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A command-line tool for producing tables of the positions of solar system objects over time.
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README.md

ephemerisCompute

ephemerisCompute is a command-line tool for producing tables of the positions of solar system objects over time.

For the Sun, Moon and planets, it extracts positions from the publicly available NASA DE405 ephemeris, which covers the time period 1600 to 2200 AD, typically with an accuracy of a few km. Outside of this time range, it solves Kepler's equation for the position of an object in an elliptical orbit, yielding results of much lower accuracy.

For asteroids, it solves Kepler's equation using orbital elements downloaded from Ted Bowell's astorb.dat catalogue.

For comets, it obtains orbital elements from the Minor Planet Center's website.

ephemerisCompute was written to produce all of the ephemerides on the website https://in-the-sky.org, which is maintained by the author.

Supported operating systems

ephemerisCompute is written in C and runs in Linux, MacOS, and other Unix-like operating systems.

License

This code is distributed under the Gnu General Public License. It is (C) Dominic Ford 2010 - 2020.

Set up

Before you start, ephemerisCompute needs to download various data from the internet, including the DE405 ephemeris files, the asteroid catalogue, and the list of comets.

This can be done with the shell script setup.sh. The total download size will be around 500 MB.

Producing an ephemeris

Running the command-line tool bin/ephem.bin will produce a default ephemeris for Jupiter between 2000 Jan 1 and 2000 Feb 1, at midnight each day:

dominic@ganymede:~/ephemerisCompute$ ./bin/ephem.bin 
2451544.500000000000    3.996320681  2.730993728  1.073274469   
2451545.500000000000    3.991757746  2.736868431  1.075903739   
2451546.500000000000    3.987185148  2.742736516  1.078530407   
...

The first time you run the tool, it needs to convert the ASCII data files you downloaded into a binary format, which will typically take a few seconds before any output is produced. The binary data is cached, leading to near instantaneous performance subsequently.

In this output, the columns are Julian day number, and the XYZ position of Jupiter, measured in AU, relative to the centre of mass of the solar system.

The following command-line arguments can be used to customise the ephemeris:

  • --jd_min [float] - Specify the Julian day number at which the ephemeris should begin.

  • --jd_max [float] - Specify the Julian day number at which the ephemeris should end.

  • --jd_step [float] - Specify the interval between the lines in the ephemeris, in days.

  • --epoch [float] - Specify the epoch of the RA/Dec coordinate system, e.g. 2451545.0 for J2000 (default).

  • --objects [string] - Specify the list of objects to produce ephemerides for. Objects should be separated by commas, e.g. "jupiter, mars" or "P301, A4, 1P/Halley". See below for an explanation of what names are accepted for objects. If multiiple objects are listed, their positions are listed in sets of columns from left to right.

  • --output_binary [int] - If zero, a text-based ephemeris is produced. If non-zero, then the data is output as a stream of binary data, with type double. The first column, the Julian day number, is omitted from binary ephemerides.

  • --output_constellations [int] - If non-zero, then the final column states the name of the constellation the object is in. Note the fetching this information is one of the slowest routines within ephemerisCompute, so this may have significant performance impact when computing large ephemerides.

  • --use_orbital_elements [int] - If zero, then the NASA JPL DE405 ephemeris is used to produce the ephemeris. This will give best accuracy (by far). If set to 1, then orbital elements for all objects are used to compute their approximate positions. If set to 2, then algorithms from Jean Meeus's book "Astronomical Algorithms" are used [not currently supported; do not use!]. The positions of comets and asteroids are always computed using orbital elements, since they are not included in DE405.

  • --output_format [int] - Selects what data should be returned. The following formats are currently supported:

    • -1: XYZ position (ecliptic coordinates at epoch of observation)
    • 0: XYZ position (in ICRS coordinates)
    • 1: RA and Dec (in radians, J2000.0 coordinates; recommended)
    • 2: X, Y, Z, RA, Dec, V-band magnitude, phase, angular size
    • 3: As for 2, but also: physical size, albedo, sun_dist, earth_dist, sun_ang_dist, theta_edo, eclLng, eclDist, eclLat

Object names

This section lists the names which are recognised by the --objects command-line argument:

  • p1, pmercury, mercury: Mercury
  • p2, pvenus, venus: Venus
  • p3, pearth, earth: Earth
  • p301, pmoon, moon: The Moon
  • p4, pmars, mars: Mars
  • p5, pjupiter, jupiter: Jupiter
  • p6, psaturn, saturn: Saturn
  • p7, puranus, uranus: Uranus
  • p8, pneptune, neptune: Neptune
  • p9, ppluto, pluto: Pluto
  • A<n>: Asteroid number n, e.g. A1 for Ceres, or A4 for Vesta
  • C/1995 O1. Comets may be referred to by their names in this format
  • 1P/Halley. Comets may be referred to by their names in this format
  • 0001P. Periodic comets may be referred to by their names in the format %4dP
  • CJ95O010. Comets may be referred to by their Minor Planet Center designations
  • C<n>: Comer number n. n is the line number within the file Soft00Cmt.txt, downloaded from the Minor Planet Center.
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