Earth's atmosphere consists mostly of nitrogen and oxygen. Tropical regions receive more energy from the Sun than polar regions, which is redistributed by atmospheric and ocean circulation. Greenhouse gases also play an important role in regulating the surface temperature. A region's climate is not only determined by latitude, but also by its proximity to moderating oceans and height among other factors. Extreme weather, such as tropical cyclones and heat waves, occurs in most areas and has a large impact on life.
+
Historically, earth has been written in lowercase. From early Middle English, its definite sense as "the globe" was expressed as the earth. By Early Modern English, many nouns were capitalized, and the earth was also written the Earth, particularly when referenced along with other heavenly bodies. More recently, the name is sometimes simply given as Earth, by analogy with the names of the other planets, though earth and forms with the remain common.[25]House styles now vary: Oxford spelling recognizes the lowercase form as the most common, with the capitalized form an acceptable variant. Another convention capitalizes "Earth" when appearing as a name (e.g. "Earth's atmosphere") but writes it in lowercase when preceded by the (e.g. "the atmosphere of the earth"). It almost always appears in lowercase in colloquial expressions such as "what on earth are you doing?"[27]
+
Occasionally, the name Terra/ˈtɛrə/ is used in scientific writing and especially in science fiction to distinguish humanity's inhabited planet from others,[28] while in poetry Tellus/ˈtɛləs/ has been used to denote personification of the Earth.[29] The Greek poetic name Gaea (Gæa) /ˈdʒiːə/ is rare, though the alternative spelling Gaia has become common due to the Gaia hypothesis, in which case its pronunciation is /ˈɡaɪə/ rather than the more Classical /ˈɡeɪə/.[30]
+
There are a number of adjectives for the planet Earth. From Earth itself comes earthly. From the Latin Terra comes Terran/ˈtɛrən/,[31] Terrestrial /təˈrɛstriəl/,[32] and (via French) Terrene/təˈriːn/,[33] and from the Latin Tellus comes Tellurian/tɛˈlʊəriən/[34] and Telluric.[35]
+
Artist's impression of the early Solar System's planetary disk
+
The oldest material found in the Solar System is dated to 4.5682+0.0002 −0.0004Ga (billion years) ago.[36] By 4.54±0.04 Ga the primordial Earth had formed.[37] The bodies in the Solar System formed and evolved with the Sun. In theory, a solar nebula partitions a volume out of a molecular cloud by gravitational collapse, which begins to spin and flatten into a circumstellar disk, and then the planets grow out of that disk with the Sun. A nebula contains gas, ice grains, and dust (including primordial nuclides). According to nebular theory, planetesimals formed by accretion, with the primordial Earth being estimated as likely taking anywhere from 70–100 million years to form.[38]
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Estimates of the age of the Moon range from 4.5 Ga to significantly younger.[39] A leading hypothesis is that it was formed by accretion from material loosed from Earth after a Mars-sized object with about 10% of Earth's mass, named Theia, collided with Earth.[40] It hit Earth with a glancing blow and some of its mass merged with Earth.[41][42] Between approximately 4.1 and 3.8 Ga, numerous asteroid impacts during the Late Heavy Bombardment caused significant changes to the greater surface environment of the Moon and, by inference, to that of Earth.[43]
+
As the molten outer layer of Earth cooled it formed the first solid crust, which is thought to have been mafic in composition. The first continental crust, which was more felsic in composition, formed by the partial melting of this mafic crust. The presence of grains of the mineral zircon of Hadean age in Eoarcheansedimentary rocks suggests that at least some felsic crust existed as early as 4.4 Ga, only 140 Ma after Earth's formation.[49] There are two main models of how this initial small volume of continental crust evolved to reach its current abundance:[50] (1) a relatively steady growth up to the present day,[51] which is supported by the radiometric dating of continental crust globally and (2) an initial rapid growth in the volume of continental crust during the Archean, forming the bulk of the continental crust that now exists,[52][53] which is supported by isotopic evidence from hafnium in zircons and neodymium in sedimentary rocks. The two models and the data that support them can be reconciled by large-scale recycling of the continental crust, particularly during the early stages of Earth's history.[54]
