nay, knows--that in the moment
of observation just passed he too has added his mite to the great
fund of human knowledge. Galileo's mummied forefinger still points
toward the stars from its little pedestal of wood in the _Museo_
at Florence, a sign to all men that he is unforgotten. But Galileo
knew on that 11th of January, 1610, that the memory of him would
never fade; that the very music of the spheres would thenceforward
be attuned to a truer note, if any would but hearken to the Jovian
harmony. For he recognized at once that the visible revolution of
these moons around Jupiter, while that planet was himself visibly
travelling through space, must deal its death-blow to the old
Ptolemaic system of the universe. Here was a great planet, the
centre of a system of satellites, and yet not the centre of the
universe. Surely, then, the earth, too, might be a mere planet like
Jupiter, and not the supposed motionless centre of all things.

The satellite discovery was published in 1610 in a little book
called "Sidereus Nuncius," usually translated "The Sidereal
Messenger." It seems to us, however, that the word "messenger" is
not strong enough; surely in Papal Italy a _nuncius_ was more than
a mere messenger. He was clothed with the very highest authority,
and we think it probable that Galileo's choice of this word in
the title of his book means that he claimed for himself similar
authority in science. At all events, the book made him at once a
great reputation and numerous enemies.

But it was not until 1616 that the Holy Office (Inquisition) issued
an edict ordering Galileo to abandon his opinion that the earth
moved, and at the same time placed Copernicus's _De Revolutionibus_
and two other books advocating that doctrine on the "Index Librorum
Prohibitorum," or list of books forbidden by the Church. These
volumes remained in subsequent editions of the "Index" down to
1821, but they no longer appear in the edition in force to-day.

Galileo's most characteristic work is entitled the "Dialogue on the
Two Chief Systems of the World." It was not published until 1632,
although the idea of the book was conceived many years earlier.
In it he gave full play to his extraordinary powers as a true
humorist, a _fine lame_ among controversialists, and a genuine man
of science, valuing naked truth above all other things. As may
be imagined, it was no small matter to obtain the authorities'
consent to this publication. Galileo was already known to hold
heretical opinions, and it was suspected that he had not laid them
aside when commanded to do so by the edict of 1616. But perhaps
Galileo's introduction to the "Dialogue" secured the censor's
_imprimatur_; it is even suspected that the Roman authorities
helped in the preparation of this introduction. Fortunately, we
have a delightful contemporary translation into English, by Thomas
Salusbury, printed at London by Leybourne in 1661. We have already
quoted from this translation, and now add from the same work part
of Galileo's masterly preface to the "Dialogue":

"Judicious reader, there was published some years since in _Rome_
a salutiferous Edict, that, for the obviating of the dangerous
Scandals of the Present Age, imposed a reasonable Silence upon the
Pythagorean (Copernican) opinion of the Mobility of the Earth.
There want not such as unadvisedly affirm, that the Decree was not
the production of a sober Scrutiny, but of an ill-formed passion;
and one may hear some mutter that Consultors altogether ignorant
of Astronomical observations ought not to clipp the wings of
speculative wits with rash prohibitions."

Galileo first states his own views, and then pretends that he will
oppose them. He goes on to say that he believes in the earth's
immobility, and takes "the contrary only for a mathematical
_Capriccio_," as he calls it; something to be considered, because
possessing an academical interest, but on no account having a real
existence. Of course any one (even a censor) ought to be able to
see that it is the Capriccio, and not its opposite, that Galileo
really advocates. Three persons appear in the "Dialogue": Salviati,
who believes in the Copernican system; Simplicio, of suggestive
name, who thinks the earth cannot move; and, finally, Sagredus,
a neutral gentleman of humorous propensities, who usually begins
by opposing Salviati, but ends by being convinced. He then helps
to punish poor Simplicio, who is one of those persons apparently
incapable of comprehending a reasonable argument. Here is an
interesting specimen of the "Dialogue" taken from Salusbury's
translation: Salviati refers to the argument, then well known, that
the earth cannot rotate on its axis, "because of the impossibility
of its moving long without wearinesse." Sagredus replies: "There
are some kinds of animals which refresh themselves after wearinesse
by rowling on the earth; and that therefore there is no need
to fear that the Terrestrial Globe should tire, nay, it may be
reasonably affirmed that it enjoyeth a perpetual and most tranquil
repose, keeping itself in an eternal rowling." Salviati's comment
on this sally is, "You are too tart and satyrical, Sagredus."

There is no doubt that the "Dialogue" finished the Ptolemaic
theory, and made that of Copernicus the only possible one. At
all events, it brought about the well-known attack upon Galileo
from the authorities of the Holy Office. We shall not recount
the often-told tale of his recantation. He was convicted (very
rightly) of being a Copernican, and was forced to abjure that
doctrine. Galileo's life may be summed up as one of those through
which the world has been made richer. A clean-cutting analytic
wit, never becoming dull: heated again and again in the fierce
blaze of controversy, it was allowed to cool only that it might
acquire a finer temper, to pierce with fatal certainty the smallest
imperfections in the armor of his adversaries.




THE PLANET OF 1898


The discovery of a new and important planet usually receives
more immediate popular attention and applause than any other
astronomical event. Philosophers are fond of referring to our
solar system as a mere atom among the countless universes that
seem to be suspended within the profound depths of space. They are
wont to point out that this solar system, small and insignificant
as a whole in comparison with many of the stellar worlds, is,
nevertheless, made up of a large number of constituent planets; and
these in turn are often accompanied with still smaller satellites,
or moons. Thus does Nature provide worlds within worlds, and it is
not surprising that public attention should be at once attracted
by any new member of our sun's own special family of planets. The
ancients were acquainted with only five of the bodies now counted
as planets, viz.: Mercury, Venus, Mars, Jupiter, and Saturn. The
dates of their discovery are lost in antiquity. To these Uranus was
added in 1781 by a brilliant effort of the elder Herschel. We are
told that intense popular excitement followed the announcement of
Herschel's first observation: he was knighted and otherwise honored
by the English King, and was enabled to lay a secure foundation for
the future distinguished astronomical reputation of his family.

Herschel's discovery quickened the restless activity of
astronomers. Persistent efforts were made to sift the heavens
more and more closely, with the strengthened hope of adding still
further to our planetary knowledge. An association of twenty-four
enthusiastic German astronomers was formed for the express purpose
of hunting planets. But it fell to the lot of an Italian, Piazzi,
of Palermo, to find the first of that series of small bodies now
known as the asteroids or minor planets. He made the discovery at
the very beginning of our century, January 1, 1801.

But news travelled slowly in those days, and it was not until
nearly April that the German observers heard from Piazzi. In the
meantime, he had himself been prevented by illness from continuing
his observations. Unfortunately, the planet had by this time moved
so near the sun, on account of its own motions and those of the
earth, that it could no longer be observed. The bright light of
the sun made observations of the new body impossible; and it was
feared that, owing to lack of knowledge of the planet's orbit,
astronomers would be unable to trace it. So there seemed, indeed,
to be danger of an almost irreparable loss to science. But in
scientific, as in other human emergencies, someone always appears
at the proper moment. A very young mathematician at Göttingen,
named Gauss, attacked the problem, and was able to devise a method
of predicting the future course of the planet on the sky, using
only the few observations made by Piazzi himself. Up to that time
no one had attempted to compute a planetary orbit, unless he had
at his disposal a series of observations extending throughout the
whole period of the planet's revolution around the sun. But the
Piazzi planet offered a new problem in astronomy. It had become
imperatively necessary to obtain an orbit from a few observations
made at nearly the same date. Gauss's work was signally triumphant,
for the planet was actually found in the position predicted by him,
as soon as a change in the relative places of the planet and earth
permitted suitable observations to be made.

But after all, Piazzi's planet belongs to a class of quite small
bodies, and is by no means as interesting as Herschel's discovery,
Uranus. Yet even this must be relegated to second rank among
planetary discoveries. On September 23, 1846, the telescope of the
Berlin Observatory was directed to a certain point on the sky for a
very special reason. Galle, the astronomer of Berlin, had received
a letter from Leverrier, of Paris, telling him that if he would
look in a certain direction he would detect a new and large planet.

Leverrier's information was based upon a mathematical calculation.
Seated in his study, with no instruments but pen and paper, he
had slowly figured out the history of a world as yet unseen.
Tiny discrepancies existed in the observed motions of Herschel's
planet Uranus. No man had explained their cause. To Leverrier's
acute understanding they slowly shaped themselves into the
possible effects of attraction emanating from some unknown planet
exterior to Uranus. Was it conceivable that these slight tremulous
imperfections in the motion of a planet could be explained in this
way? Leverrier was able to say confidently, "Yes." But we may rest
assured that Galle had but small hopes that upon his eye first, of
all the myriad eyes of men, would fall a ray of the new planet's
light. Careful and methodical, he would neglect no chance of
advancing his beloved science. He would look.

Only one who has himself often seen the morning's sunrise put an
end to a night's observation of the stars can hope to appreciate
what Galle's feelings must have been when he saw the planet. To his
trained eye it was certainly recognizable at once. And then the
good news was sent on to Paris. We can imagine Leverrier, the cool
calculator, saying to himself: "Of course he found it. It was a
mathematical certainty." Nevertheless, his satisfaction must have
been of the keenest. No triumphs give a pleasure higher than those
of the intellect. Let no one imagine that men who make researches
in the domain of pure science are under-paid. They find their
reward in pleasure that is beyond any price.

The Leverrier planet was found to be the last of the so-called
major planets, so far as we can say in the present state of
science. It received the name Neptune. Observers have found no
other member of the solar system comparable in size with such
bodies as Uranus and Neptune. More than one eager mathematician has
tried to repeat Leverrier's achievement, but the supposed planet
was not found. It has been said that figures never lie; yet such is
the case only when the computations are correctly made. People are
prone to give to the work of careless or incompetent mathematicians
the same degree of credence that is really due only to masters of
the craft. It requires the test of time to affix to any man's work
the stamp of true genius.

While, then, we have found no more large planets, quite a group of
companions to Piazzi's little one have been discovered. They are
all small, probably never exceeding about 400 miles in diameter.
All travel around the sun in orbits that lie wholly within that
of Jupiter and are exterior to that of Mars. The introduction of
astronomical photography has given a tremendous impetus to the
discovery of these minor planets, as they are called. It is quite
interesting to examine the photographic process by which such
discoveries are made possible and even easy. The matter will not
be difficult to understand if we remember that all the planets are
continually changing their places among the other stars. For the
planets travel around the sun at a comparatively small distance.
The great majority of the stars, on the contrary, are separated
from the sun by an almost immeasurable space. As a result, they do
not seem to move at all among themselves, and so we call them fixed
stars: they may, indeed, be in motion, but their great distance
prevents our detecting it in a short period of time.

Now, stellar photographs are made in much the same way as ordinary
portraits. Only, instead of using a simple camera, the astronomer
exposes his photographic plate at the eye-end of a telescope. The
sensitive surface of the plate is substituted for the human eye. We
then find on the picture a little dot corresponding to every star
within the photographed region of the sky. But, as everyone knows,
the turning of the earth on its axis makes the whole heavens,
including the sun, moon, and stars, rise and set every day. So the
stars, when we photograph them, are sure to be either climbing up
in the eastern sky or else slowly creeping down in the western. And
that makes astronomical photography very different from ordinary
portrait work.

The stars correspond to the sitter, but they don't sit still.
For this reason it is necessary to connect the telescope with a
mechanical contrivance which makes it turn round like the hour-hand
of an ordinary clock. The arrangement is so adjusted that the
telescope, once aimed at the proper object in the sky, will move
so as to remain pointed exactly the same during the whole time of
the photographic exposure. Thus, while the light of any star is
acting on the plate, such action will be continuous at a single
point. Consequently, the finished picture will show the star as a
little dot; while without this arrangement, the star would trail
out into a line instead of a dot. Now we have seen that the planets
are all moving slowly among the fixed stars. So if we make a star
photograph in a part of the sky where a planet happens to be, the
planet will make a short line on the plate; whereas, if the planet
remained quite unmoved relatively to the stars it would give a dot
like the star dots. The presence of a line, therefore, at once
indicates a planet.

This method of planet-hunting has proved most useful. More than 400
small planets similar to Piazzi's have been found, though never
another one like Uranus and Neptune. As we have said, all these
little bodies lie between Mars and Jupiter. They evidently belong
to a group or family, and many astronomers have been led to believe
that they are but fragments of a former large planet.

In August, 1898, however, one was found by Witt, of Berlin, which
will probably occupy a very prominent place in the annals of
astronomy. For this planet goes well within the orbit of Mars,
and this will bring it at times very close to the earth. In fact,
when the motions of the new planet and the earth combine to bring
them to their positions of greatest proximity, the new planet will
approach us closer than any other celestial body except our own
moon. Witt named his new planet Eros. Its size, though small, may
prove to be sufficient to bring it within the possibilities of
naked-eye observation at the time of closest approach to the earth.

To astronomers the great importance of this new planet is due to
the following circumstance: For certain reasons too technical to be
stated here in detail, the distance from the earth to any planet
can be determined with a degree of precision which is greatest for
planets that are near us. Thus in time we shall learn the distance
of Eros more accurately than we know any other celestial distance.
From this, by a process of calculation, the solar distance from the
earth is determinable. But the distance from earth to sun is the
fundamental astronomical unit of measure; so that Witt's discovery,
through its effect on the unit of measure, will doubtless
influence every part of the science of astronomy. Here we have
once more a striking instance of the reward sure to overtake
the diligent worker in science--a whole generation of men will
doubtless pass away before we shall have exhausted the scientific
advantages to be drawn from Witt's remarkable observation of 1898.




HOW TO MAKE A SUN-DIAL[A]


Long before clocks and watches had been invented, people began to
measure time with sun-dials. Nowadays, when almost everyone has a
watch in his pocket, and can have a clock, too, on the mantel-piece
of every room in the house, the sun-dial has ceased to be needed
in ordinary life. But it is still just as interesting as ever to
anyone who would like to have the means of getting time direct
from the sun, the great hour-hand or timekeeper of the sky. Any
person who is handy with tools can make a sun-dial quite easily, by
following the directions given below.

In the first place, you must know that the sun-dial gives the time
by means of the sun's shadow. If you stick a walking-cane up in the
sand on a bright, sunshiny day, the cane has a long shadow that
looks like a dark line on the ground. Now if you watch this shadow
carefully, you will see that it does not stay in the same place all
day. Slowly but surely, as the sun climbs up in the sky, the shadow
creeps around the cane. You can see quite easily that if the cane
were fastened in a board floor, and if we could mark on the floor
the places where the shadow was at different hours of the day, we
could make the shadow tell us the time just like the hour-hand of a
clock. A sun-dial is just such an arrangement as this, and I will
show you how to mark the shadow places exactly, so as to tell the
right time without any trouble whenever the sun shines.

If you were to watch very carefully such an arrangement as a cane
standing in a board floor, you would not find the creeping shadow
in just the same place at the same time every day. If you marked
the place of the shadow at exactly ten o'clock by your watch some
morning, and then went back another day at ten, you would not find
the shadow on the old mark. It would not get very far from it in
a day or two, but in a month or so it would be quite a distance
away. Now, of course, a sun-dial would be of no use if it did not
tell the time correctly every day; and in fact, it is not easy to
make a dial when the shadow is cast by a stick standing straight
up. But we can get over this difficulty very well by letting the
shadow be cast by a stick that leans over toward the floor just the
right amount, as I will explain in a moment. Of course, we should
not really use the floor for our sun-dial. It is much better to
mark out the hour-lines, as they are called, on a smooth piece of
ordinary white board, and then, after the dial is finished, it can
be screwed down to a piazza floor or railing, or it can be fastened
on a window-sill. It ought to be put in a place where the sun can
get at it most of the time, because, of course, you cannot use the
sun-dial when the sun is not shining on it. If the dial is set on a
window-sill (of a city house, for instance) you must choose a south
window if you can, so as to get the sun nearly all day. If you have
to take an east window, you can use the dial in the morning only,
and in a west window only in the afternoon. Sometimes it is best
not to try to fasten the dial to its support with screws, but just
to mark its place, and then set it out whenever you want to use
it. For if the dial is made of wood, and not painted, it might be
injured by rain or snow in bad weather if left out on a window-sill
or piazza.

[Illustration: Fig. 1.]

It is not quite easy to fasten a little stick to a board so that it
will lean over just right. So it is better not to use a stick or
a cane in the way I have described, but instead to use a piece of
board cut to just the right shape.

Fig. 1 shows what a sun-dial should look like. The lines to show
the shadow's place at the different hours of the day will be marked
on the board ABCD, and this will be put flat on the window-sill
or piazza floor. The three-cornered piece of board _abc_ is
fastened to the bottom-board ABCD by screws going through ABCD
from underneath. The edge _ab_ of the three-cornered board _abc_
then takes the place of the leaning stick or cane, and the time
is marked by the shadow cast by the edge _ab_. Of course, it is
important that this edge should be straight and perfectly flat and
even. If you are handy with tools, you can make it quite easily,
but if not, you can mark the right shape on a piece of paper very
carefully, and take it to a carpenter, who can cut the board
according to the pattern you have marked on the paper.

[Illustration: Fig. 2.]

Now I must tell you how to draw the shape of the three-cornered
board _abc_. Fig. 2 shows how it is done. The side _ac_ should
always be just five inches long. The side _bc_ is drawn at right
angles to _ac_, which you can do with an ordinary carpenter's
square. The length of _bc_ depends on the place for which the dial
is made. The following table gives the length of _bc_ for various
places in the United States, and, after you have marked out the
length of _bc_, it is only necessary to complete the three-cornered
piece by drawing the side _ab_ from _a_ to _b_.


TABLE SHOWING THE LENGTH OF THE SIDE _bc_.

  --------------------------
     Place.           _b c_
                     Inches.
  --------------------------
  Albany             4-11/16
  Baltimore          4-1/16
  Boston             4-1/2
  Buffalo            4-11/16
  Charleston         3-1/4
  Chicago            4-1/2
  Cincinnati         4-1/16
  Cleveland          4-1/2
  Denver             4-3/16
  Detroit            4-1/2
  Indianapolis       4-1/16
  Kansas City        3-15/16
  Louisville         3-15/16
  Milwaukee          3-11/16
  New Orleans        2-7/8
  New York           4-3/8
  Omaha              4-3/8
  Philadelphia       4-3/16
  Pittsburg          4-3/8
  Portland, Me       4-13/16
  Richmond           3-15/16
  Rochester          4-11/16
  San Diego          3-1/4
  San Francisco      3-15/16
  Savannah           3-1/8
  St. Louis          3-15/16
  St. Paul           5
  Seattle            5-9/16
  Washington, D. C.  4-1/16
  --------------------------

If you wish to make a dial for a place not given in the table,
it will be near enough to use the distance _bc_ as given for the
place nearest to you. But in selecting the nearest place from the
table, please remember to take that one of the cities mentioned
which is nearest to you in a north-and-south direction. It does
not matter how far away the place is in an east-and-west direction.
So, instead of taking the place that is nearest to you on the map
in a straight line, take the place to which you could travel by
going principally east or west, and very little north or south. The
figure drawn is about the right shape for New York. The board used
for the three-cornered piece should be about one-half inch thick.
But if you are making a window-sill dial, you may prefer to have it
smaller than I have described. You can easily have it half as big
by making all the sizes and lines in half-inches where the table
calls for inches.

After you have marked out the dimensions for the three-cornered
piece that is to throw the shadow, you can prepare the dial itself,
with the lines that mark the place of the shadow for every hour
of the day. This you can do in the manner shown in Fig. 3. Just
as in the case of the three-cornered piece, you can draw the dial
with a pencil directly on a smooth piece of white board, about
three-quarters of an inch thick, or you can mark it out on a paper
pattern and transfer it afterward to the board. Perhaps it will be
as well to begin by drawing on paper, as any mistakes can then be
corrected before you commence to mark your wood.

[Illustration: Fig. 3.]