+
New continental crust forms as a result of plate tectonics, a process ultimately driven by the continuous loss of heat from Earth's interior. Over the period of hundreds of millions of years, tectonic forces have caused areas of continental crust to group together to form supercontinents that have subsequently broken apart. At approximately 750 Ma, one of the earliest known supercontinents, Rodinia, began to break apart. The continents later recombined to form Pannotia at 600–540 Ma, then finally Pangaea, which also began to break apart at 180 Ma.[55]
+
The most recent pattern of ice ages began about 40 Ma,[56] and then intensified during the Pleistocene about 3 Ma.[57]High- and middle-latitude regions have since undergone repeated cycles of glaciation and thaw, repeating about every 21,000, 41,000 and 100,000 years.[58] The Last Glacial Period, colloquially called the "last ice age", covered large parts of the continents, up to the middle latitudes, in ice and ended about 11,700 years ago.[59]
+
During the Neoproterozoic, 1000 to 541 Ma, much of Earth might have been covered in ice. This hypothesis has been termed "Snowball Earth", and it is of particular interest because it preceded the Cambrian explosion, when multicellular life forms significantly increased in complexity.[70][71] Following the Cambrian explosion, 535 Ma, there have been at least five major mass extinctions and many minor ones.[72][73] Apart from the proposed current Holocene extinction event, the most recent was 66 Ma, when an asteroid impact triggered the extinction of the non-avian dinosaurs and other large reptiles, but largely spared small animals such as insects, mammals, lizards and birds. Mammalian life has diversified over the past 66 Mys, and several million years ago an African ape gained the ability to stand upright.[74] This facilitated tool use and encouraged communication that provided the nutrition and stimulation needed for a larger brain, which led to the evolution of humans. The development of agriculture, and then civilization, led to humans having an influence on Earth and the nature and quantity of other life forms that continues to this day.[75] Over 99% of all species that ever lived on Earth are extinct.[76][77]
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Because carbon dioxide (CO 2) has a long life time in the atmosphere, moderate human CO 2 emissions may postpone the next glacial inception by 100,000 years.[78] Earth's expected long-term future is tied to that of the Sun. Over the next 1.1 billion years, solar luminosity will increase by 10%, and over the next 3.5 billion years by 40%.[79] Earth's increasing surface temperature will accelerate the inorganic carbon cycle, reducing CO 2 concentration to levels lethally low for plants (10 ppm for C4 photosynthesis) in approximately 100–900 million years.[80][81] The lack of vegetation will result in the loss of oxygen in the atmosphere, making animal life impossible.[82] Due to the increased luminosity, Earth's mean tempearture may reach 100 °C (212 °F) in 1.5 billion years, and all ocean water will evaporate and be lost to space within an estimated 1.6 to 3 billion years.[83] Even if the Sun were stable, a fraction of the water in the modern oceans will descend to the mantle, due to reduced steam venting from mid-ocean ridges.[83][84]
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The Sun will evolve to become a red giant in about 5 billion years. Models predict that the Sun will expand to roughly 1 AU (150 million km; 93 million mi), about 250 times its present radius.[79][85] Earth's fate is less clear. As a red giant, the Sun will lose roughly 30% of its mass, so, without tidal effects, Earth will move to an orbit 1.7 AU (250 million km; 160 million mi) from the Sun when the star reaches its maximum radius.[79]
+
The summit of Chimborazo, the point on the Earth's surface that is farthest from the Earth's center[86]
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The shape of Earth is nearly spherical. There is a small flattening at the poles and bulging around the equator due to Earth's rotation.[87] so that a better approximation of Earth's shape is an oblate spheroid, whose equatorial diameter is 43 kilometres (27 mi) larger than the pole-to-pole diameter.[88]