In the first place you must draw a couple of lines MN and M′N′,
eight inches long, and just far enough apart to fit the edge of
your three-cornered shadow-piece. You will remember I told you
to make that one-half inch thick, so your two lines will also be
one-half inch apart. Now draw the two lines NO and N′O′ square
with MN and M′N′, and make the distances NO and N′O′ just five
inches each. The lines OK, O′K′, and the other lines forming the
outer border of the dial, are then drawn just as shown, OK and O′K′
being just eight inches long, the same as MN and M′N′. The lower
lines in the figure, which are not very important, are to complete
the squares. You must mark the lines NO and N′O′ with the figures
VI, these being the lines reached by the shadow at six o'clock in
the morning and evening. The points where the VII, VIII, and other
hour-lines cut the lines OK, O′K′, MK, and M′K′ can be found from
the table on page 78.

In using the table you will notice that the line IX falls sometimes
on one side of the corner K, and sometimes on the other. Thus for
Albany the line passes seven and seven-sixteenth inches from O,
while for Charleston it passes four and three-eighth inches from M.
For Baltimore it passes exactly through the corner K.

TABLE SHOWING HOW TO MARK THE HOUR-LINES.

  -----------------+-----------------------------+------------------------
                   | Distance from O to the line | Distance from M to the
                   | marked                      | line marked
        PLACE.     +---------+---------+---------+-------+--------+-------
                   |  VII.   |  VIII.  |   IX.   |  IX.  |   X.   |   XI.
  -----------------+---------+---------+---------+-------+--------+-------
                   | Inches. | Inches. | Inches. |Inches.|Inches. |Inches.
  Albany           | 1-15/16 | 4-3/16  | 7-7/16  |       | 3-1/16 | 1-7/16
  Baltimore        | 2-1/8   | 4-11/16 | 8       |       | 2-7/8  | 1-7/16
  Boston           | 2       | 4-5/16  | 7-7/16  |       | 3-1/16 | 1-7/16
  Buffalo          | 1-15/16 | 4-3/16  | 7-7/16  |       | 3-1/16 | 1-7/16
  Charleston       | 2-7/16  | 5-3/8   |         | 4-3/8 | 2-1/2  | 1-1/8
  Chicago          | 2       | 4-5/16  | 7-7/16  |       | 3-1/16 | 1-7/16
  Cincinnati       | 2-1/8   | 4-11/16 | 8       |       | 2-7/8  | 1-7/16
  Cleveland        | 2       | 4-5/16  | 7-7/16  |   --  | 3-1/16 | 1-7/16
  Denver           | 2-1/8   | 4-1/2   | 7-11/16 |       | 2-7/8  | 1-7/16
  Detroit          | 2       | 4-5/16  | 7-7/16  |       | 3-1/16 | 1-7/16
  Indianapolis     | 2-1/8   | 4-11/16 | 8       |       | 2-7/8  | 1-7/16
  Kansas City      | 2-1/4   | 4-11/16 | 8       |       | 2-7/8  | 1-5/16
  Louisville       | 2-1/4   | 4-11/16 | 8       |       | 2-7/8  | 1-5/16
  Milwaukee        | 1-15/16 | 4-3/16  | 7-7/16  |       | 3-1/16 | 1-7/16
  New Orleans      | 2-11/16 | 5-3/4   |         | 4-1/16| 2-5/16 | 1-1/8
  New York         | 2       | 4-5/16  | 7-11/16 |       | 3-1/16 | 1-7/16
  Omaha            | 2       | 4-5/16  | 7-11/16 |       | 3-1/16 | 1-7/16
  Philadelphia     | 2-1/8   | 4-1/2   | 7-11/16 |       | 2-7/8  | 1-7/16
  Pittsburg        | 2       | 4-5/16  | 7-11/16 |       | 3-1/16 | 1-7/16
  Portland, Me     | 1-15/16 | 4-3/16  | 7-1/8   |       | 3-3/16 | 1-1/2
  Richmond         | 2-1/4   | 4-11/16 | 8       |       | 2-7/8  | 1-5/16
  Rochester        | 1-15/16 | 4-3/16  | 7-7/16  |       | 3-1/16 | 1-7/16
  San Diego        | 2-7/16  | 5-3/8   |         | 4-3/8 | 2-1/2  | 1-1/8
  San Francisco    | 2-1/4   | 4-11/16 | 8       |       | 2-7/8  | 1-5/16
  Savannah         | 2-9/16  | 5-9/16  |         | 4-1/4 | 2-1/2  | 1-1/8
  St. Louis        | 2-1/4   | 4-11/16 | 8       |       | 2-7/8  | 1-5/16
  St. Paul         | 1-15/16 | 4-1/16  | 7-1/8   |       | 3-3/16 | 1-1/2
  Seattle          | 1-13/16 | 3-15/16 | 6-5/8   |       | 3-3/8  | 1-1/2
  Washington, D. C.| 2-1/8   | 4-11/16 | 8       |       | 2-7/8  | 1-7/16
  -----------------+---------+---------+---------+-------+--------+-------

The distance for the line marked V from O′ is just the same as the
distance from O to VII. Similarly, IV corresponds to VIII, III to
IX, II to X, and I to XI. The number XII is marked at MM′ as shown.
If you desire to add lines (not shown in Fig. 3 to avoid confusion)
for hours earlier than six in the morning, it is merely necessary
to mark off a distance on the line KO, below the point O, and equal
to the distance from O to VII. This will give the point where the
5 A.M. shadow line drawn from N cuts the line KO. A corresponding
line for 7 P.M. can be drawn from N′ on the other side of the
figure.

After you have marked out the dial very carefully, you must fasten
the three-cornered shadow-piece to it in such a way that the whole
instrument will look like Fig. 1. The edge _ac_ (Fig. 2) goes on NM
(Fig. 3). The point _a_ (Fig. 2) must come exactly on N (Fig. 3);
and as the lines NM (Fig. 3) and N′M′ (Fig. 3) have been made just
the right distance apart to fit the thickness of the three-cornered
piece _abc_ (Fig. 2), everything will go together just right. The
point _c_ (Fig. 2) will not quite reach to M (Fig. 3), but will
be on the line NM (Fig. 3) at a distance of three inches from
M. The two pieces of wood will be fastened together with three
screws going through the bottom-board ABCD (Figs. 1 and 3) and
into the edge _ac_ (Fig. 2) of the three-cornered piece. The whole
instrument will then look something like Fig. 1.

After you have got your sun-dial put together, you need only set
it in the sun in a level place, on a piazza or window-sill, and
turn it round until it tells the right time by the shadow. You can
get your local time from a watch near enough for setting up the
dial. Once the dial is set right you can screw it down or mark its
position, and it will continue to give correct solar time every day
in the year.

If you wish to adjust the dial very closely, you must go out
some fine day and note the error of the dial by a watch at about
ten in the morning, and at noon, and again at about two in the
afternoon. If the error is the same each time, the dial is rightly
set. If not, you must try, by turning the dial slightly, to get
it so placed that your three errors will be nearly the same. When
you have got them as nearly alike as you can, the dial will be
sufficiently near right. The solar or dial time may, however,
differ somewhat from ordinary watch time, but the difference will
never be great enough to matter, when we remember that sun-dials
are only rough timekeepers after all, and useful principally for
amusement.


FOOTNOTE:

[A] This chapter is especially intended for boys and girls and
others who like to make things with carpenters' tools.




PHOTOGRAPHY IN ASTRONOMY


New highways of science have been monumented now and again by the
masterful efforts of genius, working single-handed; but more often
it is slow-moving time that ripens discovery, and, at the proper
moment, opens some new path to men whose intellectual power is but
willingness to learn. So the annals of astronomical photography do
not recount the achievements of extraordinary genius. It would have
been strange, indeed, if the discovery of photography had not been
followed by its application to astronomy.

The whole range of chemical science contains no experiment of
greater inherent interest than the development of a photographic
plate. Let but the smallest ray of light fall upon its strangely
sensitive surface, and some subtle invisible change takes place.
It is then merely necessary to plunge the plate into a properly
prepared chemical bath, and the gradual process of developing
the picture begins. Slowly, very slowly, the colorless surface
darkens wherever light has touched it. Let us imagine that the
exposure has been made with an ordinary lens and camera, and that
it is a landscape seeming to grow beneath the experimenter's
eyes. At first only the most conspicuous objects make their
appearance. But gradually the process extends, until finally
every tiny detail is reproduced with marvellous fidelity to the
original. The photographic plate, when developed in this way, is
called a "negative." For in Nature luminous points, or sources
of light, are bright, while the developing negative turns dark
wherever light has acted. Thus the negative, while true to Nature,
reproduces everything in a reversed way; bright things are dark,
and shadows appear light. For ordinary purposes, therefore, the
negative has to be replaced by a new photograph made by copying it
again photographically. In this way it is again reversed, giving
us a picture corresponding correctly to the facts as seen. Such a
copy from a negative is what is ordinarily called a photograph;
technically, it is known as a "positive."

One of the remarkable things about the sensitive plate is its
complete indifference to the distance from which the light comes.
It is ready to yield obediently to the ray of some distant star
that may have journeyed, as it were, from the very vanishing
point of space, or to the bright glow of an electric light upon
the photographer's table. This quality makes its use especially
advantageous in astronomy, since we can gain knowledge of remote
stars only by a study of the light they send us. In such study the
photographic plate possesses a supreme advantage over the human
eye. If the conditions of weather and atmosphere are favorable,
an observer looking through an ordinary telescope will see nearly
as much at the first glance as he will ever see. Attentive and
continued study will enable him to fix details upon his memory,
and to record them by means of drawings and diagrams. Occasional
moments of especially undisturbed atmospheric conditions will allow
him to glimpse faint objects seldom visible. But on the whole,
telescopic astronomers add little to their harvest by continued
husbandry in the same field of stars. Photography is different.
The effect of light upon the sensitive surface of the plate is
strictly cumulative. If a given star can bring about a certain
result when it has been allowed to act upon the plate for one
minute, then in two or three minutes it will accomplish much more.
Perhaps a single minute's exposure would have produced a mark
scarcely perceptible upon the developed negative. In that case,
three or four minutes would give us a perfectly well defined black
image of the star.

[Illustration: Star-Field in Constellation Monoceros.

Photographed by Barnard, February 1, 1894.

Exposure, three hours.]

Thus, by lengthening the exposure we can make the fainter stars
impress themselves upon the plate. If their light is not able
to produce the desired effect in minutes, we can let its action
accumulate for hours. In this manner it becomes possible and
easy to photograph objects so faint that they have never been
seen, even with our most powerful telescopes. This achievement
ranks high among those which make astronomy appeal so strongly
to the imagination. Scientific men are not given to fancies; nor
should they be. But the first long-exposure photograph must have
been an exciting thing. After coming from the observatory,
the chemical development was, of course, made in a dark room, so
that no additional light might harm the plate until the process
was complete. Carrying it out then into the light, that early
experimenter cannot but have felt a thrill of triumph; for his hand
held a true picture of dim stars to the eye unlighted, lifted into
view as if by magic.

Plates have been thus exposed as long as twenty-five hours, and the
manner of doing it is very interesting. Of course, it is impossible
to carry on the work continuously for so long a period, since the
beginning of daylight would surely ruin the photograph. In fact,
the astronomer must stop before even the faintest streak of dawn
begins to redden the eastern sky. Moreover, making astronomical
negatives requires excessively close attention, and this it is
impossible to give continuously during more than a few hours.
But the exposure of a single plate can be extended over several
nights without difficulty. It is merely necessary to close the
plate-holder with a "light-tight" cover when the first night's
work is finished. To begin further exposure of the same plate
on another night, we simply aim the photographic telescope at
precisely the same point of the sky as before. The light-tight
plate-holder being again opened, the exposure can go on as if there
had been no interruption.

Astronomers have invented a most ingenious device for making sure
that the telescope's aim can be brought back again to the same
point with great exactness. This is a very important matter;
for the slightest disturbance of the plate before the second or
subsequent portions of the exposure would ruin everything. Instead
of a very complete single picture, we should have two partial ones
mixed up together in inextricable confusion.

To prevent this, photographic telescopes are made double, not
altogether unlike an opera-glass. One of the tubes is arranged for
photography proper, while the other is fitted with lenses suitable
for an ordinary visual telescope. The two tubes are made parallel.
Thus the astronomer, by looking through the visual glass, can watch
objects in the heavens even while they are being photographed. The
visual half of the instrument is provided with a pair of very fine
cross-wires movable at will in the field of view. These can be
made to bisect some little star exactly, before beginning the first
night's work. Afterward, everything about the instrument having
been left unchanged, the astronomer can always assure himself of
coming back to precisely the same point of the sky, by so adjusting
the instrument that the same little star is again bisected.

It must not be supposed, however, that the entire instrument
remains unmoved, even during the whole of a single night's
exposure. For in that case, the apparent motion of the stars as
they rise or set in the sky would speedily carry them out of the
telescope's field of view. Consequently, this motion has to be
counteracted by shifting the telescope so as to follow the stars.
This can be accomplished accurately and automatically by means
of clock-work mechanism. Such contrivances have already been
applied in the past to visual telescopes, because even then they
facilitated the observer's work. They save him the trouble of
turning his instrument every few minutes, and allow him to give his
undivided attention to the actual business of observation.

For photographic purposes the telescope needs to "follow" the
stars far more accurately than in the older kind of observing with
the eye. Nor is it possible to make a clock that will drive the
instrument satisfactorily and quite automatically. But by means of
the second or visual telescope, astronomers can always ascertain
whether the clock is working correctly at any given moment.
It requires only a glance at the little star bisected by the
cross-wires, and, if there has been the slightest imperfection in
the following by clock-work, the star will no longer be cut exactly
by the wires.

The astronomer can at once correct any error by putting in
operation a very ingenious mechanical device sometimes called
a "mouse-control." He need only touch an electric button, and
a signal is sent into the clock-work. Instantly there is a
shifting of the mechanism. For one of the regular driving wheels
is substituted, temporarily, another having an _extra tooth_.
This makes the clock run a little faster so long as the electric
current passes. In a similar way, by means of another button, the
clock can be made to run slower temporarily. Thus by watching
the cross-wires continuously, and manipulating his two electric
buttons, the photographic astronomer can compel his telescope
to follow exactly the object under observation, and he can make
certain of obtaining a perfect negative.

These long-exposure plates are intended especially for what may be
called descriptive astronomy. With them, as we have seen, advantage
is taken of cumulative light-effects on the sensitive plate, and
the telescope's light-gathering and space-penetrating powers are
vastly increased. We are enabled to carry our researches far
beyond the confines of the old visible universe. Extremely faint
objects can be recorded, even down to their minutest details, with
a fidelity unknown to older visual methods. But at present we
intend to consider principally applications of photography in the
astronomy of measurement, rather than the descriptive branch of our
subject. Instead of describing pictures made simply to see what
certain objects look like in the sky, we shall consider negatives
intended for precise measurement, with all that the word precision
implies in celestial science.

Taking up first the photography of stars, we must begin by
mentioning the work of Rutherfurd at New York. More than thirty
years ago he had so far perfected methods of stellar photography
that he was able to secure excellent pictures of stars as faint as
the ninth magnitude. In those days the modern process of dry-plate
photography had not been invented. To-day, plates exposed in the
photographic telescope are made of glass covered with a perfectly
dry film of sensitized gelatine. But in the old wet-plate process
the sensitive film was first wetted with a chemical solution; and
this solution could not be allowed to dry during the exposure.
Consequently, Rutherfurd was limited to exposures a few minutes
in length, while nowadays, as we have said, their duration can be
prolonged at will.

When we add to this the fact that the old plates were far less
sensitive to light than those now available, it is easy to see
what were the difficulties in the way of photographing faint stars
in Rutherfurd's time. Nor did he possess the modern ingenious
device of a combined visual and photographic instrument. He had no
electric controlling apparatus. In fact, the younger generation of
astronomers can form no adequate idea of the patience and personal
skill Rutherfurd must have had at his command. For he certainly did
produce negatives that are but little inferior to the best that can
be made to-day. His only limitation was that he could not obtain
images of stars much below the ninth magnitude.

To understand just what is meant here by the ninth magnitude, it
is necessary to go back in imagination to the time of Hipparchus,
the father of sidereal astronomy. (See page 39.) He adopted the
convenient plan of dividing all the stars visible to the naked eye
(of course, he had no telescope) into six classes, according to
their brilliancy. The faintest visible stars were put in the sixth
class, and all the others were assigned somewhat arbitrarily to one
or the other of the brighter classes.

Modern astronomers have devised a more scientific system, which has
been made to conform very nearly to that of Hipparchus, just as
it has come down to us through the ages. We have adopted a certain
arbitrary degree of luminosity as the standard "first-magnitude";
compared with sunlight, this may be represented roughly by a
fraction of which the numerator is 1, and the denominator about
eighty thousand millions. The standard second-magnitude star is one
whose light, compared with a first-magnitude, may be represented
approximately by the fraction ⅖. The third magnitude, in turn, may
be compared with the second by the same fraction ⅖; and so the
classification is extended to magnitudes below those visible to the
unaided eye. Each magnitude compares with the one above it, as the
light of two candles would compare with the light of five.

Rutherfurd did not stop with mere photographs. He realized very
clearly the obvious truth that by making a picture of the sky we
simply change the scene of our operations. Upon the photograph
we can measure that which we might have studied directly in the
heavens; but so long as they remain unmeasured, celestial pictures
have a potential value only. Locked within them may lie hidden
some secret of our universe. But it will not come forth unsought.
Patient effort must precede discovery, in photography, as elsewhere
in science. There is no royal road. Rutherfurd devised an elaborate
measuring-machine in which his photographs could be examined under
the microscope with the most minute exactness. With this machine
he measured a large number of his pictures; and it has been shown
quite recently that the results obtained from them are comparable
in accuracy with those coming from the most highly accredited
methods of direct eye-observation.

And photographs are far superior in ease of manipulation.
Convenient day-observing under the microscope in a comfortable
astronomical laboratory is substituted for all the discomforts
of a midnight vigil under the stars. The work of measurement can
proceed in all weathers, whereas formerly it was limited strictly
to perfectly clear nights. Lastly, the negatives form a permanent
record, to which we can always return to correct errors or
re-examine doubtful points.

Rutherfurd's stellar work extended down to about 1877, and
included especially parallax determinations and the photography of
star-clusters. Each of these subjects is receiving close attention
from later investigators, and, therefore, merits brief mention
here. Stellar parallax is in one sense but another name for stellar
distance. Its measurement has been one of the important problems
of astronomy for centuries, ever since men recognized that the
Copernican theory of our universe requires the determination of
stellar distances for its complete demonstration.

If the earth is swinging around the sun once a year in a mighty
path or orbit, there must be changes of its position in space
comparable in size with the orbit itself. And the stars ought to
shift their apparent places on the sky to correspond with these
changes in the terrestrial observer's position. The phenomenon is
analogous to what occurs when we look out of a room, first through
one window, and then through another. Any object on the opposite
side of the street will be seen in a changed direction, on account
of the observer's having shifted his position from one window to
the other. If the object seemed to be due north when seen from
the first window, it will, perhaps, appear a little east of north
from the other. But this change of direction will be comparatively
small, if the object under observation is very far away, in
comparison with the distance between the two windows.

This is what occurs with the stars. The earth's orbit, vast as
it is, shrinks into almost absolute insignificance when compared
with the profound distances by which we are sundered from even the
nearest fixed stars. Consequently, the shifting of their positions
is also very small--so small as to be near the extreme limit
separating that which is measurable from that which is beyond human
ken.

Photography lends itself most readily to a study of this matter.
Suppose a certain star is suspected of "having a parallax." In
other words, we have reason to believe it near enough to admit of
a successful measurement of distance. Perhaps it is a very bright
star; and, other things being equal, it is probably fair to assume
that brightness signifies nearness. And astronomers have certain
other indications of proximity that guide them in the selection of
proper objects for investigation, though such evidence, of course,
never takes the place of actual measurement.

The star under examination is sure to have near it on the sky a
number of stars so very small that we may safely take them to be
immeasurably far away. The parallax star is among them, but not
of them. We see it projected upon the background of the heavens,
though it may in reality be quite near us, astronomically speaking.
If this is really so, and the star, therefore, subject to the
slight parallactic shifting already mentioned, we can detect it by
noting the suspected star's position among the surrounding small
stars. For these, being immeasurably remote, will remain unchanged,
within the limits of our powers of observation, and thus serve
as points of reference for marking the apparent shifting of the
brighter star we are actually considering.

We have merely to photograph the region at various seasons of the
year. Careful examination of the photographs under the microscope
will then enable us to measure the slightest displacement of the
parallax star. From these measures, by a process of calculation,
astronomers can then obtain the star's distance. It will not
become known in miles; we shall only ascertain how many times the
distance between the earth and sun would have to be laid down like
a measuring-rod, in order to cover the space separating us from the
star: and the subsequent evaluation of this distance "earth to sun"
in miles is another important problem in whose solution photography
promises to be most useful.

The above method of measuring stellar distance is, of course,
subject to whatever slight uncertainty arises from the assumption
that the small stars used for comparison are themselves beyond the
possibility of parallactic shifting. But astronomy possesses no
better method. Moreover, the number of small stars used in this
way is, of course, much larger in photography than it ever can be
in visual work. In the former process, all surrounding stars can
be photographed at once; in the latter each star must be measured
separately, and daylight soon intervenes to impose a limit on
numbers. Usually only two can be used; so that here photography
has a most important advantage. It minimizes the chance of our
parallax being rendered erroneous, by the stars of comparison not
being really infinitely remote. This might happen, perhaps, in the
case of one or two; but with an average result from a large number
we know it to be practically impossible.

Cluster work is not altogether unlike "parallax hunting" in its
preliminary stage of securing the photographic observations. The
object is to obtain an absolutely faithful picture of a star group,
just as it exists in the sky. We have every reason to suppose
that a very large number of stars condensed into one small spot
upon the heavens means something more than chance aggregation.
The Pleiades group (page 10) contains thousands of massive stars,
doubtless held together by the force of their mutual gravitational
attraction. If this be true, there must be complex orbital motion
in the cluster; and, as time goes on, we should actually see the
separate components change their relative positions, as it were,
before our eyes. The details of such motion upon the great scale
of cosmic space offer one of the many problems that make astronomy
the grandest of human sciences.

We have said that time must pass before we can see these things;
there may be centuries of waiting. But one way exists to hurry on
the perfection of our knowledge; we must increase the precision of
observations. Motions that would need the growth of centuries to
become visible to the older astronomical appliances, might yield
in a few decades to more delicate observational processes. Here
photography is most promising. Having once obtained a surpassingly
accurate picture of a star-cluster, we can subject it easily to
precise microscopic measurement. The same operations repeated at a
later date will enable us to compare the two series of measures,
and thus ascertain the motions that may have occurred in the
interval. The Rutherfurd photographs furnish a veritable mine of
information in researches of this kind; for they antedate all other
celestial photographs of precision by at least a quarter-century,
and bring just so much nearer the time when definite knowledge
shall replace information based on reasoning from probabilities.

Rutherfurd's methods showed the advantages of photography as
applied to individual star-clusters. It required only the attention
of some astronomer disposing of large observational facilities,
and accustomed to operations upon a great scale, to apply similar
methods throughout the whole heavens. In the year 1882 a bright
comet was very conspicuous in the southern heavens. It was
extensively observed from the southern hemisphere, and especially
at the British Royal Observatory at the Cape of Good Hope.

Gill, director of that institution, conceived the idea that this
comet might be bright enough to photograph. At that time, comet
photography had been attempted but little, if at all, and it was
by no means sure that the experiment would be successful. Nor was
Gill well acquainted with the work of Rutherfurd; for the best
results of that astronomer had lain dormant many years. He was one
of those men with whom personal modesty amounts to a fault. Loath
to put himself forward in any way, and disliking to rush into
print, Rutherfurd had given but little publicity to his work. This
peculiarity has, doubtless, delayed his just reputation; but he
will lose nothing in the end from a brief postponement. Gill must,
however, be credited with more penetration than would be his due
if Rutherfurd had made it possible for others to know that he had
anticipated many of the newer ideas.

However this may be, the comet was photographed with the help of
a local portrait photographer named Allis. When Gill and Allis
fastened a simple portrait camera belonging to the latter upon the
tube of one of the Cape telescopes, and pointed it at the great
comet, they little thought the experiment would lead to one of the
greatest astronomical works ever attempted by men. Yet this was
destined to occur. The negative they obtained showed an excellent
picture of the comet; but what was more important for the future of
sidereal astronomy, it was also quite thickly dotted with little
black points corresponding to stars. The extraordinary ease with
which the whole heavens could be thus charted photographically
was brought home to Gill as never before. It was this comet
picture that interested him in the application of photography
to star-charting; and without his interest the now famous
astro-photographic catalogue of the heavens would probably never
have been made.

After considerable preliminary correspondence, a congress
of astronomers was finally called to meet at Paris in 1887.
Representatives of the principal observatories and civilized
governments were present. They decided that the end of the
nineteenth century should see the making of a great catalogue
of all the stars in the sky, upon a scale of completeness and
precision surpassing anything previously attempted. It is
impossible to exaggerate the importance of such a work; for upon
our star-catalogues depends ultimately the entire structure of
astronomical science.

The work was far too vast for the powers of any observatory alone.
Therefore, the whole sky, from pole to pole, was divided into
eighteen belts or zones of approximately equal area; and each of
these was assigned to a single observatory to be photographed. A
series of telescopes was specially constructed, so that every
part of the work should be done with the same type of instrument.
As far as possible, an attempt was made to secure uniformity of
methods, and particularly a uniform scale of precision. To cover
the entire sky upon the plan proposed no less than 44,108 negatives
are required; and most of these have now been finished. The further
measurement of the pictures and the drawing up of a vast printed
star-catalogue are also well under way. One of the participating
observatories, that at Potsdam, Germany, has published the first
volume of its part of the catalogue. It is estimated that this
observatory alone will require twenty quarto volumes to contain
merely the final results of its work on the catalogue. Altogether
not less than two million stars will find a place in this, our
latest directory of the heavens.

Such wholesale methods of attacking problems of observational
astronomy are particularly characteristic of photography. The great
catalogue is, perhaps, the best illustration of this tendency; but
of scarcely smaller interest, though less important in reality, is
the photographic method of dealing with minor planets. We have
already said (page 63) that in the space between the orbits of Mars
and Jupiter several hundred small bodies are moving around the sun
in ordinary planetary orbits. These bodies are called asteroids,
or minor planets. The visual method of discovering unknown members
of this group was painfully tedious; but photography has changed
matters completely, and has added immensely to our knowledge of the
asteroids.

Wolf, of Heidelberg, first made use of the new process for
minor-planet discovery. His method is sufficiently ingenious to
deserve brief mention again. A photograph of a suitable region
of the sky was made with an exposure lasting two or three hours.
Throughout all this time the instrument was manipulated so as
to follow the motion of the heavens in the way we have already
explained, so that each star would appear on the negative as a
small, round, black dot.

But if a minor planet happened to be in the region covered by the
plate, its photographic image would be very different. For the
orbital motion of the planet about the sun would make it move a
little among the stars even in the two or three hours during which
the plate was exposed. This motion would be faithfully reproduced
in the picture, so that the planet would appear as a short curved
line rather than a well-defined dot like a star. Thus the presence
of such a line-image infallibly denotes an asteroid.

Subsequent calculations are necessary to ascertain whether the
object is a planet already known or a genuine new discovery. Wolf,
and others using his method in recent years, have made immense
additions to our catalogue of asteroids. Indeed, the matter was
beginning to lose interest on account of the frequency and sameness
of these discoveries, when the astronomical world was startled by
the finding of the Planet of 1898. (Page 58.)

On August 27, 1898, Witt, of Berlin, discovered the small body
that bears the number "433" in the list of minor planets, and has
received the name Eros. Its important peculiarity consists in the
exceptional position of the orbit. While all the other asteroids
are farther from the sun than Mars, and less distant than Jupiter,
Eros can pass within the orbit of the former. At times, therefore,
it will approach our earth more closely than any other permanent
member of the solar system, excepting our own moon. So it is, in
a sense, our nearest neighbor; and this fact alone makes it the
most interesting of all the minor planets. The nineteenth century
was opened by Piazzi's well-known discovery of the first of these
bodies (page 59); it is, therefore, fitting that we should find
the most important one at its close. We are almost certain that it
will be possible to make use of Eros to solve with unprecedented
accuracy the most important problem in all astronomy. This is
the determination of our earth's distance from the sun. When
considering stellar parallax, we have seen how our observations
enable us to measure some of the stars' distances in terms of the
distance "earth to sun" as a unit. It is, indeed, the fundamental
unit for all astronomical measures, and its exact evaluation has
always been considered the basal problem of astronomy. Astronomers
know it as the problem of Solar Parallax.

We shall not here enter into the somewhat intricate details of
this subject, however interesting they may be. The problem offers
difficulties somewhat analogous to those confronting a surveyor
who has to determine the distance of some inaccessible terrestrial
point. To do this, it is necessary first to measure a "base-line,"
as we call it. Then the measurement of angles with a theodolite
will make it possible to deduce the required distance of the
inaccessible point by a process of calculation. To insure accuracy,
however, as every surveyor knows, the base-line must be made long
enough; and this is precisely what is impossible in the case of the
solar parallax.

For we are necessarily limited to marking out our base-line on
the earth; and the entire planet is too small to furnish one of
really sufficient size. The best we can do is to use the distance
between two observatories situated, as near as may be, on opposite
sides of the earth. But even this base is wofully small. However,
the smallness loses some of its harmful effect if we operate upon
a planet that is comparatively near us. We can measure such
a planet's distance more accurately than any other; and this
being known, the solar distance can be computed by the aid of
mathematical considerations based upon Newton's law of gravitation
and observational determinations of the planetary orbital elements.

Photography is by no means limited to investigations in the older
departments of astronomical observation. Its powerful arm has
been stretched out to grasp as well the newer instruments of
spectroscopic study. Here the sensitive plate has been substituted
for the human eye with even greater relative advantage. The
accurate microscopic measurement of difficult lines in stellar
spectra was indeed possible by older methods; but photography
has made it comparatively easy; and, above all, has rendered
practicable series of observations extensive enough in numbers to
furnish statistical information of real value. Only in this way
have we been able to determine whether the stars, in their varied
and unknown orbits, are approaching us or moving farther away. Even
the speed of this approach or recession has become measurable,
and has been evaluated in the case of many individual stars. (See
page 21.)

[Illustration: Solar Corona. Total Eclipse.

Photographed by Campbell, January 22, 1898; Jeur, India.]

The subject of solar physics has become a veritable department of
astronomy in the hands of photographic investigators. Ingenious
spectro-photographic methods have been devised, whereby we have
secured pictures of the sun from which we have learned much that
must have remained forever unknown to older methods.

Especially useful has photography proved itself in the observation
of total solar eclipses. It is only when the sun's bright disk
is completely obscured by the interposed moon that we can see
the faintly luminous structure of the solar corona, that great
appendage of our sun, whose exact nature is still unexplained. Only
during the few minutes of total eclipse in each century can we
look upon it; and keen is the interest of astronomers when those
few minutes occur. But it is found that eye observations made in
hurried excitement have comparatively little value. Half a dozen
persons might make drawings of the corona during the same eclipse,
yet they would differ so much from one another as to leave the
true outline very much in doubt. But with photography we can obtain
a really correct picture whose details can be studied and discussed
subsequently at leisure.

If we were asked to sum up in one word what photography has
accomplished, we should say that observational astronomy has
been revolutionized. There is to-day scarcely an instrument of
precision in which the sensitive plate has not been substituted
for the human eye; scarcely an inquiry possible to the older
method which cannot now be undertaken upon a grander scale.
Novel investigations formerly not even possible are now entirely
practicable by photography; and the end is not yet. Valuable as are
the achievements already consummated, photography is richest in
its promise for the future. Astronomy has been called the "perfect
science"; it is safe to predict that the next generation will
wonder that the knowledge we have to-day should ever have received
so proud a title.




TIME STANDARDS OF THE WORLD


The question is often asked, "What is the practical use of
astronomy?" We know, of course, that men would profit greatly from
a study of that science, even if it could not be turned to any
immediate bread-and-butter use; for astronomy is essentially the
science of big things, and it makes men bigger to fix their minds
on problems that deal with vast distances and seemingly endless
periods of time. No one can look upon the quietly shining stars
without being impressed by the thought of how they burned--then
as now--before he himself was born, and so shall continue after
he has passed away--aye, even after his latest descendants shall
have vanished from the earth. Of all the sciences, astronomy is
at once the most beautiful poetically, and yet the one offering
the grandest and most difficult problems to the intellect. A study
of these problems has ever been a labor of love to the greatest
minds; their solution has been counted justly among man's loftiest
achievements.

And yet of all the difficult and abstruse sciences, astronomy is,
perhaps, the one that comes into the ordinary practical daily
life of the people more definitely and frequently than any other.
There exist at least three things we owe to astronomy that must
be regarded as quite indispensable, from a purely practical point
of view. In the first place, let us consider the maps in a work
on geography. How many people ever think to ask how these maps
are made? It is true that the ordinary processes of the surveyor
would enable us to draw a map showing the outlines of a part of
the earth's surface. Even the locations of towns and rivers might
be marked in this way. But one of the most important things of all
could not be added without the aid of astronomical observations.
The latitude and longitude lines, which are essential to show the
relation of the map to the rest of the earth, we owe to astronomy.
The longitude lines, particularly, as we shall see farther on, play
a most important part in the subject of time.

The second indispensable application of astronomy to ordinary
business affairs relates to the subject of navigation. How do ships
find their way across the ocean? There are no permanent marks on
the sea, as there are on the land, by which the navigator can guide
his course. Nevertheless, seamen know their path over the trackless
ocean with a certainty as unerring as would be possible on shore;
and it is all done by the help of astronomy. The navigator's
observations of the sun are astronomical observations; the tables
he uses in calculating his observations--the tables that tell him
just where he is and in what direction he must go--are astronomical
tables. Indeed, it is not too much to say that without astronomy
there could be no safe ocean navigation.

But the third application of astronomy is of still greater
importance in our daily life--the furnishing of correct time
standards for all sorts of purposes. It is to this practical use of
astronomical science that we would direct particular attention. Few
persons ever think of the complicated machinery that must be put
in motion in order to set a clock. A man forgets some evening to
wind his watch at the accustomed hour. The next morning he finds it
run down. It must be re-set. Most people simply go to the nearest
clock, or ask some friend for the time, so as to start the watch
correctly. More careful persons, perhaps, visit the jeweller's
and take the time from his "regulator." But the regulator itself
needs to be regulated. After all, it is nothing more than any other
clock, except that greater care has been taken in the mechanical
construction and arrangement of its various parts. Yet it is but
a machine built by human hands, and, like all human works, it is
necessarily imperfect. No matter how well it has been constructed,
it will not run with perfectly rigid accuracy. Every day there will
be a variation from the true time by a small amount, and in the
course of days or weeks the accumulation of these successive small
amounts will lead to a total of quite appreciable size.

Just as the ordinary citizen looks to the jeweller's regulator to
correct his watch, so the jeweller applies to the astronomer for
the correction of his regulator. Ever since the dawn of astronomy,
in the earliest ages of which we have any record, the principal
duty of the astronomer has been the furnishing of accurate time
to the people. We shall not here enter into a detailed account,
however interesting it would be, of the gradual development by
which the very perfect system at present in use has been reached;
but shall content ourselves with a description of the methods now
employed in nearly all the civilized countries of the world.

In the first place, every observatory is, of course, provided with
what is known as an astronomical clock. This instrument, from the
astronomer's point of view, is something very different from the
ordinary popular idea. To the average person an astronomical clock
is a complicated and elaborate affair, giving the date, day of the
week, phases of the moon, and other miscellaneous information. But
in reality the astronomer wants none of these things. His one and
only requirement is that the clock shall keep as near uniform time
as may be possible to a machine constructed by human hands. No
expense is spared in making the standard clock for an observatory.
Real artists in mechanical construction--men who have attained a
world-wide celebrity for delicate skill in fashioning the parts of
a clock--such are the astronomer's clock-makers.

To increase precision of motion in the train of wheels, it is
necessary that the mechanism be as simple as possible. For this
reason all complications of date, etc., are left out. We have
even abandoned the usual convenient plan of having the hour and
minute hands mounted at the same centre; for this kind of mounting
makes necessary a slightly more intricate form of wheelwork. The
astronomer's clock usually has the centres of the second hand,
minute hand, and hour hand in a straight line, and equally distant
from each other. Each hand has its own dial; all drawn, of course,
upon the same clock-face.

Even after such a clock has been made as accurately as possible,
it will, nevertheless, not give the very best performance unless
it is taken care of properly. It is necessary to mount it very
firmly indeed. It should not be fastened to an ordinary wall, but
a strong pier of masonry or brick must be built for it on a very
solid foundation. Moreover, this pier is best placed underground
in a cellar, so that the temperature of the clock can be kept
nearly uniform all the year round; for we find that clocks do
not run quite the same in hot weather as they do in cold. Makers
have, indeed, tried to guard against this effect of temperature,
by ingenious mechanical contrivances. But these are never quite
perfect in their action, and it is best not to test them too
severely by exposing the clock to sharp changes of heat and cold.

Another thing affecting the going of fine clocks, strange as it may
seem, is the variation of barometric pressure. There is a slight
but noticeable difference in their running when the barometer is
high and when it is low. To prevent this, some of our best clocks
have been enclosed in air-tight cases, so that outside barometric
changes may not be felt in the least by the clock itself.

But even after all this has been accomplished, and the astronomer
is in possession of a clock that may be called a masterpiece of
mechanical construction, he is not any better off than was the
jeweller with his regulator. After all, even the astronomical clock
needs to be set, and its error must be determined from time to
time. A final appeal must then be had to astronomical observations.
The clock must be set by the stars and sun. For this purpose the
astronomer uses an instrument called a "transit." This is simply
a telescope of moderate size, possibly five or six feet long, and
firmly attached to an axis at right angles to the tube of the
telescope.

This axis is supported horizontally in such a way that it points
as nearly as may be exactly east and west. The telescope itself
being square with the axis, always points in a north-and-south
direction. It is possible to rotate the telescope about its axis so
as to reach all parts of the sky that are directly north or south
of the observatory. In the field of view of the telescope certain
very fine threads are mounted so as to form a little cross. As the
telescope is rotated this cross traces out, as it were, a great
circle on the sky; and this great circle is called the astronomical
meridian.

Now we are in possession of certain star-tables, computed from
the combined observations of astronomers in the last 150 years.
These tables tell us the exact moment of time when any star is
on the meridian. To discover, therefore, whether our clock is
right on any given night, it is merely necessary to watch a star
with the telescope, and note the exact instant by the clock when
it reaches the little cross in the field of view. Knowing from
the astronomical tables the time when the star ought to have been
on the meridian, and having observed the clock time when it is
actually there, the difference is, of course, the error of the
clock. The result can be checked by observations of other stars,
and the slight personal errors of observation can be rendered
harmless by taking the mean from several stars. By an hour's work
on a fine night it is possible to fix the clock error quite easily
within the one-twentieth part of a second.

We have not space to enter into the interesting details of the
methods by which the astronomical transit is accurately set in
the right position, and how any slight residual error in its
setting can be eliminated from our results by certain processes
of computation. It must suffice to say that practically all time
determinations in the observatory depend substantially upon the
procedure outlined above.