+
The point on the surface farthest from Earth's center of mass is the summit of the equatorial Chimborazo volcano in Ecuador (6,384.4 km or 3,967.1 mi).[89][90][91] The average diameter of the reference spheroid is 12,742 kilometres (7,918 mi). Local topography deviates from this idealized spheroid, although on a global scale these deviations are small compared to Earth's radius: the maximum deviation of only 0.17% is at the Mariana Trench (10,925 metres or 35,843 feet below local sea level),[92] whereas Mount Everest (8,848 metres or 29,029 feet above local sea level) represents a deviation of 0.14%.[n 6][94]
+In geodesy, the exact shape that Earth's oceans would adopt in the absence of land and perturbations such as tides and winds is called the geoid. More precisely, the geoid is the surface of gravitational equipotential at mean sea level.[95]
+
Earth's mass is approximately 5.97×1024kg (5,970 Yg). It is composed mostly of iron (32.1%), oxygen (30.1%), silicon (15.1%), magnesium (13.9%), sulphur (2.9%), nickel (1.8%), calcium (1.5%), and aluminum (1.4%), with the remaining 1.2% consisting of trace amounts of other elements. Due to mass segregation, the core region is estimated to be primarily composed of iron (88.8%), with smaller amounts of nickel (5.8%), sulphur (4.5%), and less than 1% trace elements.[98]
+
The most common rock constituents of the crust are nearly all oxides: chlorine, sulphur, and fluorine are the important exceptions to this and their total amount in any rock is usually much less than 1%. Over 99% of the crust is composed of 11 oxides, principally silica, alumina, iron oxides, lime, magnesia, potash and soda.[99][98]
+
Earth's interior, like that of the other terrestrial planets, is divided into layers by their chemical or physical (rheological) properties. The outer layer is a chemically distinct silicate solid crust, which is underlain by a highly viscous solid mantle. The crust is separated from the mantle by the Mohorovičić discontinuity.[102] The thickness of the crust varies from about 6 kilometres (3.7 mi) under the oceans to 30–50 km (19–31 mi) for the continents. The crust and the cold, rigid, top of the upper mantle are collectively known as the lithosphere, which is divided into independently moving tectonic plates.[103]
+
Beneath the lithosphere is the asthenosphere, a relatively low-viscosity layer on which the lithosphere rides. Important changes in crystal structure within the mantle occur at 410 and 660 km (250 and 410 mi) below the surface, spanning a transition zone that separates the upper and lower mantle. Beneath the mantle, an extremely low viscosity liquid outer core lies above a solid inner core.[104] Earth's inner core may be rotating at a slightly higher angular velocity than the remainder of the planet, advancing by 0.1–0.5° per year, although both somewhat higher and much lower rates have also been proposed.[105] The radius of the inner core is about one fifth of that of Earth.
+Density increases with depth, as described in the table on the right.
+
The major heat-producing isotopes within Earth are potassium-40, uranium-238, and thorium-232.[106] At the center, the temperature may be up to 6,000 °C (10,830 °F),[107] and the pressure could reach 360 GPa (52 million psi).[108] Because much of the heat is provided by radioactive decay, scientists postulate that early in Earth's history, before isotopes with short half-lives were depleted, Earth's heat production was much higher. At approximately 3 Gyr, twice the present-day heat would have been produced, increasing the rates of mantle convection and plate tectonics, and allowing the production of uncommon igneous rocks such as komatiites that are rarely formed today.[109][110]
+
+The mean heat loss from Earth is 87 mW m−2, for a global heat loss of 4.42×1013 W.[111] A portion of the core's thermal energy is transported toward the crust by mantle plumes, a form of convection consisting of upwellings of higher-temperature rock. These plumes can produce hotspots and flood basalts.[112] More of the heat in Earth is lost through plate tectonics, by mantle upwelling associated with mid-ocean ridges. The final major mode of heat loss is through conduction through the lithosphere, the majority of which occurs under the oceans because the crust there is much thinner than that of the continents.[113]
Earth's mechanically rigid outer layer, the lithosphere, is divided into tectonic plates. These plates are rigid segments that move relative to each other at one of three boundaries types: at convergent boundaries, two plates come together; at divergent boundaries, two plates are pulled apart; and at transform boundaries, two plates slide past one another laterally. Along these plate boundaries, earthquakes, volcanic activity, mountain-building, and oceanic trench formation can occur.[115] The tectonic plates ride on top of the asthenosphere, the solid but less-viscous part of the upper mantle that can flow and move along with the plates.[116]
+
As the tectonic plates migrate, oceanic crust is subducted under the leading edges of the plates at convergent boundaries. At the same time, the upwelling of mantle material at divergent boundaries creates mid-ocean ridges. The combination of these processes recycles the oceanic crust back into the mantle. Due to this recycling, most of the ocean floor is less than 100 Ma old. The oldest oceanic crust is located in the Western Pacific and is estimated to be 200 Ma old.[117][118] By comparison, the oldest dated continental crust is 4,030 Ma,[119] although zircons have been found preserved as clasts within Eoarchean sedimentary rocks that give ages up to 4,400 Ma, indicating that at least some continental crust existed at that time.[49]
+
The seven major plates are the Pacific, North American, Eurasian, African, Antarctic, Indo-Australian, and South American. Other notable plates include the Arabian Plate, the Caribbean Plate, the Nazca Plate off the west coast of South America and the Scotia Plate in the southern Atlantic Ocean. The Australian Plate fused with the Indian Plate between 50 and 55 Ma. The fastest-moving plates are the oceanic plates, with the Cocos Plate advancing at a rate of 75 mm/a (3.0 in/year)[120] and the Pacific Plate moving 52–69 mm/a (2.0–2.7 in/year). At the other extreme, the slowest-moving plate is the South American Plate, progressing at a typical rate of 10.6 mm/a (0.42 in/year).[121]
+
Current Earth without water, elevation greatly exaggerated (click/enlarge to "spin" 3D-globe).
+
The total surface area of Earth is about 510 million km2 (197 million sq mi).[15] Of this, 70.8%,[15] or 361.13 million km2 (139.43 million sq mi), is below sea level and covered by ocean water.[122] Below the ocean's surface are much of the continental shelf, mountains, volcanoes,[88] oceanic trenches, submarine canyons, oceanic plateaus, abyssal plains, and a globe-spanning mid-ocean ridge system. The remaining 29.2%, or 148.94 million km2 (57.51 million sq mi), not covered by water has terrain that varies greatly from place to place and consists of mountains, deserts, plains, plateaus, and other landforms. The elevation of the land surface varies from the low point of −418 m (−1,371 ft) at the Dead Sea, to a maximum altitude of 8,848 m (29,029 ft) at the top of Mount Everest. The mean height of land above sea level is about 797 m (2,615 ft).[123]
+
The continental crust consists of lower density material such as the igneous rocks granite and andesite. Less common is basalt, a denser volcanic rock that is the primary constituent of the ocean floors.[124] Sedimentary rock is formed from the accumulation of sediment that becomes buried and compacted together. Nearly 75% of the continental surfaces are covered by sedimentary rocks, although they form about 5% of the crust.[125] The third form of rock material found on Earth is metamorphic rock, which is created from the transformation of pre-existing rock types through high pressures, high temperatures, or both. The most abundant silicate minerals on Earth's surface include quartz, feldspars, amphibole, mica, pyroxene and olivine.[126] Common carbonate minerals include calcite (found in limestone) and dolomite.[127]
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The pedosphere is the outermost layer of Earth's continental surface and is composed of soil and subject to soil formation processes. The total arable land is 10.9% of the land surface, with 1.3% being permanent cropland.[130][131] Close to 40% of Earth's land surface is used for agriculture, or an estimated 16.7 million km2 (6.4 million sq mi) of cropland and 33.5 million km2 (12.9 million sq mi) of pastureland.[132]
+
Earth's gravity measured by NASA's GRACE mission, showing deviations from the theoretical gravity. Red shows where gravity is stronger than the smooth, standard value, and blue shows where it is weaker.