The observatory clock having been once set right by observations
of the sky, its error can be re-determined every few days quite
easily. Thus even the small irregularities of its nearly perfect
mechanism can be prevented from accumulating until they might reach
a harmful magnitude. But we obtain in this way only a correct
standard of time within the observatory itself. How can this be
made available for the general public? The problem is quite simple
with the aid of the electric telegraph. We shall give a brief
account of the methods now in use in New York City, and these may
be taken as essentially representative of those employed elsewhere.

Every day, at noon precisely, an electric signal is sent out by
the United States Naval Observatory in Washington. The signal is
regulated by the standard clock of the observatory, of course
taking account of star observations made on the next preceding
fine night. This signal is received in the central New York office
of the telegraph company, where it is used to keep correct a
very fine clock, which may be called the time standard of the
telegraph company. This clock, in turn, has automatic electric
connections, by means of which it is made to send out signals
over what are called "time wires" that go all over the city.
Jewellers, and others who desire correct time, can arrange to have
a small electric sounder in their offices connected with the time
wires. Thus the ticks of the telegraph company's standard clock
are repeated automatically in the jeweller's shop, and used for
controlling the exactness of his regulator. This, in brief, is the
method by which the astronomer's careful determination of correct
time is transferred and distributed to the people at large.

Having thus outlined the manner of obtaining and distributing
correct time, we shall now consider the question of time
differences between different places on the earth. This is a matter
which many persons find most perplexing, and yet it is essentially
quite simple in principle. Travellers, of course, are well
acquainted with the fact that their watches often need to be reset
when they arrive at their destination. Yet few ever stop to ask the
cause.

Let us consider for a moment our method of measuring time. We go
by the sun. If we leave out of account some small irregularities
of the sun's motion that are of no consequence for our present
purpose, we may lay down this fundamental principle: When the sun
reaches its highest position in the sky it is twelve o'clock or
noon.

The sun, as everyone knows, rises each morning in the east, slowly
goes up higher and higher in the sky, and at last begins to descend
again toward the west. But it is clear that as the sun travels
from east to west, it must pass over the eastern one of any two
cities sooner than the western one. When it reaches its greatest
height over a western city it has, therefore, already passed its
greatest height over an eastern one. In other words, when it is
noon, or twelve o'clock, in the western city, it is already after
noon in the eastern city. This is the simple and evident cause of
time differences in different parts of the country. Of any two
places the eastern one always has later time than the western.
When we consider the matter in this way there is not the slightest
difficulty in understanding how time differences arise. They will,
of course, be greatest for places that are very far apart in an
east-and-west direction. And this brings us again to the subject of
longitude, which, as we have already said, plays an important part
in all questions relating to time; for longitude is used to measure
the distance in an east-and-west direction between different parts
of the earth.

If we consider the earth as a large ball we can imagine a series of
great circles drawn on its surface and passing directly from the
North Pole to the South Pole. Such a circle could be drawn through
any point on the earth. If we imagine a pair of them drawn through
two cities, such as New York and London, the longitude difference
of these two cities is defined as the angle at the North Pole
between the two great circles in question. The size of this angle
can be expressed in degrees. If we then wish to know the difference
in time between New York and London in hours, we need only divide
their longitude difference in degrees by the number 15. In this
simple way we can get the time difference of any two places. We
merely measure the longitude difference on a map, and then divide
by 15 to get the time difference. These time differences can
sometimes become quite large. Indeed, for two places differing 180
degrees in longitude, the time difference will evidently be no less
than twelve hours.

Most civilized nations have agreed informally to adopt some one
city as the fundamental point from which all longitudes are to
be counted. Up to the present we have considered only longitude
differences; but when we speak of the longitude of a city we mean
its longitude difference from the place chosen by common consent as
the origin for measuring longitudes. The town almost universally
used for this purpose is Greenwich, near London, England. Here
is situated the British Royal Observatory, one of the oldest and
most important institutions of its kind in the world. The great
longitude circle passing through the centre of the astronomical
transit at the Greenwich observatory is the fundamental longitude
circle of the earth. The longitude of any other town is then simply
the angle at the pole between the longitude circle through that
town and the fundamental Greenwich one here described.

Longitudes are counted both eastward and westward from Greenwich.
Thus New York is in 74 degrees west longitude, while Berlin is in
14 degrees east longitude. This has led to a rather curious state
of affairs in those parts of the earth the longitudes of which are
nearly 180 degrees east or west. There are a number of islands
in that part of the world, and if we imagine for a moment one
whose longitude is just 180 degrees, we shall have the following
remarkable result as to its time difference from Greenwich.

We have seen that of any two places the eastern always has the
later time. Now, since our imaginary island is exactly 180 degrees
from Greenwich, we can consider it as being either 180 degrees east
or 180 degrees west. But if we call it 180 degrees east, its time
will be twelve hours later than Greenwich, and if we call it 180
degrees west, its time will be twelve hours earlier than Greenwich.
Evidently there will be a difference of just twenty-four hours, or
one whole day, between these two possible ways of reckoning its
time. This circumstance has actually led to considerable confusion
in some of the islands of the Pacific Ocean. The navigators who
discovered the various islands naturally gave them the date which
they brought from Europe. And as some of these navigators sailed
eastward, around the Cape of Good Hope, and others westward, around
Cape Horn, the dates they gave to the several islands differed by
just one day.

The state of affairs at the present time has been adjusted by a
sort of informal agreement. An arbitrary line has been drawn on
the map near the 180th longitude circle, and it has been decided
that the islands on the east side of this line shall count their
longitudes west from Greenwich, and those west of the line shall
count longitude east from Greenwich. Thus Samoa is nearly 180
degrees west of Greenwich, while the Fiji Islands are nearly 180
degrees east. Yet the islands are very near each other, though the
arbitrary line passes between them. As a result, when it is Sunday
in Samoa it is Monday in the Fiji Islands. The arbitrary line
described here is sometimes called the International Date-Line.

It does not pass very near the Philippine Islands, which are
situated in about 120 degrees east longitude, and, therefore, use
a time about eight hours later than Greenwich. New York, being
about 74 degrees west of Greenwich, is about five hours earlier in
time. Consequently, as we may remark in passing, Philippine time is
about thirteen hours later than New York time. Thus, five o'clock,
Sunday morning, May 1st, in Manila, would correspond to four
o'clock, Saturday afternoon, April 30th, in New York.

There is another kind of time which we shall explain briefly--the
so-called "standard," or railroad time, which came into general
use in the United States some few years ago, and has since been
generally adopted throughout the world. It requires but a few
moments' consideration to see that the accidental situation of
the different large cities in any country will cause their local
times to differ by odd numbers of hours, minutes, and seconds.
Thus a great deal of inconvenience has been caused in the past.
For instance, a train might leave New York at a certain hour by
New York time. It would then arrive in Buffalo some hours later by
New York time. But it would leave Buffalo by Buffalo time, which
is quite different. Thus there would be a sort of jump in the
time-table at Buffalo, and it would be a jump of an odd number of
minutes.

It would be different in different cities, and very hard to
remember. Indeed, as each railway usually ran its trains by the
time used in the principal city along its line, it might happen
that three or four different railroad times would be used in a
single city where several roads met. This has all been avoided by
introducing the standard time system. According to this the whole
country is divided into a series of time zones, fifteen degrees
wide, and so arranged that the middle line of each zone falls at
a point whose longitude from Greenwich is 60, 75, 90, 105, or 120
degrees. The times at these middle lines are, therefore, earlier
than Greenwich time by an even number of hours. Thus, for instance,
the 75-degree line is just five even hours earlier than Greenwich
time. All cities simply use the time of the nearest one of these
special lines.

This does not result in doing away with time differences
altogether--that would, of course, be impossible in the nature
of things--but for the complicated odd differences in hours
and minutes, we have substituted the infinitely simpler series
of differences in even hours. The traveller from Chicago to New
York can reset his watch by putting it just one hour later on
his arrival--the minute hand is kept unchanged, and no New York
timepiece need be consulted to set the watch right on arriving.
There can be no doubt that this standard-time system must be
considered one of the most important contributions of astronomical
science to the convenience of man.

Its value has received the widest recognition, and its use has now
extended to almost all civilized countries--France is the only
nation of importance still remaining outside the time-zone system.
In the following table we give the standard time of the various
parts of the earth as compared with Greenwich, together with the
date of adopting the new time system. It will be noticed that in
certain cases even half-hours have been employed to separate the
time-zones, instead of even hours as used in the United States.


TABLE OF THE WORLD'S TIME STANDARDS

  ----------------+---------------------++------------------
  When it is Noon |                     || Date of Adopting
   at Greenwich   |        In           ||  Standard Time
      it is       |                     ||      System.
  ----------------+---------------------++------------------
  Noon            | Great Britain.      ||
                  | Belgium.            || May, 1892.
                  | Holland.            || May, 1892.
                  | Spain.              || January, 1901.
   1 P.M.         | Germany.            || April, 1893.
                  | Italy.              || November, 1893.
                  | Denmark.            || January, 1894.
                  | Switzerland.        || June, 1894.
                  | Norway.             || January, 1895.
                  | Austria (railways). ||
   1.30 P.M.      | Cape Colony.        || 1892.
                  | Orange River Colony.|| 1892.
                  | Transvaal.          || 1892.
   2 P.M.         | Natal.              || September, 1895.
                  | Turkey (railways).  ||
                  | Egypt.              || October, 1900.
   8 P.M.         | West Australia.     || February, 1895.
   9 P.M.         | Japan.              || 1896.
   9.30 P.M.      | South Australia.    || May, 1899.
  10 P.M.         | Victoria.           || February, 1895.
                  | New South Wales.    || February, 1895.
                  | Queensland.         || February, 1895.
  11 P.M.         | New Zealand.        ||
  ----------------+---------------------++------------------

  In the United States and Canada it is
    4  A.M. by Pacific Time when it is Noon at Greenwich.
    5  A.M.  " Mountain  "    "    "     "  "      "
    6  A.M.  " Central   "    "    "     "  "      "
    7  A.M.  " Eastern   "    "    "     "  "      "
    8  A.M.  " Colonial  "    "    "     "  "      "




MOTIONS OF THE EARTH'S POLE


Students of geology have been puzzled for many years by traces
remaining from the period when a large part of the earth was
covered with a heavy cap of ice. These shreds of evidence all seem
to point to the conclusion that the centre of the ice-covered
region was quite far away from the present position of the north
pole of the earth. If we are to regard the pole as very near the
point of greatest cold, it becomes a matter of much interest to
examine whether the pole has always occupied its present position,
or whether it has been subject to slow changes of place upon
the earth's surface. Therefore, the geologists have appealed to
astronomers to discover whether they are in possession of any
observational evidence tending to show that the pole is in motion.

Now we may say at once that astronomical research has not as yet
revealed the evidence thus expected. Astronomy has been unable to
come to the rescue of geological theory. From about the year 1750,
which saw the beginning of precise observation in the modern sense,
down to very recent times, astronomers were compelled to deny the
possibility of any appreciable motion of the pole. Observational
processes, it is true, furnished slightly divergent pole positions
from time to time. Yet these discrepancies were always so minute as
to be indistinguishable from those slight personal errors that are
ever inseparable from results obtained by the fallible human eye.

But in the last few years improved methods of observation, coupled
with extreme diligence in their application by astronomers
generally, have brought to light a certain small motion of the pole
which had never before been demonstrated in a reliable way. This
motion, it is true, is not of the character demanded by geological
theory, for the geologists had been led to expect a motion which
would be continuous in the same direction, no matter how slow might
be its annual amount; for the vast extent of geologic time would
give even the slowest of motions an opportunity to produce large
effects, provided its results could be continuously cumulative.
Given time enough, and the pole might move anywhere on the earth,
no matter how slow might be its tortoise speed.

But the small motion we have discovered is neither cumulative nor
continuous in one direction. It is what we call a periodic motion,
the pole swinging now to one side, and now to the other, of its
mean or average position. Thus this new discovery cannot be said
to unravel the mysterious puzzle of the geologists. Yet it is not
without the keenest interest, even from their point of view; for
the proof of any form of motion in a pole previously supposed to be
absolutely at rest may mean everything. No man can say what results
will be revealed by the further observations now being continued
with great diligence.

In the first place, it is important to explain that any such
motions as we have under consideration will show themselves to
ordinary observational processes principally in the form of changes
of terrestrial latitudes. Let us imagine a pair of straight lines
passing through the centre of the earth and terminating, one at the
observer's station on the earth's surface, and the other at that
point of the equator which is nearest the observer. Then, according
to the ordinary definition of latitude, the angle between these two
imaginary lines is called the latitude of the point of observation.
Now we know, of course, that the equator is everywhere just 90
degrees from the pole. Consequently, if the pole is subject to any
motion at all, the equator must also partake of the motion.

Thus the angle between our two imaginary lines will be affected
directly by polar movement, and the latitude obtained by
astronomical observation will be subject to quite similar changes.
To clear up the whole question, so far as this can be done by
the gathering of observational evidence, it is only necessary to
keep up a continual series of latitude determinations at several
observatories. These determinations should show small variations
similar in magnitude to the wabblings of the pole.

Let us now consider for a moment what is meant by the axis of the
earth. It has long been known that the planet has in general the
shape of a ball or sphere. That this is so can be seen at once
from the way ships at sea disappear at the horizon. As they go
farther and farther from us, we first lose sight of the hull, and
then slowly and gradually the spars and sails seem to sink down
into the ocean. This proves that the earth's surface is curved.
That it is more or less like a sphere is evident from the fact that
it always casts a round shadow in eclipses. Sometimes the earth
passes between the sun and eclipsed moon. Then we see the earth's
black shadow projected on the moon, which would otherwise be quite
bright. This shadow has been observed in a very large number of
such eclipses, and it has always been found to have a circular edge.

While, therefore, the earth is nearly a round ball, it must not be
supposed that it is exactly spherical in form. We may disregard
the small irregularities of its surface, for even the greatest
mountains are insignificant in height when compared with the
entire diameter of the earth itself. But even leaving these out of
account, the earth is not perfectly spherical. We can describe it
best as a flattened sphere. It is as though one were to press a
round rubber ball between two smooth boards. It would be flattened
at the top and bottom and bulged out in the middle. This is the
shape of the earth. It is flattened at the poles and bulges out
near the equator. The shortest straight line that can be drawn
through the earth's centre and terminated by the flattened parts of
its surface may be called the earth's axis of figure; and the two
points where this axis meets the surface are called the poles of
figure.

But the earth has another axis, called the axis of rotation. This
is the one about which the planet turns once in a day, giving
rise to the well-known phenomena called the rising and setting of
sun, moon, and stars. For these motions of the heavenly bodies
are really only apparent ones, caused by an actual motion of the
observer on the earth. The observer turns with the earth on its
axis, and is thus carried past the sun and stars.

This daily turning of the earth, then, takes place about the axis
of rotation. Now, it so happens that all kinds of astronomical
observations for the determination of latitude lead to values based
on the rotation axis of the earth, and not on its axis of figure.
We have seen how the earth's equator, from which we count our
latitudes, is everywhere 90 degrees distant from the pole. But this
pole is the pole of rotation, or the point at which the rotation
axis pierces the earth's surface. It is not the pole of figure.

It is clear that the latitude of any observatory will remain
constant only if the pole of figure and the rotation pole maintain
absolutely the same positions relatively one to the other. These
two poles are actually very near together; indeed, it was supposed
for a very long time that they were absolutely coincident, so that
there could not be any variations of latitude. But it now appears
that they are separated slightly.

Strange to say, one of them is revolving about the other in a
little curve. The pole of figure is travelling around the pole of
rotation. The distance between them varies a little, never becoming
greater than about fifty feet, and it takes about fourteen months
to complete a revolution. There are some slight irregularities in
the motion, but, in the main, it takes place in the manner here
stated. In consequence of this rotation of the one pole about the
other, the pole of figure is now on one side of the rotation pole
and now on the opposite side, but it never travels continuously
in one direction. Thus, as we have already seen, the sort of
continuous motion required to explain the observed geological
phenomena has not yet been found by astronomers.

Observations for the study of latitude variations have been made
very extensively within recent years both in Europe and the United
States. It has been found practically most advantageous to carry
out simultaneous series of observations at two observatories
situated in widely different parts of the earth, but having very
nearly the same latitude. It is then possible to employ the same
stars for observation in both places, whereas it would be necessary
to use different sets of stars if there were much difference in the
latitudes.

There is a special advantage in using the same stars in both
places. We can then determine the small difference in latitude
between the two participating observatories in a manner which
will make it quite free from any uncertainty in our knowledge of
the positions on the sky of the stars observed; for, strange
as it may seem, our star-catalogues do not contain absolutely
accurate numbers. Like all other data depending on fallible
human observation, they are affected with small errors. But if
we can determine simply the difference in latitude of the two
observatories, we can discover from its variation the path in
which the pole is moving. If, for instance, the observatories are
separated by one-quarter the circumference of the globe, the pole
will be moving directly toward one of them, when it is not changing
its distance from the other one at all.

This method was used for seven years with good effect at the
observatories of Columbia University in New York, and the Royal
Observatory at Naples, Italy. For obtaining its most complete
advantages it is, of course, better to establish several observing
stations on about the same parallel of latitude. This was done
in 1899 by the International Geodetic Association. Two stations
are in the United States, one in Japan, and one in Sicily. We
can, therefore, hope confidently that our knowledge as to the
puzzling problem of polar motion will soon receive very material
advancement.




SATURN'S RINGS


The death of James E. Keeler, Director of the Lick Observatory, in
California (p. 32), recalls to mind one of the most interesting
and significant of later advances in astronomical science.
Only seven years have elapsed since Keeler made the remarkable
spectroscopic observations which gave for the first time an ocular
demonstration of the true character of those mysterious luminous
rings surrounding the brilliant planet Saturn. His results have not
yet been made sufficiently accessible to the public at large, nor
have they been generally valued at their true worth. We consider
this work of Keeler's interesting, because the problem of the rings
has been a classic one for many generations; and we have been
particular, also, to call it significant, because it is pregnant
with the possibilities of newer methods of spectroscopic research,
applied in the older departments of observational astronomy.

The troubles of astronomers with the rings began with the invention
of the telescope itself. They date back to 1610, when Galileo
first turned his new instrument to the heavens (p. 49). It may
be imagined easily that the bright planet Saturn was among the
very first objects scrutinized by him. His "powerful" instrument
magnified only about thirty times, and was, doubtless, much
inferior to our pocket telescopes of to-day. But it showed, at
all events, that something was wrong with Saturn. Galileo put it,
"_Ultimam planet am tergeminam observavi_" ("I have observed the
furthest planet to be triple").

It is easy to understand now how Galileo's eyes deceived him. For
a round luminous ball like Saturn, surrounded by a thin flat ring
seen nearly edgewise, really looks as if it had two little attached
appendages. Strange, indeed, it is to-day to read a scientific
book so old that the planet Saturn could be called the "furthest"
planet. But it was the outermost known in Galileo's day, and
for nearly two centuries afterward. Not until 1781 did William
Herschel discover Uranus (p. 59); and Neptune was not disclosed by
the marvellous mathematical perception of Le Verrier until 1846 (p.
61).

Galileo's further observations of Saturn bothered him more and
more. The planet's behavior became much worse as time went on.
"Has Saturn devoured his children, according to the old legend?"
he inquired soon afterward; for the changed positions of earth
and planet in the course of their motions around the sun in their
respective orbits had become such that the ring was seen quite
edgewise, and was, therefore, perfectly invisible to Galileo's
"optic tube." The puzzle remained unsolved by Galileo; it was left
for another great man to find the true answer. Huygens, in 1656,
first announced that the ring _is_ a ring.

The manner in which this announcement was made is characteristic
of the time; to-day it seems almost ludicrous. Huygens published a
little pamphlet in 1656 called "_De Saturni Luna Observatio Nova_"
or, "A New Observation of Saturn's Moon." He gave the explanation
of what had been observed by himself and preceding astronomers in
the form of a puzzle, or "logogriph." Here is what he had to say of
the phenomenon in question:

"aaaaaaa ccccc d eeeee g h iiiiiii llll mm nnnnnnnnn oooo pp q rr s
ttttt uuuuu."