+
The gravity of Earth is the acceleration that is imparted to objects due to the distribution of mass within Earth. Near Earth's surface, gravitational acceleration is approximately 9.8 m/s2 (32 ft/s2). Local differences in topography, geology, and deeper tectonic structure cause local and broad, regional differences in Earth's gravitational field, known as gravity anomalies.[133]
+
The main part of Earth's magnetic field is generated in the core, the site of a dynamo process that converts the kinetic energy of thermally and compositionally driven convection into electrical and magnetic field energy. The field extends outwards from the core, through the mantle, and up to Earth's surface, where it is, approximately, a dipole. The poles of the dipole are located close to Earth's geographic poles. At the equator of the magnetic field, the magnetic-field strength at the surface is 3.05×10−5T, with a magnetic dipole moment of 7.79×1022 Am2 at epoch 2000, decreasing nearly 6% per century.[134] The convection movements in the core are chaotic; the magnetic poles drift and periodically change alignment. This causes secular variation of the main field and field reversals at irregular intervals averaging a few times every million years. The most recent reversal occurred approximately 700,000 years ago.[135][136]
+
Schematic of Earth's magnetosphere. The solar wind flows from left to right
+
The extent of Earth's magnetic field in space defines the magnetosphere. Ions and electrons of the solar wind are deflected by the magnetosphere; solar wind pressure compresses the dayside of the magnetosphere, to about 10 Earth radii, and extends the nightside magnetosphere into a long tail.[137] Because the velocity of the solar wind is greater than the speed at which waves propagate through the solar wind, a supersonic bow shock precedes the dayside magnetosphere within the solar wind.[138]Charged particles are contained within the magnetosphere; the plasmasphere is defined by low-energy particles that essentially follow magnetic field lines as Earth rotates.[139][140] The ring current is defined by medium-energy particles that drift relative to the geomagnetic field, but with paths that are still dominated by the magnetic field,[141] and the Van Allen radiation belts are formed by high-energy particles whose motion is essentially random, but contained in the magnetosphere.[142][143]
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During magnetic storms and substorms, charged particles can be deflected from the outer magnetosphere and especially the magnetotail, directed along field lines into Earth's ionosphere, where atmospheric atoms can be excited and ionized, causing the aurora.[144]
+
Earth's rotation imaged by DSCOVR EPIC on 29 May 2016, a few weeks before a solstice.
+
Earth's rotation period relative to the Sun—its mean solar day—is 86,400 seconds of mean solar time (86,400.0025 SI seconds).[145] Because Earth's solar day is now slightly longer than it was during the 19th century due to tidal deceleration, each day varies between 0 and 2 ms longer than the mean solar day.[146][147]
+
Earth's rotation period relative to the fixed stars, called its stellar day by the International Earth Rotation and Reference Systems Service (IERS), is 86,164.0989 seconds of mean solar time (UT1), or 23h 56m 4.0989s.[4][n 10] Earth's rotation period relative to the precessing or moving mean March equinox (when the Sun is at 90° on the equator), is 86,164.0905 seconds of mean solar time (UT1) (23h 56m 4.0905s).[4] Thus the sidereal day is shorter than the stellar day by about 8.4 ms.[148]
+
Apart from meteors within the atmosphere and low-orbiting satellites, the main apparent motion of celestial bodies in Earth's sky is to the west at a rate of 15°/h = 15'/min. For bodies near the celestial equator, this is equivalent to an apparent diameter of the Sun or the Moon every two minutes; from Earth's surface, the apparent sizes of the Sun and the Moon are approximately the same.[149][150]
+
The Pale Blue Dot photo taken in 1990 by the Voyager 1 spacecraft showing Earth (center right) from nearly 6.0 billion km (3.7 billion mi) away, about 5.6 hours at light speed.[151]
+
Earth orbits the Sun at an average distance of about 150 million km (93 million mi) every 365.2564 mean solar days, or one sidereal year. This gives an apparent movement of the Sun eastward with respect to the stars at a rate of about 1°/day, which is one apparent Sun or Moon diameter every 12 hours. Due to this motion, on average it takes 24 hours—a solar day—for Earth to complete a full rotation about its axis so that the Sun returns to the meridian. The orbital speed of Earth averages about 29.78 km/s (107,200 km/h; 66,600 mph), which is fast enough to travel a distance equal to Earth's diameter, about 12,742 km (7,918 mi), in seven minutes, and the distance to the Moon, 384,000 km (239,000 mi), in about 3.5 hours.[5]
+
The Moon and Earth orbit a common barycenter every 27.32 days relative to the background stars. When combined with the Earth–Moon system's common orbit around the Sun, the period of the synodic month, from new moon to new moon, is 29.53 days. Viewed from the celestial north pole, the motion of Earth, the Moon, and their axial rotations are all counterclockwise. Viewed from a vantage point above the north poles of both the Sun and Earth, Earth orbits in a counterclockwise direction about the Sun. The orbital and axial planes are not precisely aligned: Earth's axis is tilted some 23.44 degrees from the perpendicular to the Earth–Sun plane (the ecliptic), and the Earth–Moon plane is tilted up to ±5.1 degrees against the Earth–Sun plane. Without this tilt, there would be an eclipse every two weeks, alternating between lunar eclipses and solar eclipses.[5][152]
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The Hill sphere, or the sphere of gravitational influence, of Earth is about 1.5 million km (930,000 mi) in radius.[153][n 11] This is the maximum distance at which Earth's gravitational influence is stronger than the more distant Sun and planets. Objects must orbit Earth within this radius, or they can become unbound by the gravitational perturbation of the Sun.[153]
+
Earth, along with the Solar System, is situated in the Milky Way and orbits about 28,000 light-years from its center. It is about 20 light-years above the galactic plane in the Orion Arm.[154]
+
The axial tilt of Earth is approximately 23.439281°[4] with the axis of its orbit plane, always pointing towards the Celestial Poles. Due to Earth's axial tilt, the amount of sunlight reaching any given point on the surface varies over the course of the year. This causes the seasonal change in climate, with summer in the Northern Hemisphere occurring when the Tropic of Cancer is facing the Sun, and winter taking place when the Tropic of Capricorn in the Southern Hemisphere faces the Sun. During the summer, the day lasts longer, and the Sun climbs higher in the sky. In winter, the climate becomes cooler and the days shorter.[155] Above the Arctic Circle and below the Antarctic Circle there is no daylight at all for part of the year, causing a polar night, and this night extends for several months at the poles themselves. These same latitudes also experience a midnight sun, where the sun remains visible all day.[156][157]
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By astronomical convention, the four seasons can be determined by the solstices—the points in the orbit of maximum axial tilt toward or away from the Sun—and the equinoxes, when Earth's rotational axis is aligned with its orbital axis. In the Northern Hemisphere, winter solstice currently occurs around 21 December; summer solstice is near 21 June, spring equinox is around 20 March and autumnal equinox is about 22 or 23 September. In the Southern Hemisphere, the situation is reversed, with the summer and winter solstices exchanged and the spring and autumnal equinox dates swapped.[158]
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The angle of Earth's axial tilt is relatively stable over long periods of time. Its axial tilt does undergo nutation; a slight, irregular motion with a main period of 18.6 years.[159] The orientation (rather than the angle) of Earth's axis also changes over time, precessing around in a complete circle over each 25,800 year cycle; this precession is the reason for the difference between a sidereal year and a tropical year. Both of these motions are caused by the varying attraction of the Sun and the Moon on Earth's equatorial bulge. The poles also migrate a few meters across Earth's surface. This polar motion has multiple, cyclical components, which collectively are termed quasiperiodic motion. In addition to an annual component to this motion, there is a 14-month cycle called the Chandler wobble. Earth's rotational velocity also varies in a phenomenon known as length-of-day variation.[160]