It was not until 1659, three years later, in a book entitled
"_Systema Saturnium_," that Huygens rearranged the above letters in
their proper order, giving the Latin sentence:

"_Annulo cingitur, tenui plano, nusquam cohaerente, ad eclipticam
inclinato._" Translated into English, this sentence informs us that
the planet "is girdled with a thin, flat ring, nowhere touching
Saturn, and inclined to the ecliptic"!

This was a perfectly correct and wonderfully sagacious explanation
of those complex and exasperatingly puzzling phenomena that had
been too difficult for no less a person than Galileo himself. It
was an explanation that _explained_. The reason for its preliminary
announcement in the above manner must have been the following:
Huygens was probably not quite sure of his ground in 1656,
while three years afterward he had become quite certain. By the
publication of the logogriph of 1656 he secured for himself the
credit of what he had done. If any other astronomer had published
the true explanation after 1656, Huygens could have proved his
claim to priority by rearranging the letters of his puzzle. On the
other hand, if further researches showed him that he was wrong,
he would never have made known the true meaning of his logogriph,
and would thus have escaped the ignominy of making an erroneous
explanation. Thus, the method of announcement was comparable in
ingenuity with the Huygenian explanation itself.

We are compelled to pass over briefly the entertaining history of
subsequent observations of the ring, in order to explain the new
work of Keeler and others. Cassini, about 1675, been able to show
that the ring was double; that there are really two independent
rings, with a distinct dark space between them. It was a case of
wheels within wheels. To our own eminent countryman, W. C. Bond,
of Cambridge, Mass., we owe the further discovery (Harvard College
Observatory, November, 1850) of the third ring. This is also
concentric with the other two, and interior to them, but difficult
to observe, because of its much smaller luminosity.

It is almost transparent, and the brilliant light of the planet's
central ball is capable of shining directly through it. For this
reason the inner ring is called the "gauze" or "crape" ring. If we
add to the above details the fact that our modern large telescopes
show slight irregularities in the surface of the rings, especially
when seen edgewise, we have a brief statement of all that the
telescope has been able to reveal to us since Galileo's time.

But of far greater interest than the mere fact of their existence
is the important cosmic question as to the constitution, structure,
and, above all, durability of the ring system. Astronomers often
use the term "stability" with regard to celestial systems like the
ring system of Saturn. By this they mean permanent durability. A
system is stable if its various parts can continue in their present
relationship to one another, without violating any of the known
laws of astronomy. Whenever we study any collection of celestial
objects, and endeavor to explain their motions and peculiarities,
we always seek some explanation not inconsistent with the continued
existence of the phenomena in question. For this there is, perhaps,
no sufficient philosophical basis. Probably much of the great
celestial procession is but a passing show, to be but for a moment
in the endless vista of cosmic time.

However this may be, we are bound to assume as a working theory
that Saturn has always had these rings, and will always have them;
and it is for us to find out how this is possible. The problem has
been attacked mathematically by various astronomers, including
Laplace; but no conclusive mathematical treatment was obtained
until 1857, when James Clerk Maxwell proved in a masterly manner
that the rings could be neither solid nor liquid. He showed,
indeed, that they would not last if they were continuous bodies
like the planets. A big solid wheel would inevitably be torn
asunder by any slight disturbance, and then precipitated upon
the planet's surface. Therefore, the rings must be composed of
an immense number of small detached particles, revolving around
Saturn in separate orbits, like so many tiny satellites.

This mathematical theory of the ring system being once established,
astronomers were more eager than ever to obtain a visual
confirmation of it. We had, indeed, a sort of analogy in the
assemblage of so-called "minor planets" (p. 64), which are known
to be revolving around our sun in orbits situated between Mars and
Jupiter. Some hundreds of these are known to exist, and probably
there are countless others too small for us to see. Such a swarm
of tiny particles of luminous matter would certainly give the
impression of a continuous solid body, if seen from a distance
comparable to that separating us from Saturn. But arguments founded
on analogy are of comparatively little value.

Astronomers need direct and conclusive telescopic evidence, and
this was lacking until Keeler made his remarkable spectroscopic
observation in 1895. The spectroscope is a peculiar instrument,
different in principle from any other used in astronomy; we study
distant objects with it by analyzing the light they send us, rather
than by examining and measuring the details of their visible
surfaces. The reader will recall that according to the modern
undulatory theory, light consists simply of a series of waves.
Now, the nature of waves is very far from being understood in the
popular mind. Most people, for instance, think that the waves of
ocean consist of great masses of water rolling along the surface.

This notion doubtless arises from the behavior of waves when
they break upon the shore, forming what we call surf. When a
wave meets with an immovable body like a sand beach, the wave is
broken, and the water really does roll upon the beach. But this
is an exceptional case. Farther away from the shore, where the
waves are unimpeded, they consist simply of particles of water
moving straight up and down. None of the water is carried by mere
wave-action away from the point over which it was situated at first.

Tides or other causes may move the water, but not simple
wave-motion alone. That this is so can be proved easily. If a chip
of wood be thrown overboard from a ship at sea it will be seen to
rise and fall a long time on the waves, but it will not move.
Similarly, wind-waves are often quite conspicuous on a field of
grain; but they are caused by the individual grain particles moving
up and down. The grain certainly cannot travel over the ground,
since each particle is fast to its own stalk.

But while the particles do not travel, the wave-disturbance
does. At times it is transmitted to a considerable distance from
the point where it was first set in motion. Thus, when a stone
is dropped into still water, the disturbance (though not the
water) travels in ever-widening circles, until at last it becomes
too feeble for us to perceive. Light is just such a travelling
wave-disturbance. Beginning, perhaps, in some distant star, it
travels through space, and finally the wave impinges on our eyes
like the ocean-wave breaking on a sand beach. Such a light-wave
affects the eye in some mysterious way. We call it "seeing."

The spectroscope (p. 21) enables us to measure and count the waves
reaching us each second from any source of light. No matter how far
away the origin of stellar light may be, the spectroscope examines
the character of that light, and tells us the number of waves set
up every second. It is this characteristic of the instrument that
has enabled us to make some of the most remarkable observations of
modern times. If the distant star is approaching us in space, more
light-waves per second will reach us than we should receive from
the same star at rest. Thus if we find from the spectroscope that
there are too many waves, we know that the star is coming nearer;
and if there are too few, we can conclude with equal certainty that
the star is receding.

Keeler was able to apply the spectroscope in this way to the planet
Saturn and to the ring system. The observations required dexterity
and observational manipulative skill in a superlative degree. These
Keeler had; and this work of his will always rank as a classic
observation. He found by examining the light-waves from opposite
sides of the planet that the luminous ball rotated; for one side
was approaching us and the other receding. This observation was,
of course, in accord with the known fact of Saturn's rotation
on his axis. With regard to the rings, Keeler showed in the same
way the existence of an axial rotation, which appears not to have
been satisfactorily proved before, strange as it may seem. But the
crucial point established by his spectroscope was that the interior
part of the rings rotates _faster_ than the exterior.

The velocity of rotation diminishes gradually from the inside to
the outside. This fact is absolutely inconsistent with the motion
of a solid ring; but it fits in admirably with the theory of a
ring comprised of a vast assemblage of small separate particles.
Thus, for the first time, astronomy comes into possession of an
observational determination of the nature of Saturn's rings, and
Galileo's puzzle is forever solved.




THE HELIOMETER


Astronomical discoveries are always received by the public with
keen interest. Every new fact read in the great open book of nature
is written eagerly into the books of men. For there exists a strong
curiosity to ascertain just how the greater world is built and
governed; and it must be admitted that astronomers have been able
to satisfy that curiosity with no small measure of success. But it
is seldom that we hear of the means by which the latest and most
refined astronomical observations are effected. Popular imagination
pictures the astronomer, as he doubtless once was, an aged
gentleman, usually having a long white beard, and spending entire
nights staring at the sky through a telescope.

But the facts to-day are very different. The working astronomer
is an active man in the prime of life, often a young man. He
wastes no time in star-gazing. His observations consist of
exact measurements made in a precise, systematic, and almost
business-like manner. A night's "watch" at the telescope is
seldom allowed to exceed about three hours, since it is found
that more continued exertions fatigue the eye and lead to less
accurate results. To this, of course, there have been many notable
exceptions, for endurance of sight, like any form of physical
strength, differs greatly in different individuals. Astronomical
research does not include "picking out" the constellations, and
learning the Arabic names of individual stars. These things are not
without interest; but they belong to astronomy's ancient history,
and are of little value except to afford amusement and instruction
to successive generations of amateurs.

Among the instruments for carefully planned measurements of
precision the heliometer probably takes first rank. It is at
once the most exquisitely accurate in its results, and the most
fatiguing to the observer, of all the varied apparatus employed by
the astronomer. The principle upon which its construction depends
is very peculiar, and applies to all telescopes, even ordinary
ones for terrestrial purposes. If part of a telescope lens be
covered up with the hand, it will still be possible to see through
the instrument. The glass lens at the end of the tube farthest from
the observer's eye helps to magnify distant objects and make them
seem nearer by gathering to a single point, or focus, a greater
amount of their light than could be brought together by the far
smaller lens in the unaided eye.

The telescope might very properly be likened to an enlarged eye,
which can see more than we can, simply because it is bigger. If
a telescope lens has a surface one hundred times as large as
that of the lens in our eye, it will gather and bring to a focus
one hundred times as much light from a distant object. Now, if
any part of this telescope be covered, the remaining part will,
nevertheless, gather and focus light just as though the whole lens
were in action; only, there will be less light collected at the
focus within the tube. The small lens at the telescope's eye-end is
simply a magnifier to help our eye examine the image of any distant
object formed at the focus by the large lens at the farther end of
the instrument. For of this simple character is the operation of
any telescope: the large glass lens at one end collects a distant
planet's light, and brings it to a focus near the other end of the
tube, where it forms a tiny picture of the planet, which, in turn,
is examined with the little magnifier at the eye-end.

Having arrived at the fundamental principle that part of a lens
will act in a manner similar to a whole one, it is easy to explain
the construction of a heliometer. An ordinary telescope lens is
sawed in half by means of a thin round metal disk revolved rapidly
by machinery, and fed continually with emery and water at its
edge. The cutting effect of emery is sufficient to make such a
disk enter glass much as an ordinary saw penetrates wood. The two
"semi-lenses," as they are called, are then mounted separately in
metal holders. These are attached to one end of the heliometer,
called the "head," in such a way that the two semi-lenses can slide
side by side upon metal guides. This head is then fastened to one
end of a telescope tube mounted in the usual way. The "head" end
of the instrument is turned toward the sky in observing, and at
the eye-end is placed the usual little magnifier we have already
described.

The observer at the eye-end has control of certain rods by means
of which he can push the semi-lenses on their slides in the head
at the other end of the tube. Now, if he moves the semi-lenses so
as to bring them side by side exactly, the whole arrangement will
act like an ordinary telescope. For the semi-lenses will then fit
together just as if the original glass had never been cut. But
if the half-lenses are separated a little on their slides, each
will act by itself. Being slightly separated, each will cover a
different part of the sky. The whole affair acts as if the observer
at the eye-end were looking through two telescopes at once. For
each semi-lens acts independently, just as if it were a complete
glass of only half the size.

Now, suppose there were a couple of stars in the sky, one in the
part covered by the first semi-lens, and one in the part covered
by the second. The observer would, of course, see both stars at
once upon looking into the little magnifier at the eye-end of the
heliometer.

We must remember that these stars will appear in the field of view
simply as two tiny points of light. The astronomer, as we have
said, is provided with a simple system of long rods, by means of
which he can manipulate the semi-lenses while the observation is
being made. If he slides them very slowly one way or the other, the
two star-points in the field of view will be seen to approach each
other. In this way they can at last be brought so near together
that they will form but a single dot of light. Observation with
the heliometer consists in thus bringing two star-images together,
until at last they are superimposed one upon the other, and we see
one image only. Means are provided by which it is then possible to
measure the amount of separation of the two half-lenses. Evidently
the farther asunder on the sky are the two stars under observation,
the greater will be the separation of the semi-lenses necessary
to make a single image of their light. Thus, measurement of the
lenses' separation becomes a means of determining the separation
of the stars themselves upon the sky.

The two slides in the heliometer head are supplied with a pair of
very delicate measures or "scales" usually made of silver. These
can be examined from the eye-end of the instrument by looking
through a long microscope provided for this special purpose. Thus
an extremely precise value is obtained both of the separation of
the sliders and of the distance on the sky between the stars under
examination. Measures made in this way with the heliometer are
counted the most precise of astronomical observations.

Having thus described briefly the kind of observations obtained
with the heliometer, we shall now touch upon their further
utilization. We shall take as an example but one of their many
uses--that one which astronomers consider the most important--the
measurement of stellar distances. (See also p. 94.)

Even the nearest fixed star is almost inconceivably remote from
us. And astronomers are imprisoned on this little earth; we cannot
bridge the profound distance separating us from the stars, so as
to use direct measurement with tape-line or surveyor's chain. We
are forced to have recourse to some indirect method. Suppose a
certain star is suspected, on account of its brightness, or for
some other reason, of being near us in space, and so giving a
favorable opportunity for a determination of distance. A couple of
very faint stars are selected close by. These, on account of their
faintness, the astronomer may regard as quite immeasurably far
away. He then determines with his heliometer the exact position on
the sky of the bright star with respect to the pair of faint ones.
Half a year is then allowed to pass. During that time the earth has
been swinging along in its annual path or orbit around the sun.
Half a year will have sufficed to carry the observer on the earth
to the other side of that path, and he is then 185,000,000 miles
away from his position at the first observation.

Another determination is made of the bright star's position as
referred to the two faint ones. Now, if all these stars were
equally distant, their relative positions at the second observation
would be just the same as at the former one. But if, as is very
probable, the bright star is very much nearer us than are the two
faint ones, we shall obtain a different position from our second
observation. For the change of 185,000,000 miles in the observer's
location will, of course, affect the direction in which we see
the near star, while it will leave the distant ones practically
unchanged. Without entering into technical details, we may say that
from a large number of observations of this kind, we can obtain
the distance of the bright star by a process of calculation. The
only essential is to have an instrument that can make the actual
observations of position accurately enough; and in this respect the
heliometer is still unexcelled.




OCCULTATIONS


Scarcely anyone can have watched the sky without noticing how
different is the behavior of our moon from that of any other object
we can see. Of course, it has this in common with the sun and stars
and planets, that it rises in the eastern horizon, slowly climbs
the dome of the sky, and again goes down and sets in the west.
This motion of the heavenly bodies is known to be an apparent one
merely, and caused by the turning of our own earth upon its axis. A
man standing upon the earth's surface can look up and see above him
one-half the great celestial vault, gemmed with twinkling stars,
and bearing, perhaps, within its ample curve one or two serenely
shining planets and the lustrous moon. But at any given moment the
observer can see nothing of the other half of the heavenly sphere.
It is beneath his feet, and concealed by the solid bulk of the
earth.

The earth, however, is turning on an axis, carrying the observer
with it. And so it is continually presenting him to a new part of
the sky. At any moment he sees but a single half-sphere; yet the
very next instant it is no longer the same; a small portion has
passed out of sight on one side by going down behind the turning
earth, while a corresponding new section has come into view on
the opposite side. It is this coming into view that we call the
rising of a star; and the corresponding disappearance on the other
side is called setting. Thus rising and setting are, of course,
due entirely to a turning of the earth, and not at all to actual
motions of the stars; and for this reason, all objects in the sky,
without exception, must rise and set again. But the moon really has
a motion of its own in addition to this apparent one caused by the
earth's rotation.

Somewhere in the dawn of time early watchers of the stars thought
out those fancied constellations that survive even down to our own
day. They placed the mighty lion, king of beasts, upon the face
of night, and the great hunter, too, armed with club and dagger,
to pursue him. Among these constellations the moon threads her
destined way, night after night, so rapidly that the unaided eye
can see that she is moving. It needs but little power of fancy's
magic to recall from the dim past a picture of some aged astronomer
graving upon his tablets the Records of the Moon. "To-night she is
near the bright star in the eye of the Bull." And again: "To-night
she rides full, and near to the heart of the Virgin."

And why does the moon ride thus through the stars of night? Modern
science has succeeded in disentangling the intricacies of her
motion, until to-day only one or two of the very minutest details
of that motion remain unexplained. But it has been a hard problem.
Someone has well said that lunar theory should be likened to a
lofty cliff upon whose side the intellectual giants among men can
mark off their mental stature, but whose height no one of them may
ever hope to scale.

But for our present purpose it is unnecessary to pursue the subject
of lunar motion into its abstruser details. To understand why the
moon moves rapidly among the stars, it is sufficient to remember
that she is whirling quickly round the earth, so as to complete
her circuit in a little less than a month. We see her at all times
projected upon the distant background of the sky on which are set
the stellar points of light, as though intended for beacons to
mark the course pursued by moon and planets. The stars themselves
have no such motions as the moon; situated at a distance almost
inconceivably great, they may, indeed, be travellers through empty
space, yet their journeys shrink into insignificance as seen from
distant earth. It requires the most delicate instruments of the
astronomer to so magnify the tiny displacements of the stars on the
celestial vault that they may be measured by human eyes.

Let us again recur to our supposed observer watching the moon night
after night, so as to record the stars closely approached by her.
Why should he not some time or other be surprised by an approach so
close as to amount apparently to actual contact? The moon covers
quite a large surface on the sky, and when we remember the nearly
countless numbers of the stars, it would, indeed, be strange if
there were not some behind the moon as well as all around her.

A moment's consideration shows that this must often be the case;
and in fact, if the moon's advancing edge be scrutinized carefully
through a telescope, small stars can be seen frequently to
disappear behind it. This is a most interesting observation. At
first we see the moon and star near each other in the telescope's
field of view. But the distance between them lessens perceptibly,
even quickly, until at last, with a startling suddenness, the star
goes out of sight behind the moon. After a time, ranging from a few
moments to, perhaps, more than an hour, the moon will pass, and we
can see the star suddenly reappear from behind the other edge.

These interesting observations, while not at all uncommon, can
be made only very rarely by naked-eye astronomers. The reason is
simple. The moon's light is so brilliant that it fairly overcomes
the stars whenever they are at all near, except in the case of
very bright ones. Small stars that can be followed quite easily
up to the moon's edge in a good telescope, disappear from a
naked-eye view while the moon is still a long distance away. But
the number of very bright stars is comparatively small, so that
it is quite unusual to find anyone not a professional astronomer
who has actually seen one of these so-called "occultations."
Moreover, most people are not informed in advance of the occurrence
of an opportunity to make such observations, although they can be
predicted quite easily by the aid of astronomical calculations.
Sometimes we have occultations of planets, and these are the most
interesting of all. When the moon passes between us and one of the
larger planets, it is worth while to observe the phenomenon even
without a telescope.

Up to this point we have considered occultations chiefly as being
of interest to the naked-eye astronomer. Yet occultations have a
real scientific value. It is by their means that we can, perhaps,
best measure the moon's size. By noting with a telescope the time
of disappearance and reappearance of known stars, astronomers can
bring the direct measurement of the moon's diameter within the
range of their numerical calculations. Sometimes the moon passes
over a condensed cluster of stars like the Pleiades. The results
obtainable on these occasions are valuable in a very high degree,
and contribute largely to making precise our knowledge of the lunar
diameter.

There is another thing of scientific interest about occultations,
though it has lost some of its importance in recent years. When
such an event has been observed, the agreement of the predicted
time with that actually recorded by the astronomer offers a most
searching test of the correctness of our theory of lunar motion. We
have already called attention to the great inherent difficulty of
this theory. It is easy to see that by noting the exact instant of
disappearance of a star at a place on the earth the latitude and
longitude of which are known, we can both check our calculations
and gather material for improving our theory. The same principle
can be used also in the converse direction. Within the limits of
precision imposed by the state of our knowledge of lunar theory,
we can utilize occultations to help determine the position on
the earth of places whose longitude is unknown. It is a very
interesting bit of history that the first determination of the
longitude of Washington was made by means of occultations, and that
this early determination led to the founding of the United States
Naval Observatory.