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In modern times, Earth's perihelion occurs around 3 January, and its aphelion around 4 July. These dates change over time due to precession and other orbital factors, which follow cyclical patterns known as Milankovitch cycles. The changing Earth–Sun distance causes an increase of about 6.8% in solar energy reaching Earth at perihelion relative to aphelion.[161][n 12] Because the Southern Hemisphere is tilted toward the Sun at about the same time that Earth reaches the closest approach to the Sun, the Southern Hemisphere receives slightly more energy from the Sun than does the northern over the course of a year. This effect is much less significant than the total energy change due to the axial tilt, and most of the excess energy is absorbed by the higher proportion of water in the Southern Hemisphere.[162]
+
The Moon is a relatively large, terrestrial, planet-like natural satellite, with a diameter about one-quarter of Earth's. It is the largest moon in the Solar System relative to the size of its planet, although Charon is larger relative to the dwarf planetPluto.[163][164] The natural satellites of other planets are also referred to as "moons", after Earth's.[165] The most widely accepted theory of the Moon's origin, the giant-impact hypothesis, states that it formed from the collision of a Mars-size protoplanet called Theia with the early Earth. This hypothesis explains (among other things) the Moon's relative lack of iron and volatile elements and the fact that its composition is nearly identical to that of Earth's crust.[41]
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The gravitational attraction between Earth and the Moon causes tides on Earth.[166] The same effect on the Moon has led to its tidal locking: its rotation period is the same as the time it takes to orbit Earth. As a result, it always presents the same face to the planet.[167] As the Moon orbits Earth, different parts of its face are illuminated by the Sun, leading to the lunar phases.[168] Due to their tidal interaction, the Moon recedes from Earth at the rate of approximately 38 mm/a (1.5 in/year). Over millions of years, these tiny modifications—and the lengthening of Earth's day by about 23 µs/yr—add up to significant changes.[169] During the Ediacaran period, for example, (approximately 620 Ma) there were 400±7 days in a year, with each day lasting 21.9±0.4 hours.[170]
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The Moon may have dramatically affected the development of life by moderating the planet's climate. Paleontological evidence and computer simulations show that Earth's axial tilt is stabilized by tidal interactions with the Moon.[171] Some theorists think that without this stabilization against the torques applied by the Sun and planets to Earth's equatorial bulge, the rotational axis might be chaotically unstable, exhibiting large changes over millions of years, as is the case for Mars, though this is disputed.[172][173]
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Viewed from Earth, the Moon is just far enough away to have almost the same apparent-sized disk as the Sun. The angular size (or solid angle) of these two bodies match because, although the Sun's diameter is about 400 times as large as the Moon's, it is also 400 times more distant.[150] This allows total and annular solar eclipses to occur on Earth.[174]
+
As of April 2020[update], there are 2,666 operational, human-made satellites orbiting Earth.[8] There are also inoperative satellites, including Vanguard 1, the oldest satellite currently in orbit, and over 16,000 pieces of tracked space debris.[n 3] Earth's largest artificial satellite is the International Space Station.[180]
+
Water is transported to various parts of the hydrosphere via the water cycle.