On March 28, 1810, Mr. Pitkin, of Connecticut, reported to
the House of Representatives on "laying a foundation for the
establishment of a first meridian for the United States, by which a
further dependence on Great Britain or any other foreign nation for
such meridian may be entirely removed." This report was the result
of a memorial presented by one William Lambert, who had deduced the
longitude of the Capitol from an occultation observed October 20,
1804. Various proceedings were had in Congress and in committee,
until at last, in 1821, Lambert was appointed "to make astronomical
observations by lunar occultations of fixed stars, solar eclipses,
or any approved method adapted to ascertain the longitude of the
Capitol from Greenwich." Lambert's reports were made in 1822
and 1823, but ten years passed before the establishment of a
formal Naval Observatory under Goldsborough, Wilkes, and Gilliss.
But to Lambert belongs the honor of having marked out the first
fundamental official meridian of longitude in the United States.




MOUNTING GREAT TELESCOPES


There are many interesting practical things about an astronomical
observatory with which the public seldom has an opportunity to
become acquainted. Among these, perhaps, the details connected
with setting up a great telescope take first rank. The writer
happened to be present at the Cape of Good Hope Observatory when
the photographic equatorial telescope was being mounted, and the
operation of putting it in position may be taken as typical of
similar processes elsewhere. (See also p. 86.)

[Illustration: Forty-Inch Telescope, Yerkes Observatory, University
of Chicago.]

In the first place, it is necessary to explain what is meant by an
"equatorial" telescope. One of the chief difficulties in making
ordinary observations arises from the rising and setting of the
stars. They are all apparently moving across the face of the sky,
usually climbing up from the eastern horizon, only to go down again
and set in the west. If, therefore, we wish to scrutinize any
given object for a considerable time, we must move the telescope
continuously so as to keep pace with the motion of the heavens. For
this purpose, the tube must be attached to axles, so that it can be
turned easily in any direction. The equatorial mounting is a device
that permits the telescope to be thus aimed at any part of the sky,
and at the same time facilitates greatly the operation of keeping
it pointed correctly after a star has once been brought into the
field of view.

To understand the equatorial mounting it is necessary to remember
that the rising and setting motions of the heavenly bodies are
apparent ones only, and due in reality to the turning of the earth
on its own axis. As the earth goes around, it carries observer,
telescope, and observatory past the stars fixed upon the distant
sky. Consequently, to keep a telescope pointed continuously at a
given star, it is merely necessary to rotate it steadily backward
upon a suitable axis just fast enough to neutralize exactly the
turning of our earth.

By a suitable axis for this purpose we mean one so mounted as
to be exactly parallel to the earth's own axis of rotation. A
little reflection shows how simply such an arrangement will work.
All the heavenly bodies may be regarded, for practical purposes,
as excessively remote in comparison with the dimensions of our
earth. The entire planet shrinks into absolute insignificance
when compared with the distances of the nearest objects brought
under observation by astronomers. It follows that if we have our
telescope attached to such a rotation-axis as we have described,
it will be just the same for purposes of observation as though the
telescope's axis were not only parallel to the earth's axis, but
actually coincident with it. The two axes may be separated by a
distance equal to that between the earth's surface and its centre;
but, as we have said, this distance is insignificant so far as our
present object is concerned.

There is another way to arrive at the same result. We know that
the stars in rising and setting all seem to revolve about the pole
star, which itself seems to remain immovable. Consequently, if we
mount our telescope so that it can turn about an axis pointing at
the pole, we shall be able to neutralize the rotation of the stars
by simply turning the telescope about the axis at the proper speed
and in the right direction. Astronomical considerations teach us
that an axis thus pointing at the pole will be parallel to the
earth's own axis. Thus we arrive at the same fundamental principle
for mounting an astronomical telescope from whichever point of view
we consider the subject.

Every large telescope is provided with such an axis of rotation;
and for the reason stated it is called the "polar axis." The
telescope itself is then called an "equatorial." The advantage
of this method of mounting is very evident. Since we can follow
the stars' motions by turning the telescope about one axis only,
it becomes a very simple matter to accomplish this turning
automatically by means of clock-work.

The "following" of a star being thus provided for by the device
of a polar axis, it is, of course, also necessary to supply some
other motion so as to enable us to aim the tube at any point in the
heavens. For it is obvious that if it were rigidly attached to the
polar axis, we could, indeed, follow any star that happened to be
in the field of view, but we could not change this field of view at
will so as to observe other stars or planets. To accomplish this,
the telescope is attached to the polar axis by means of a pivot.
By turning the telescope around its polar axis, and also on this
pivot, we can find any object in the heavens; and once found, we
can leave to the polar axis and its automatic clock-work the task
of keeping that object before the observer's eye.

In setting up the Cape of Good Hope instrument the astronomers were
obliged to do a large part of the work of adjustment personally.
Far away from European instrument-makers, the parts of the
mounting and telescope had to be "assembled," or put together, by
the astronomers of the Cape Observatory. A heavy pier of brick
and masonry had been prepared in advance. Upon this was placed a
massive iron base, intended to support the superstructure of polar
axis and telescope. This base rested on three points, one of which
could be screwed in and out, so as to tilt the whole affair a
little forward or backward. By means of this screw we effected the
final adjustment of the polar axis to exact parallelism with that
of the earth. Other screws were provided with which the base could
be twisted a little horizontally either to the right or left. Once
set up in a position almost correct, it was easy to perfect the
adjustment by the aid of these screws.

Afterward the tube and lenses were put in place, and the clock
properly attached inside the big cast-iron base. This clock-work
looked more like a piece of heavy machinery than a delicate clock
mechanism. But it had heavy work to do, carrying the massive
telescope with its weighty lenses, and needed to be correspondingly
strong. It had a driving-weight of about 2,000 pounds, and was so
powerful that turning the telescope affected it no more than the
hour-hand of an ordinary clock affects the mechanism within its
case.

The final test of the whole adjustment consisted in noting whether
stars once brought into the telescopic field of view could be
maintained there automatically by means of the clock. This object
having been attained successfully, the instrument stood ready to be
used in the routine business of the observatory.

Before leaving the subject of telescope-mountings, we must mention
the giant instrument set up at the Paris Exposition of 1900. The
project of having a _Grande Lunette_ had been hailed by newspapers
throughout the world and by the general public in their customary
excitable way. It was tremendously over-advertised; exaggerated
notions of the instrument's powers were spread abroad and eagerly
credited; the moon was to be dragged down, as it were, from its
customary place in the sky, so near that we should be able almost
to touch its surface. As to the planets--free license was given to
the journalistic imagination, and there was no effective limitation
to the magnificence of astronomical discovery practically within
our grasp, beyond the necessity for printed space demanded by
sundry wars, pestilences, and other mundane trifles.

[Illustration: Yerkes Observatory, University of Chicago.]

Now, the present writer is very far from advocating a lessening of
the attention devoted to astronomy. Rather would he magnify his
office than diminish it. But let journalistic astronomy be as good
an imitation of sober scientific truth as can be procured at space
rates; let editors encourage the public to study those things
in the science that are ennobling and cultivating to the mind;
let there be an end to the frenzied effort to fabricate a highly
colored account of alleged discoveries of yesterday, capable of
masquerading to-day under heavy head-lines as News.

The manner in which the big telescope came to be built is not
without interest, and shows that enterprise is far from dead, even
in the old countries. A stock company was organized--we should
call it a corporation--under the name _Société de l'Optique_. It
would appear that shares were regularly put on the market, and
that a prospectus, more or less alluring, was widely distributed.
We may say at once that the investing public did not respond
with obtrusive alacrity; but at all events, the promoters'
efforts received sufficient encouragement to enable them to begin
active work. From the very first a vigorous attempt was made to
utilize both the resources of genuine science and the devices of
quasi-charlatanry. It was announced that the public were to be
admitted to look through the big glass (apparently at so much an
eye), and many, doubtless, expected that the man in the street
would be able to make personal acquaintance with the man in the
moon. A telescopic image of the sun was to be projected on a big
screen, and exhibited to a concourse of spectators assembled in
rising tiers of seats within a great amphitheatre. And when clouds
or other circumstances should prevent observing the planets or
scrutinizing the sun, a powerful stereopticon was to be used.
Artificial pictures of the wonders of heaven were to be projected
on the screen, and the public would never be disappointed. It
was arranged that skilled talkers should be present to explain
all marvels: and, in short, financial profit was to be combined
with machinery for advancing scientific discovery. Astronomers
the world over were "circularized," asked to become shareholders,
and, in default of that, to send lantern-slides or photographs of
remarkable celestial objects for exhibition in the magic-lantern
part of the show.

The project thus brought to the attention of scientific men three
years ago did not have an attractive air. It savored too much of
charlatanism. But it soon appeared that effective government
sanction had been given to the enterprise; and, above all,
that men of reputation were allowing the use of their names in
connection with the affair. More important still, we learned that
the actual construction had been undertaken by Gautier, of Paris,
that finances were favorable, and that real work on parts of the
instrument was to commence without delay.

Gautier is a first-class instrument-builder; he has established
his reputation by constructing successfully several telescopes of
very large size, including the _equatorial coudé_ of the Paris
Observatory, a unique instrument of especial complexity. The
present writer believes that, if sufficient time and money were
available, the _Grande Lunette_ would stand a reasonable chance of
success in the hands of such a man. And by a reasonable chance, we
mean that there is a large enough probability of genuine scientific
discovery to justify the necessary financial outlay. But the
project should be divorced from its "popular" features, and every
kind of advertising and charlatanism excluded with rigor.

As planned originally, and actually constructed, the _Grande
Lunette_ presents interesting peculiarities, distinguishing it
from other telescopes. Previous instruments have been built on the
principle of universal mobility. It is possible to move them in
all directions, and thus bring any desired star under observation,
irrespective of its position in the sky. But this general mobility
offers great difficulties in the case of large and ponderous
telescopes. Delicacy of adjustment is almost destroyed when the
object to be adjusted weighs several tons. And the excessive
weight of telescopes is not due to unavoidably heavy lenses alone.
It is essential that the tube be long; and great length involves
appreciable thickness of material, if stiffness and solidity are to
remain unsacrificed. Length in the tube is necessitated by certain
peculiar optical defects of all lenses, into the nature of which we
shall not enter at present. The consequences of these defects can
be rendered harmless only if the instrument is so arranged that the
observer's eye is far from the other end of the tube. The length
of a good telescope should be at least twelve times the diameter
of its large lens. If the relative length can be still further
increased, so much the better; for then the optical defects can be
further reduced.

In the case of the Paris instrument a radical departure consists
in making the tube of unprecedented length, 197 feet, with a
lens diameter of 49¼ inches. This great length, while favorable
optically, precludes the possibility of making the instrument
movable in the usual sense. In fact, the entire tube is attached
to a fixed horizontal base, and no attempt is made to change its
position. Outside the big lens, and disconnected altogether from
the telescope proper, is mounted a smooth mirror, so arranged that
it can be turned in any direction, and thus various parts of the
sky examined by reflection in the telescope.

While this method unquestionably has the advantage of leaving
the optician quite free as to how long he will make his tube, it
suffers from the compensating objection that a new optical surface
is introduced into the combination, viz., the mirror. Any slight
unavoidable imperfection in the polishing of its surface will
infallibly be reproduced on a magnified scale in the image of a
distant star brought before the observer's eye.

But it is not yet possible to pronounce definitely upon the merit
of this form of instrument, since, as we have said, the maker
has not been given time enough to try the idea to the complete
satisfaction of scientific men. In the early part of August, 1900,
when the informant of the present writer left Paris, after serving
as a member of the international jury for judging instruments of
precision at the Exposition, the condition of the _Grande Lunette_
was as follows: Two sets of lenses had been contemplated, one
intended for celestial photography, and the other to be used for
ordinary visual observation. Only the photographic lenses had been
completed, however, and for this reason the public could not be
permitted to look through the instrument. The photographic lenses
were in place in the tube, but at that time their condition was
such that, though some photographs had been obtained, it was
not thought advisable to submit them to the jury. Consequently,
the _Lunette_ did not receive a prize. Since that time various
newspapers have reported wonderful results from the telescope;
but, disregarding the fusillade from the sensational press, we may
sum up the present state of affairs very briefly. Gautier is still
experimenting; and, given sufficient time and money, he may succeed
in producing what astronomers hope for--an instrument capable of
advancing our knowledge, even if that advance be only a small one.




THE ASTRONOMER'S POLE


The pole of the frozen North is not the only pole sought with
determined effort by more than one generation of scientific men. Up
in the sky astronomers have another pole which they are following
up just as vigorously as ever Arctic explorer struggled toward
the difficult goal of his terrestrial journeying. The celestial
pole is, indeed, a fundamentally important thing in astronomical
science, and the determination of its exact position upon the
sky has always engaged the closest attention of astronomers.
Quite recently new methods of research have been brought to
bear, promising a degree of success not hitherto attained in the
astronomers' pursuit of their pole.

In the first place, we must explain what is meant by the celestial
pole. We have already mentioned the poles of the earth (p. 136).
Our planet turns once daily upon an axis passing through its
centre, and it is this rotation that causes all the so-called
diurnal phenomena of the heavens. Rising and setting of sun, moon,
and stars are simply results of this turning of the earth. Heavenly
bodies do not really rise; it is merely the man on the earth who is
turned round on an axis until he is brought into a position from
which he can see them. The terrestrial poles are those two points
on the earth's surface where it is pierced by the rotation axis of
the planet. Now we can, if we choose, imagine this axis lengthened
out indefinitely, further and further, until at last it reaches the
great round vault of the sky. Here it will again pierce out two
polar points; and these are the celestial poles.

The whole thing is thus quite easy to understand. On the sky the
poles are marked by the prolongation of the earth's axis, just as
on the earth the poles are marked by the axis itself. And this
explains at once why the stars seem nightly to revolve about the
pole. If the observer is being turned round the earth's axis, of
course it will appear to him as if the stars were rotating around
the same axis in the opposite direction, just as houses and fields
seem to fly past a person sitting in a railway train, unless he
stops to remember that it is really himself who is in motion, and
not the trees and houses.

The existence of such a centre of daily motions among the stars
once recognized, it becomes of interest to ascertain whether the
centre itself always retains precisely the same position in the
sky. It was discovered as early as the time of Hipparchus (p. 39)
that such is not the case, and that the celestial pole is subject
to a slow motion among the stars on the sky. If a given star were
to-day situated exactly at the pole, it would no longer be there
after the lapse of a year's time; for the pole would have moved
away from it.

This motion of the pole is called precession. It means that certain
forces are continually at work, compelling the earth's axis to
change its position, so that the prolongation of that axis must
pierce the sky at a point which moves as time goes on. These forces
are produced by the gravitational attractions of the sun, moon,
and planets upon the matter composing our earth. If the earth
were perfectly spherical in shape, the attractions of the other
heavenly bodies would not affect the direction of the earth's
rotation-axis in the least. But the earth is not quite globular in
form; it is flattened a little at the poles and bulges out somewhat
at the equator. (See p. 135.)

This protuberant matter near the equator gives the other bodies
in the solar system an opportunity to disturb the earth's
rotation. The general effect of all these attractions is to make
the celestial pole move upon the sky in a circle having a radius
of about 23½ degrees; and it requires 25,800 years to complete
a circuit of this precessional cycle. One of the most striking
consequences of this motion will be the change of the polar star.
Just at present the bright star Polaris in the constellation of
the Little Bear is very close to the pole. But after the lapse of
sufficient ages the first-magnitude star Vega of the constellation
Lyra will in its turn become Guardian of the Pole.

It must not be supposed, however, that the motion of the pole
proceeds quite uniformly, and in an exact circle; the varying
positions of the heavenly bodies whose attractions cause
the phenomena in question are such as to produce appreciable
divergencies from exact circular motion. Sometimes the pole
deviates a little to one side of the precessional circle, and
sometimes it deviates on the other side. The final result is a
sort of wavy line, half on one side and half on the other of an
average circular curve. It takes only nineteen years to complete
one of these little waves of polar motion, so that in the
whole precessional cycle of 25,800 years there are about 1,400
indentations. This disturbance of the polar motion is called by
astronomers nutation.

The first step in a study of polar motion is to devise a method of
finding just where the pole is on any given date. If the astronomer
can ascertain by observational processes just where the pole is
among the stars at any moment, and can repeat his observations year
after year and generation after generation, he will possess in
time a complete chart of a small portion at least of the celestial
pole's vast orbit. From this he can obtain necessary data for a
study of the mathematical theory of attractions, and thus, perhaps,
arrive at an explanation of the fundamental laws governing the
universe in which we live.

The instrument which has been used most extensively for the
study of these problems is the transit (p. 118) or the "meridian
circle." This latter consists of a telescope firmly attached to a
metallic axis about which it can turn. The axis itself rests on
massive stone supports, and is so placed that it points as nearly
as possible in an east-and-west direction. Consequently, when the
telescope is turned about its axis, it will trace out on the sky
a great circle (the meridian) which passes through the north and
south points of the horizon and the point directly overhead. The
instrument has also a metallic circle very firmly fastened to
the telescope and its axis. Let into the surface of this circle
is a silver disk upon which are engraved a series of lines or
graduations by means of which it is possible to measure angles.

Observers with the meridian circle begin by noting the exact
instant when any given star passes the centre of the field of
view of the telescope. This centre is marked with a cross made
by fastening into the focus some pieces of ordinary spider's web,
which give a well-marked, delicate set of lines, even under the
magnifying power of the telescope's eye-piece. In addition to thus
noting the time when the star crosses the field of the telescope,
the astronomer can measure by means of the circle, how high up it
was in the sky at the instant when it was thus observed.

If the telescope of the meridian circle be turned toward the north,
and we observe stars close to the pole, it is possible to make two
different observations of the same star. For the close polar stars
revolve in such small circles around the pole of the heavens that
we can observe them when they are on the meridian either above the
pole or below it. Double observations of this class enable us to
obtain the elevation of the pole above the horizon, and to fix its
position with respect to the stars.

Now, there is one very serious objection to this method. In order
to secure the two necessary observations of the same star, it is
essential to be stationed at the instrument at two moments of time
separated by exactly twelve hours; and if one of the observations
occurs in the night, the other corresponding observation will occur
in daylight.

It is a fact not generally known that the brighter stars can be
seen with a telescope, even when the sun is quite high above
the horizon. Unfortunately, however, there is only one star
close to the pole which is bright enough to be thus observed in
daylight--the polar star already mentioned under the name Polaris.
The fact that we are thus limited to observations of a single
star has made it difficult even for generations of astronomers
to accumulate with the meridian circle a very large quantity of
observational material suitable for the solution of our problem.

The new method of observation to which we have referred above
consists in an application of photography to the polar problem. If
we aim at the pole a powerful photographic telescope, and expose a
photographic plate throughout the entire night, we shall find that
all stars coming within the range of the plate will mark out little
circles or "trails" upon the developed negative. It is evident that
as the stars revolve about the pole on the sky, tracing out their
daily circular orbits, these same little circles must be reproduced
faithfully upon the photographic plate. The only condition is that
the stars shall be bright enough to make their light affect the
sensitive gelatine surface.

But even if observations of this kind are continued throughout
all the hours of darkness, we do not obtain complete circles,
but only those portions of circles traced out on the sky between
sunset and sunrise. If the night is twelve hours in length, we
get half-circles on the plate; if it is eighteen hours long, we
get circles that lack only one-quarter of being complete. In
other words, we get a series of circular arcs, one corresponding
to each close polar star. There are no fewer than sixteen stars
near enough to the pole to come within the range of a photographic
plate, and bright enough to cause measurable impressions upon
the sensitive surface. The fact that the circular arcs are not
complete circles does not in the least prevent our using them for
ascertaining the position of their common centre; and that centre
is the pole. Moreover, as the arcs are distributed at all sorts
of distances from the pole and in all directions, corresponding to
the accidental positions of the stars on the sky, we have a state
of affairs extremely favorable to the accurate determination of the
pole's place among the stars by means of microscopic measurements
of the plate.

It will be perceived that this method is extremely simple, and,
therefore, likely to be successful; though its simplicity is
slightly impaired by the phenomenon known to astronomers as
"atmospheric refraction." The rays of light coming down to our
telescopes from a distant star must pass through the earth's
atmosphere before they reach us; and in passing thus from the
nothingness of outer space into the denser material of the air,
they are bent out of their straight course. The phenomenon is
analogous to what we see when we push a stick down through the
surface of still water; we notice that the stick appears to be bent
at the point where it pierces the surface of the water; and in
just the same way the rays of light are bent when they pierce into
the air. Fortunately, the mathematical theory of this atmospheric
bending of light is well understood, so that it is possible to
remove the effects of refraction from our results by a process of
calculation. In other words, we can transform our photographic
measures into what they would have been if no such thing as
atmospheric refraction existed. This having been done, all the arcs
on the plate should be exactly circular, and their common centre
should be the position of the pole among the stars on the night
when the photograph was made.

It is possible to facilitate the removal of refraction effects
very much by placing our photographic telescope at some point on
the earth situated in a very high latitude. The elevation of the
pole above the horizon is greatest in high latitudes. Indeed, if
Arctic voyagers could ever reach the pole of the earth they would
see the pole of the heavens directly overhead. Now, the higher up
the pole is in the sky, the less will be the effects of atmospheric
refraction; for the rays of light will then strike the atmosphere
in a direction nearly perpendicular to its surface, which is
favorable to diminishing the amount of bending.

There is also another very important advantage in placing the
telescope in a high latitude; in the middle of winter the nights
are very long there; if we could get within the Arctic. Circle
itself, there would be nights when the hours of darkness would
number twenty-four, and we could substitute complete circles
for our broken arcs. This would, indeed, be most favorable from
the astronomical point of view; but the essential condition of
convenience for the observer renders an expedition to the frozen
Arctic regions unadvisable.

But it is at least possible to place the telescope as far north
as is consistent with retaining it within the sphere of civilized
influences. We can put it in that one of existing observatories
on the earth which has the highest latitude; and this is the
observatory of Helsingfors, in Finland, which belongs to a great
university, is manned by competent astronomers, and has a latitude
greater than 60 degrees.

Dr. Anders Donner, Director of the Helsingfors Observatory, has
at its disposal a fine photographic telescope, and with this some
preliminary experimental "trail" photographs were made in 1895.
These photographs were sent to Columbia University, New York, and
were there measured under the writer's direction. Calculations
based on these measures indicate that the method is promising in
a very high degree; and it was, therefore, decided to construct a
special photographic telescope better adapted to the particular
needs of the problem in hand.

The desirability of a new telescope arises from the fact that we
wish the instrument to remain absolutely unmoved during all the
successive hours of the photographic exposure. It is clear that
if the telescope moves while the stars are tracing out their
little trails on the plate, the circularity of the curves will
be disturbed. Now, ordinary astronomical telescopes are always
mounted upon very stable foundations, well adapted to making the
telescope stand still; but the polar telescope which we wish to use
in a research fundamental to the entire science of astronomy ought
to possess immobility and stability of an order higher than that
required for ordinary astronomical purposes.

It is a remarkable peculiarity of the instrument needed for the
new trail photographs that it is never moved at all. Once pointed
at the pole, it is ready for all the observations of successive
generations of astronomers. It should have no machinery, no
pivots, axes, circles, clocks, or other paraphernalia of the usual
equatorial telescope. All we want is a very heavy stone pier,
with a telescope tube firmly fastened to it throughout its entire
length. The top of the pier having been cut to the proper angle of
the pole's elevation, and the telescope cemented down, everything
is complete from the instrumental side; and just such an instrument
as this is now ready for use at Helsingfors.

The late Miss Catharine Wolfe Bruce, of New York, was much
interested in the writer's proposed polar investigations, and
in October, 1898, she contributed funds for the construction of
the new telescope, and the Russian authorities have generously
undertaken the expense of a building to hold the instrument and the
granite foundation upon which it rests. Photographs are now being
secured with the new instrument, and they will be sent to Columbia
University, New York, for measurement and discussion. It is hoped
that they will carry out the promise of the preliminary photographs
made in 1895 with a less suitable telescope of the ordinary form.




THE MOON HOAX


The public attitude toward matters scientific is one of the
mysteries of our time. It can be described best by the single
word, Credulity; simple, absolute credulity. Perfect confidence
is the most remarkable characteristic of this unbelieving age.
No charlatan, necromancer, or astrologer of three centuries ago
commanded more respectful attention than does his successor of
to-day.

Any person can be a scientific authority; he has but to call
himself by that title, and everyone will give him respectful
attention. Numerous instances can be adduced from the experience
of very recent years to show how true are these remarks. We have
had the Keeley motor and the liquid-air power schemes for making
something out of nothing. Extracting gold from sea-water has been
duly heralded on scientific authority as an easy source of fabulous
wealth for the million. Hard-headed business men not only believe
in such things, but actually invest in them their most valued
possession, capital. Venders of nostrums and proprietary medicines
acquire wealth as if by magic, though it needs but a moment's
reflection to realize that these persons cannot possibly be in
possession of any drugs, or secret methods of compounding drugs,
that are unknown to scientific chemists.

If the world, then, will persistently intrust its health and wealth
into the safe-keeping of charlatans, what can we expect when things
supposedly of far less value are at stake? The famous Moon Hoax, as
we now call it, is truly a classic piece of lying. Though it dates
from as long ago as 1835, it has never had an equal as a piece of
"modern" journalism. Nothing could be more useful than to recall it
to public attention at least once every decade; for it teaches an
important lesson that needs to be iterated again and again.

On November 13, 1833, Sir John Herschel embarked on the Mountstuart
Elphinstone, bound for the Cape of Good Hope. He took with him a
collection of astronomical instruments, with which he intended to
study the heavens of the southern hemisphere, and thus extend his
father's great work to the south polar stars. An earnest student
of astronomy, he asked no better than to be left in peace to
seek the truth in his own fashion. Little did he think that his
expedition would be made the basis for a fabrication of alleged
astronomical discoveries destined to startle a hemisphere. Yet that
is precisely what happened. Some time about the middle of the year
1835 the New York _Sun_ began the publication of certain articles,
purporting to give an account of "Great Astronomical Discoveries,
lately made by Sir John Herschel at the Cape of Good Hope." It was
alleged that these articles were taken from a supplement to the
Edinburgh _Journal of Science_; yet there is no doubt that they
were manufactured entirely in the United States, and probably in
New York.

The hoax begins at once in a grandiloquent style, calculated to
attract popular attention, and well fitted to the marvels about
to be related. Here is an introductory remark, as a specimen:
"It has been poetically said that the stars of heaven are the
hereditary regalia of man as the intellectual sovereign of the
animal creation. He may now fold the zodiac around him with a
loftier consciousness of his mental supremacy." Then follows a
circumstantial and highly plausible account of the manner in
which early and exclusive information was obtained from the Cape.
This was, of course, important in order to make people believe
in the genuineness of the whole; but we pass at once to the more
interesting account of Herschel's supposed instrument.

Nothing could be more skilful than the way in which an air of
truth is cast over the coming account of marvellous discoveries
by explaining in detail the construction of the imaginary
Herschelian instrument. Sir John is supposed to have had an
interesting conversation in England "with Sir David Brewster,
upon the merits of some ingenious suggestion by the latter, in
his article on optics in the Edinburgh Encyclopædia (p. 644), for
improvements in the Newtonian reflectors." The exact reference to
a particular page is here quite delightful. After some further
talk, "the conversation became directed to that all-invincible
enemy, the paucity of light in powerful magnifiers. After a few
moments' silent thought, Sir John diffidently inquired whether
it would not be possible to effect a _transfusion of artificial
light through the focal object of vision_! Sir David, somewhat
startled at the originality of the idea, paused awhile, and then
hesitatingly referred to the refrangibility of rays, and the angle
of incidence.... Sir John continued, 'Why cannot the illuminated
microscope, say the hydro-oxygen, be applied to render distinct,
and, if necessary, even to magnify the focal object?' Sir David
sprang from his chair in an ecstasy of conviction, and leaping
half-way to the ceiling, exclaimed, 'Thou art the man.' "This
absurd imaginary conversation contains nothing but an assemblage
of optical jargon, put together without the slightest intention of
conveying any intelligible meaning to scientific people. Yet it was
well adapted to deceive the public; and we should not be surprised
if it would be credited by many newspaper readers to-day.

The authors go on to explain how money was raised to build the
new instrument, and then describe Herschers embarkation and the
difficulties connected with transporting his gigantic machines
to the place selected for the observing station. "Sir John
accomplished the ascent to the plains by means of two relief
teams of oxen, of eighteen each, in about four days, and, aided
by several companies of Dutch boors [_sic_], proceeded at once to
the erecting of his gigantic fabric." The place really selected
by Herschel cannot be described better than in his own words,
contained in a genuine letter dated January 21, 1835: "A perfect
paradise in rich and magnificent mountain scenery, sheltered from
all winds.... I must reserve for my next all description of the
gorgeous display of flowers which adorn this splendid country, as
well as the astonishing brilliancy of the constellations." The
author of the hoax could have had no knowledge of Herschers real
location, as described in this letter.

The present writer can bear witness to the correctness of
Herschel's words. Feldhausen is truly an ideal secluded spot
for astronomical study. A small obelisk under the sheer cliff
of far-famed Table Mountain now marks the site of the great
reflecting telescope. Here Herschel carried on his scrutiny of the
Southern skies. He observed 1,202 double stars and 1,708 nebulæ
and clusters, of which only 439 were already known. He studied the
famous Magellanic clouds, and made the first careful drawings of
the "keyhole" nebula in the constellation Argo.

Very recent researches of the present royal astronomer at the Cape
have shown that changes of import have certainly taken place in
this nebula since Herschel's time, when a sudden blazing up of the
wonderful star Eta Argus was seen within the nebula. This object
has, perhaps, undergone more remarkable changes of light than
any other star in the heavens. It is as though there were some
vast conflagration at work, now blazing into incandescence, and
again sinking almost into invisibility. In 1843 Maclear estimated
the brilliancy of Eta to be about equal to that of Sirius, the
brightest star in the whole sky. Later it diminished in light,
and cannot be seen to-day with the naked eye, though the latest
telescopic observations indicate that it is again beginning to
brighten.

Such was Herschel's quiet study of his beloved science, in glaring
contrast to the supposed discoveries of the "Hoax." Here are a few
things alleged to have been seen on the moon. The first time the
instrument was turned upon our satellite "the field of view was
covered throughout its entire area with a beautifully distinct and
even vivid representation of basaltic rock." There were forests,
too, and water, "fairer shores never angels coasted on a tour
of pleasure. A beach of brilliant white sand, girt with wild
castellated rocks, apparently of green marble."

There was animal life as well; "we beheld continuous herds of
brown quadrupeds, having all the external characteristics of the
bison, but more diminutive than any species of the bos genus in our
natural history." There was a kind of beaver, that "carries its
young in its arms like a human being," and lives in huts. "From the
appearance of smoke in nearly all of them, there is no doubt of its
(the beaver's) being acquainted with the use of fire." Finally,
as was, of course, unavoidable, human creatures were discovered.
"Whilst gazing in a perspective of about half a mile, we were
thrilled with astonishment to perceive four successive flocks of
large-winged creatures, wholly unlike any kind of birds, descend
with a slow, even motion from the cliffs on the western side, and
alight upon the plain.... Certainly they were like human beings,
and their attitude in walking was both erect and dignified."

We have not space to give more extended extracts from the hoax,
but we think the above specimens will show how deceptive the whole
thing was. The rare reprint from which we have extracted our
quotations contains also some interesting "Opinions of the American
Press Respecting the Foregoing Discovery." The _Daily Advertiser_
said: "No article, we believe, has appeared for years, that will
command so general a perusal and publication. Sir John has added
a stock of knowledge to the present age that will immortalize his
name and place it high on the page of science." The _Mercantile
Advertiser_ said: "Discoveries in the Moon.--We commence to-day
the publication of an interesting article which is stated to have
been copied from the Edinburgh _Journal of Science_, and which
made its first appearance here in a contemporary journal of this
city. It appears to carry intrinsic evidence of being an authentic
document." Many other similar extracts are given. The New York
_Evening Post_ did not fall into the trap. The _Evening Post's_
remarks were as follows: "It is quite proper that the _Sun_ should
be the means of shedding so much light on the _Moon_. That there
should be winged people in the moon does not strike us as more
wonderful than the existence of such a race of beings on the
earth; and that there does or did exist such a race rests on the
evidence of that most veracious of voyagers and circumstantial of
chroniclers, Peter Wilkins, whose celebrated work not only gives an
account of the general appearance and habits of a most interesting
tribe of flying Indians, but also of all those more delicate and
engaging traits which the author was enabled to discover by reason
of the conjugal relations he entered into with one of the females
of the winged tribe."

We shall limit our extracts from the contemporary press to the few
quotations here given, hoping that enough has been said to direct
attention once more to that important subject, the Possibility of
Being Deceived.




THE SUN'S DESTINATION


Three generations of men have come and gone since the Marquis
de Laplace stood before the Academy of France and gave his
demonstration of the permanent stability of our solar system. There
was one significant fault in Newton's superbly simple conception of
an eternal law governing the world in which we live. The labors of
mathematicians following him had shown that the planets must trace
out paths in space whose form could be determined in advance with
unerring certainty by the aid of Newton's law of gravitation. But
they proved just as conclusively that these planetary orbits, as
they are called, could not maintain indefinitely the same shapes or
positions. Slow indeed might be the changes they were destined to
undergo; slow, but sure, with that sureness belonging to celestial
science alone. And so men asked: Has this magnificent solar system
been built upon a scale so grand, been put in operation subject
to a law sublime in its very simplicity, only to change and change
until at length it shall lose every semblance of its former self,
and end, perhaps, in chaos or extinction?

Laplace was able to answer confidently, "No." Nor was his answer
couched in the enthusiastic language of unbalanced theorists who
work by the aid of imagination alone. Based upon the irrefragable
logic of correct mathematical reasoning, and clad in the sober garb
of mathematical formulæ, his results carried conviction to men of
science the world over. So was it demonstrated that changes in our
solar system are surely at work, and shall continue for nearly
countless ages; yet just as surely will they be reversed at last,
and the system will tend to return again to its original form and
condition. The objection that the Newtonian law meant ultimate
dissolution of the world was thus destroyed by Laplace. From that
day forward the law of gravitation has been accepted as holding
sway over all phenomena visible within our planetary world.

The intricacies of our own solar system being thus illumined, the
restless activity of the human intellect was stimulated to search
beyond for new problems and new mysteries. Even more fascinating
than the movements of our sun and planets are all those questions
that relate to the clustered stellar congeries hanging suspended
within the deep vault of night. Does the same law of gravitation
cast its magic spell over that hazy cloud of Pleiades, binding
them, like ourselves, with bonds indissoluble? Who shall answer,
yes or no? We can only say that astronomers have as yet but stepped
upon the threshold of the universe, and fixed the telescope's great
eye upon that which is within.

Let us then begin by reminding the reader what is meant by the
Newtonian law of gravitation. It appears all things possess the
remarkable property of attracting or pulling each other. Newton
declared that all substances, solid, liquid, or even gaseous--from
the massive cliff of rock down to the invisible air--all matter can
no more help pulling than it can help existing. His law further
formulates certain conditions governing the manner in which this
gravitational attraction is exerted; but these are mere matters of
detail; interest centres about the mysterious fact of attraction
itself. How can one thing pull another with no connecting link
through which the pull can act? Just here we touch the point
that has never yet been explained. Nature withholds from science
her ultimate secrets. They that have pondered longest, that have
descended farthest of all men into the clear well of knowledge,
have done so but to sound the depths beyond, never touching bottom.

This inability of ours, to give a good physical explanation of
gravitation, has led certain makers of paradoxes to doubt or even
deny that there is any such thing. But, fortunately, we have a
simple laboratory experiment that helps us. Unexplained it may
ever remain, but that there can be attraction between physical
objects connected by no visible link is proved by the behavior
of an ordinary magnet. Place a small piece of steel or iron near
a magnetized bar, and it will at once be so strongly attracted
that it will actually fly to the magnet. Anyone who has seen this
simple experiment can never again deny the possibility, at least,
of the law of attraction as stated by Newton. Its possibility
once admitted, the fact that it can predict the motions of all
the planets, even down to their minutest details, transforms the
possibility of its truth into a certainty as strong as any human
certainty can ever be.

But this demonstration of Newton's law is limited strictly to
the solar system itself. We may, indeed, reason by analogy, and
take for granted that a law which holds within our immediate
neighborhood is extremely likely to be true also of the entire
visible universe. But men of science are loath to reason thus; and
hence the fascination of researches in cosmic astronomy. Analogy
points out the path. The astronomer is not slow to follow; but he
seeks ever to establish upon incontrovertible evidence those truths
which at first only his daring imagination had led him to half
suspect.

If we are to extend the law of gravitation to the utmost, we must
be careful to consider the law itself in its most complete form.
A heavenly body like the sun is often said to govern the motions
of its family of planets; but such a statement is not strictly
accurate. The governing body is no despot; 'tis an abject slave of
law and order, as much as the tiniest of attendant planets. The
action of gravitation is mutual, and no cosmic body can attract
another without being itself in turn subject to that other's
gravitational action.

If there were in our solar system but two bodies, sun and planet,
we should find each one pursuing a path in space under the
influence of the other's attraction. These two paths or orbits
would be oval, and if the sun and planet were equally massive, the
orbits would be exactly alike, both in shape and size. But if the
sun were far larger than the planet, the orbits would still be
similar in form, but the one traversed by the larger body would be
small. For it is not reasonable to expect a little planet to keep
the big sun moving with a velocity as great as that derived by
itself from the attraction of the larger orb.

Whenever the preponderance of the larger body is extremely great,
its orbit will be correspondingly insignificant in size. This is
in fact the case with our own sun. So massive is it in comparison
with the planets that the orbit is too small to reveal its actual
existence without the aid of our most refined instruments. The path
traced out by the sun's centre would not fill a space as large as
the sun's own bulk. Nevertheless, true orbital motion is there.

So we may conclude that as a necessary consequence of the law of
gravitation every object within the solar system is in motion. To
say that planets revolve about the sun is to neglect as unimportant
the small orbit of the sun itself. This may be sufficiently
accurate for ordinary purposes; but it is unquestionably necessary
to neglect no factor, however small, if we propose to extend our
reasoning to a consideration of the stellar universe. For we shall
then have to deal with systems in which the planets are of a size
comparable with the sun; and in such systems all the orbits will
also be of comparatively equal importance.

Mathematical analysis has derived another fact from discussion
of the law of gravitation which, perhaps, transcends in simple
grandeur everything we have as yet mentioned. It matters not how
great may be the number of massive orbs threading their countless
interlacing curved paths in space, there yet must be in every
cosmic system one single point immovable. This point is called the
Centre of Gravity. If it should so happen that in the beginning of
things, some particle of matter were situated at this centre, then
would that atom ever remain unmoved and imperturbable throughout
all the successive vicissitudes of cosmic evolution. It is doubtful
whether the mind of man can form a conception of anything grander
than such an immovable atom within the mysterious intricacies of
cosmic motion.

But in general, we cannot suppose that the centres of gravity in
the various stellar systems are really occupied by actual physical
bodies. The centre may be a mere mathematical point in space,
situated among the several bodies composing the system, but,
nevertheless, endowed, in a certain sense, with the same remarkable
property of relative immobility.

Having thus defined the centre of gravity in its relation to the
constituent parts of any cosmic system, we can pass easily to its
characteristic properties in connection with the inter-relation of
stellar systems with one another. It can be proved mathematically
that our solar system will pull upon distant stars just as though
the sun and all the planets were concentrated into one vast sphere
having its centre in the centre of gravity of the whole. It is this
property of the centre of gravity which makes it pre-eminently
important in cosmic researches. For, while we know that centre to
be at rest relatively to all the planets in the system, it may,
nevertheless, in its quality as a sort of concentrated essence of
them all, be moving swiftly through space under the pull of distant
stars. In that case, the attendant bodies will go with it--but they
will pursue their evolutions within the system, all unconscious
that the centre of gravity is carrying them on a far wider circuit.

What is the nature of that circuit? This question has been for
many years the subject of earnest study by the clearest minds
among astronomers. The greatest difficulty in the way is the
comparatively brief period during which men have been able to
make astronomical observations of precision. Space and time are
two conceptions that transcend the powers of definition possessed
by any man. But we can at least form a notion of how vast is the
extent of time, if we remember that the period covered by man's
written records is registered but as a single moment upon the
great revolving dial of heaven's dome. One hundred and fifty years
have elapsed since James Bradley built the foundations of modern
sidereal astronomy upon his masterly series of observations at the
Royal Observatory of Greenwich, in England. Yet so slowly do the
movements of the stars unroll themselves upon the firmament, that
even to this day no one of them has been seen by men to trace out
more than an infinitesimal fraction of its destined path through
the voids of space.

Travellers upon a railroad cannot tell at any given moment whether
they are moving in a straight line, or whether the train is
turning upon some curve of huge size. The St. Gothard railway
has several so-called "corkscrew" tunnels, within which the rails
make a complete turn in a spiral, the train finally emerging from
the tunnel at a point almost vertically over the entrance. In this
way the train is lifted to a higher level. Passengers are wont to
amuse themselves while in these tunnels by watching the needle of
an ordinary pocket-compass. This needle, of course, always points
to the north; and as the train turns upon its curve, the needle
will make a complete revolution. But the passenger could not know
without the compass that the train was not moving in a perfectly
straight line. Just so we passengers on the earth are unaware of
the kind of path we are traversing, until, like the compass, the
astronomer's instruments shall reveal to us the truth.

But as we have seen, astronomical observations of precision have
not as yet extended through a period of time corresponding to the
few minutes during which the St. Gothard traveller watches the
compass. We are still in the dark, and do not know as yet whether
mankind shall last long enough upon the earth to see the compass
needle make its revolution. We are compelled to believe that the
motion in space of our sun is progressing upon a curved path; but
so far as precise observations allow us to speak, we can but say
that we have as yet moved through an infinitesimal element only of
that mighty curve. However, we know the point upon the sky toward
which this tiny element of our path is directed, and we have an
approximate knowledge of the speed at which we move.

More than a century ago Sir William Herschel was able to fix
roughly what we call the apex of the sun's way in space, or the
point among the stars toward which that way is for the moment
directed. We say for the moment, but we mean that moment of which
Bradley saw the beginning in 1750, and upon whose end no man of
those now living shall ever look. Herschel found that a comparison
of old stellar observations seemed to indicate that the stars in a
certain part of the sky were opening out, as it were, and that the
constellations in the opposite part of the heavens seemed to be
drawing in, or becoming smaller. There can be but one reasonable
explanation of this. We must be moving toward that part of the sky
where the stars are separating. Just so a man watching a regiment
of soldiers approaching, will see at first only a confused body of
men; but as they come nearer, the individual soldiers will seem to
separate, until at length each one is seen distinct from all the
others.

Herschel fixed the position of the apex at a point in the
constellation Hercules. The most recent investigations of Newcomb
and others have, on the whole, verified Herschel's conclusions.
With the intuitive power of rare genius, Herschel had been able to
sift truth out of error. The observational data at his disposal
would now be called rude, but they disclosed to the scrutiny of
his acute understanding the germ of truth that was in them. Later
investigators have increased the precision of our knowledge, until
we can now say that the present direction of the solar motion is
known within very narrow limits. A tiny circle might be drawn on
the sky, to which an astronomer might point his hand and say:
"Yonder little circle contains the goal toward which the sun and
planets are hastening to-day." Even the speed of this motion has
been subjected to measurement, and found to be about ten miles per
second.

The objective point and the rate of motion thus stated, exact
science holds her peace. Here genuine knowledge stops; and we can
proceed further only by the aid of that imagination which men of
science need to curb at every moment. But let no one think that the
sun will ever reach the so-called apex. To do so would mean cosmic
motion upon a straight line, while every consideration of celestial
mechanics points to motion upon a curve. When shall we turn
sufficiently upon that curve to detect its bending? 'Tis a problem
we must leave as a rich heritage to later generations that are to
follow us. The visionary theorist's notion of a great central sun,
controlling our own sun's way in space, must be dismissed as far
too daring. But for such a central sun we may substitute a central
centre of gravity belonging to a great system of which our sun
is but an insignificant member. Then we reach a conception that
has lost nothing in the grandeur of its simplicity, and is yet
in accord with the probabilities of sober mechanical science. We
cease to be a lonely world, and stretch out the bonds of a common
relationship to yonder stars within the firmament.




INDEX

                         PAGE

  Airy, Astronomer Royal, 1

  Allis, photographs comet, 101

  Andromeda nebula, 28
    temporary star, 28, 29, 45

  Apex, of solar motion, explained, 221

  Aquila, constellation, temporary star in, 40

  Arctic regions, position of pole in, 194

  Argo, constellation, variable star in, 205

  Association, international geodetic, 139

  Asteroids, first discovery by Piazzi, 59, 106
    discovery by photography, 64
    group of, 63
    photography of, invented by Wolf, 104

  Astronomer, royal, 1
    working, description of, 152

  ASTRONOMER'S POLE, THE, 184

  Astronomy, journalistic, 176
    practical uses of, 112

  Atmospheric refraction, explained, 193

  Axis, of figure of the earth, 136
    of rotation of the earth, 136
    polar, of telescope, 173


  Barnard, discovers satellite of Jupiter, 51

  Bessel, measures Pleiades, 15

  Bond, discovers crape ring of Saturn, 144

  Bradley, observes at Greenwich, 219

  Brahe, Tycho, his temporary star, 40

  Bruce, endows polar photography, 197


  Campbell, observes Pole-star, 18

  Cape of Good Hope, observatory, photography at, 101
    telescope, 170, 174

  _Capriccio_, Galileo's, 55

  Cassini, shows Saturn's rings to be double, 144

  Cassiopeia, temporary star in, 40

  Celestial pole, 184

  Central sun theory, 223

  Centre of gravity, 217

  Chart-room, on ship-board, 5

  Chronometer, invention of, 8

  Circle, meridian, explained, 189

  Clerk Maxwell, discusses Saturn's rings, 146

  Clock, affected by temperature, 117
    affected by barometric pressure, 117
    astronomical, 115
    astronomical, how mounted, 116
    astronomical, its dial, 116
    error of, determined with transit, 118
    jeweller's regulator, 114
    of telescope, 175

  Clusters of stars, photography of, 98

  Columbia University Observatory, latitude observations, 139
    polar photography, 196

  Common, his reflecting telescope, 32

  Confusion of dates, in Pacific Ocean, 125

  Congress of Astronomers, Paris, 1887, 102

  Constellations, 162

  Control, "mouse," for photography, 88

  Copernican theory of universe, 53, 56
    demonstration, 94

  Corkscrew tunnels, 220

  Crape ring of Saturn, 144

  Cumulative effect, in photography, 84


  Date, confusion of, in Pacific Ocean, 125

  Date-line, international, explained, 126

  Development of photograph, 81

  Dial, of astronomical clock, 116

  "Dialogue" of Galileo, 53

  Differences of time, explained, 121

  Directions, telescopic measurement of, 21

  Directory of the heavens, 103

  Distance, of light-source in photography, 83
    of stars, 94, 106, 158
    of Sun, 67, 97, 106

  Donner, polar photography, 195

  Double telescopes, for photography, 86


  Earth, motions of its pole, 131
    rotation of, 136, 162, 171, 184
    shape of, 135

  Eclipses, photography of, 109

  Elkin, measures Pleiades, 15

  Equatorial telescope, explained, 170

  Eros, discovered by Witt, 66, 105
    its importance, 67

  Error of clock, determined by transit, 118

  Exposure, length of, in photography, 84


  Feldhausen, Herschel's observatory near Capetown, 204

  Fiji Islands, their date, 126

  Fixed polar telescope, 197

  "Following" the stars, 88, 173

  Four-day cycle of pole-star, 24

  France, outside time-zone system, 129

  Fundamental longitude meridian, 124


  GALILEO, 47
    and the Church, 48
    discoveries of, 49
    observes Saturn, 141

  Galle, discovers Neptune, 61

  Gauss, computes first asteroid orbit, 60

  Gautier, Paris, constructs big telescope, 179

  Geodetic Association, international, 139

  Geography, maps, astronomical side of, 112

  Geology, polar motion in, 131

  Gill, photographs comet, 100

  Gilliss, at Naval Observatory, Washington, 169

  Goldsborough, at Naval Observatory, Washington, 169

  _Grande Lunette_, Paris, 1900, 176, 180

  Gravitation, 13
    in Pleiades, 14, 212
    law of, Newton's, 212

  Gravity, centre of, 217

  Greenwich, origin of longitudes, 7, 124
    time, 7

  Groombridge, English astronomer, 1


  Harrison, inventor of chronometer, 8

  Head, of heliometer, 156

  Heidelberg, photography at, 104

  HELIOMETER, 152
    head of, 156
    how used, 157
    principle of, 154
    scales of, 158
    semi-lenses of, 155

  Helsingfors observatory, polar photography at, 195

  Henry, measures Pleiades, 11, 17

  Hercules, constellation, solar motion toward, 222

  Herschel, discovers apex of solar motion, 221
    discovers Uranus, 59, 141
    John, the moon hoax, 200

  Hipparchus, discovers precession, 186
    early star-catalogue, 21, 39
    invents star magnitudes, 91

  Huygens, announces rings of Saturn, 142
    his logogriph, 143


  Ice-cap, of Earth, 131

  _Index Librorum Prohibitorum_, 53

  International, date-line, explained, 126
    geodetic association, 139

  Inter-stellar motion, in clusters, 98
    in Pleiades, 14

  Islands of Pacific, their longitude and time, 125


  Japan, latitude station in, 139

  Jewellers' correct time, 121

  Journalistic astronomy, 176

  Jupiter's satellites, discovered by Galileo, 50
    discovered by Barnard, 51


  Keeler, observes Saturn's rings, 140, 147, 150
    photographs nebulæ, 32

  "Keyhole" nebula, 205


  Lambert, determines longitude of Washington, 168

  Laplace, discusses Saturn's rings, 146
    nebular hypothesis, 33
    stability of solar system, 210

  Latitude, changes of, 133, 138
    definition of, 134
    determining the, 6

  Leverrier, predicts discovery of Neptune, 61, 142

  Lick Observatory, Keeler's observations, 140

  Light, undulatory theory of, 19, 148

  Light-waves, measuring length of, 20, 149

  Logogriph, by Huygens, 143

  Long-exposure photography, 85

  Longitude, counted East and West, 125
    determining, 6
    determining by occultations, 167
    effect on time differences, 123
    explained, 123
    of Washington, first determined, 168


  Maclear, observes Eta Argus, 205

  Magnitudes, stellar, 91

  Manila, its time, 127

  Maps, astronomical side of, 112

  Meridian circle, explained, 189

  Milky-way, poor in nebulæ, 33

  Minor Planets, see Asteroids.

  MOON, HOAX, 199
    motion among stars, 163
    mountains discovered by Galileo, 49
    size of, measured, 166

  Motion of moon, 163

  MOTIONS of the EARTH'S Pole, 131

  MOUNTING GREAT TELESCOPES, 170


  Naked-eye nebulæ, 28

  Naples, Royal Observatory, latitude observations, 139

  Naval Observatory, Washington, noon signal, 120

  NAVIGATION, 1
    before chronometers, 3
    use of astronomy in, 113

  NEBULÆ, 27

  Nebula, in Andromeda, 28
    in Orion, 30
    "keyhole", 205

  Nebular, hypothesis, 33
    structure in Pleiades, 17

  Nebulous stars, 31

  Negative, and positive, in photography, 82

  Neptune, discovery predicted by Leverrier, 61, 142
    discovery by Galle, 61

  Newcomb, fixes apex of solar motion, 222

  Newton, law of gravitation, 212
    longitude commission, 8

  New York, its telegraphic time system, 120

  Noon Signal, Washington, 120

  Number, of nebulæ, 31, 33
    of temporary stars, 38

  Nutation, explained, 188


  Occultations, 161
    explained, 165

  Occultations, use of, 166, 167

  Orion nebula, 30


  Pacific islands, their longitude and time, 125

  Parallax, solar, 67, 106
    stellar, 94, 106
    measured with heliometer, 158

  Paris, congress of astronomers, 1887, 102
    exposition of 1900, 176

  Periodic motion of earth's pole, 133

  Perseus, constellation, temporary star in, 46

  Philippine Islands, their time, 127

  Photography, asteroid, invented by Wolf, 104
    congress of astronomical, 102
    cumulative effect of light, 84
    distance of light-source, 83
    double telescopes for, 86
    general star-catalogue, 102
    IN ASTRONOMY, 81
    in discovery of asteroids, 64, 104
    in solar physics, 109
    in spectroscopy, 108
    length of exposure, 84
    measuring-machine, Rutherfurd, 93
    motion of telescope for, 87
    "mouse" control of telescope, 88
    of eclipses, 109
    of inter-stellar motion, 99
    Paris congress, 1877, 102
    polar, 191
    Rutherfurd pioneer in, 90
    star-clusters, 98
    star-distances measured by, 94
    summarized, 110
    wholesale methods in, 103

  Piazzi, discovers first asteroid, 59, 106

  Pitkin, report to House of Representatives, 168

  Planetary nebulæ, 31

  PLANET OF 1898, 58

  Planetoids, see Asteroids.

  Planets known to ancients, 58

  PLEIADES, 10
    gravitation among, 212
    motion among, 14, 16, 98
    nebular structure, 17
    number visible, 11

  Polar axis, of telescope, 173

  Polar photography, 191
    at Helsingfors, 195

  Pole, celestial, 184
    of the earth, motions of, 131
    THE ASTRONOMER'S, 184

  POLE-STAR, 18
    as a binary, 25
    as a triple, 18, 26
    change of, 187
    its four-day cycle, 24
    motion toward us, 24

  Positive, and negative, in photography, 82

  Potsdam, observatory, photographic star-catalogue, 103

  Practical uses of astronomy, 112

  Precession, explained, 186

  Prize, for invention of chronometer, 8

  Ptolemaic theory of universe, 56

  Ptolemy, writes concerning Hipparchus, 39


  Railroad time, explained, 127

  Refraction, atmospheric, explained, 193

  "Regulator," the jeweller's clock, 114

  Ring-nebulæ, 31

  Rings, of Saturn, see Saturn's rings.

  Roberts, Andromeda nebula, 28

  Rotation, of Earth, 136, 162, 171, 184
    of Saturn, 150

  Royal Astronomer, his duties, 2

  Royal Observatory, Greenwich, 124
    Greenwich, Bradley's observations, 219
    Naples, latitude observations, 139

  Rutherfurd, cluster photography, 99
    invents photographic apparatus, 93
    pioneer in photography, 90
    stellar parallax, 94


  Sagredus, character in Galileo's Dialogue, 55

  Salusbury, Galileo's translator, 50, 54

  Salviati, character in Galileo's Dialogue, 55

  Samoa, its date, 126

  SATURN'S RINGS, 140
    analogy to planetoids, 147
    announced by Huygens, 142
    observed with spectroscope, 147
    shown to be double by Cassini, 144
    structure and stability, 145

  Scales, of heliometer, 158

  Scorpio, constellation, temporary star in, 39

  Semi-lenses of heliometer, 155

  Sextant, how used, 4

  Sicily, latitude station in, 139

  _Sidereus Nuncius_, published by Galileo, 52

  Simplicio, character in Galileo's Dialogue, 55

  Sirius, brightest star, 205

  Size of Moon, measured, 166

  _Société de l'Optique_, 177

  Solar parallax, see Sun's distance.
    physics, by photography, 109
    system, stability of, 210

  Spectroscope, its use explained, 147
    used on pole-star, 19
    to observe Saturn's rings, 147

  Spiral nebulæ, 31

  Stability, of Saturn's rings, 145
    of Solar System, 210

  Standards, time, of the world, 111
    table of, 130

  "Standard" time, explained, 127

  Star-catalogue, general photographic, 102

  Star-clusters, photography of, 98

  Star-distances 94, 106
    measured with heliometer, 158
    Rutherfurd, 94

  Star magnitudes, 91

  Star-motion, toward us, 21

  Star-tables, astronomical, 118

  Stars, variable, 42

  St Gothard railway, tunnels, 220

  Sun, newspaper, the moon hoax, 201

  SUN-DIAL, HOW TO MAKE A, 69

  SUN'S, DESTINATION, 210
    distance, compared with star distance, 97
    measured with Eros, 67, 106
    motion, apex of, 221

  Sun-spots, discovered by Galileo, 49

  _Systema Saturnium_, Huygens, 143


  Telescope, clock, 175
    at Paris Exposition, 176, 180
    double, for photography, 86
    equatorial, explained, 170
    first used by Galileo, 49
    motion of, 87
    mounting great, 170
    unmoving, for polar photography, 197

  TEMPORARY STARS, 37
    in Andromeda nebula, 28, 29, 45
    in Aquila, 40
    in Cassiopeia, 40
    in Perseus, 46
    in Scorpio, 39
    their number, 38
    theory of, 42

  Time, correct, determined astronomically, 113
    differences between different places, 121

  TIME STANDARDS OF THE WORLD, 111
    standards of the World, table of, 130
    system, in New York, 120
    zones, explained, 128

  Trails, photographic, 191

  Transit, for determining clock error, 118

  Tycho Brahe, his temporary star, 40


  Ulugh Beg, early star-catalogue, 21

  Undulatory theory, of light, 19, 148

  Universe, theories of, 34, 53, 56

  Uranus, discovered by Herschel, 59, 142

  Use of occultations, 166, 167

  Uses of astronomy, practical, 112


  Variable stars, 42
    in Argo, 205

  Vega, future pole-star, 187

  Visibility of stars, in day-time, 191

  Vision, phenomenon of, 20, 149


  Washington, its longitude first determined, 168

  Waves, explained, 148
    of light, 20, 148

  Wilkes, at Naval Observatory, Washington, 169

  Wilkins, imaginary voyage of, 208

  Witt, discovers Eros, 66, 105

  Wolf, M, invents asteroid photography, 104
    measures Pleiades, 11

  World's time standards, table of, 130


  Yale College, Pleiades measured at, 15


  Zones, time, explained, 128




  TRANSCRIBER'S NOTE

  Italic text is denoted by _underscores_.

  Fractions in the two tables on pg 74 and pg 78 are displayed in the form
  "a-b/c" as 4-1/2 or 2-7/16 for example. The original text in the
  tables used the form "a b-c". A few other basic fractions in the text
  such as ½ and ⅖ are displayed in this same form in the etext.

  There is only one Footnote in this book, with its anchor on pg 69.
  It has been placed at the end of the chapter containing the anchor.

  Obvious typographical errors and punctuation errors have been
  corrected after careful comparison with other occurrences within
  the text and consultation of external sources.

  Except for those changes noted below, all misspellings in the text,
  and inconsistent or archaic usage, have been retained. For example,
  time zone, time-zone; Le Verrier, Leverrier; light wave, light-wave;
  intrust; wabbling; unexcelled; crape; monumented.

  Pg 146, 'James Clark-Maxwell' replaced by 'James Clerk Maxwell'.
  Pg 189, 'impossible to measure' replaced by 'possible to measure'.






End of Project Gutenberg's Practical Talks by an Astronomer, by Harold Jacoby

*** 