+
The abundance of water on Earth's surface is a unique feature that distinguishes the "Blue Planet" from other planets in the Solar System. Earth's hydrosphere consists chiefly of the oceans, but technically includes all water surfaces in the world, including inland seas, lakes, rivers, and underground waters down to a depth of 2,000 m (6,600 ft). The mass of the oceans is approximately 1.35×1018metric tons or about 1/4400 of Earth's total mass. The oceans cover an area of 361.8 million km2 (139.7 million sq mi) with a mean depth of 3,682 m (12,080 ft), resulting in an estimated volume of 1.332 billion km3 (320 million cu mi).[181] If all of Earth's crustal surface were at the same elevation as a smooth sphere, the depth of the resulting world ocean would be 2.7 to 2.8 km (1.68 to 1.74 mi).[182] About 97.5% of the water is saline; the remaining 2.5% is fresh water. Most fresh water, about 68.7%, is present as ice in ice caps and glaciers.[183]
+
In Earth's coldest regions, snow survives over the summer and changes into ice. This accumulated snow and eyes eventually forms into glaciers, bodies of ice that flow under the influence of their own gravity. Alpine glaciers form in mountainous areas, whereas vast ice sheets form over land in polar regions. The flow of glaciers erodes the surface changing it dramatically, with the formation of U-shaped valleys and other landforms.[184]Sea ice in the Arctic covers an area about as big as the United States, although it is quickly retreating as a consequence of climate change.[185]
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The average salinity of Earth's oceans is about 35 grams of salt per kilogram of sea water (3.5% salt).[186] Most of this salt was released from volcanic activity or extracted from cool igneous rocks.[187] The oceans are also a reservoir of dissolved atmospheric gases, which are essential for the survival of many aquatic life forms.[188] Sea water has an important influence on the world's climate, with the oceans acting as a large heat reservoir.[189] Shifts in the oceanic temperature distribution can cause significant weather shifts, such as the El Niño–Southern Oscillation.[190]
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The atmospheric pressure at Earth's sea level averages 101.325 kPa (14.696 psi),[191] with a scale height of about 8.5 km (5.3 mi).[5] A dry atmosphere is composed of 78.084% nitrogen, 20.946% oxygen, 0.934% argon, and trace amounts of carbon dioxide and other gaseous molecules.[191]Water vapor content varies between 0.01% and 4%[191] but averages about 1%.[5] The height of the troposphere varies with latitude, ranging between 8 km (5 mi) at the poles to 17 km (11 mi) at the equator, with some variation resulting from weather and seasonal factors.[192]
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Earth's biosphere has significantly altered its atmosphere. Oxygenic photosynthesis evolved 2.7 Gya, forming the primarily nitrogen–oxygen atmosphere of today.[61] This change enabled the proliferation of aerobic organisms and, indirectly, the formation of the ozone layer due to the subsequent conversion of atmospheric O 2 into O 3. The ozone layer blocks ultravioletsolar radiation, permitting life on land.[193] Other atmospheric functions important to life include transporting water vapor, providing useful gases, causing small meteors to burn up before they strike the surface, and moderating temperature.[194] This last phenomenon is known as the greenhouse effect: trace molecules within the atmosphere serve to capture thermal energy emitted from the ground, thereby raising the average temperature. Water vapor, carbon dioxide, methane, nitrous oxide, and ozone are the primary greenhouse gases in the atmosphere. Without this heat-retention effect, the average surface temperature would be −18 °C (0 °F), in contrast to the current +15 °C (59 °F),[195] and life on Earth probably would not exist in its current form.[196]
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Earth's atmosphere has no definite boundary, slowly becoming thinner and fading into outer space. Three-quarters of the atmosphere's mass is contained within the first 11 km (6.8 mi) of the surface. This lowest layer is called the troposphere. Energy from the Sun heats this layer, and the surface below, causing expansion of the air. This lower-density air then rises and is replaced by cooler, higher-density air. The result is atmospheric circulation that drives the weather and climate through redistribution of thermal energy.[197]
+
Massive clouds above the Mojave Desert, February 2016
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The primary atmospheric circulation bands consist of the trade winds in the equatorial region below 30° latitude and the westerlies in the mid-latitudes between 30° and 60°.[198]Ocean currents are also important factors in determining climate, particularly the thermohaline circulation that distributes thermal energy from the equatorial oceans to the polar regions.[199]
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The amount of solar energy reaching Earth's surface decreases with increasing latitude. At higher latitudes, the sunlight reaches the surface at lower angles, and it must pass through thicker columns of the atmosphere. As a result, the mean annual air temperature at sea level decreases by about 0.4 °C (0.7 °F) per degree of latitude from the equator.[200] Earth's surface can be subdivided into specific latitudinal belts of approximately homogeneous climate. Ranging from the equator to the polar regions, these are the tropical (or equatorial), subtropical, temperate and polar climates.[201]
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Further factors that affect a location's climates are its proximity to oceans, the oceanic and atmospheric circulation, and topology.[202] Places close to oceans typically have colder summers and warmer winters, due to the fact that oceans can the store large amounts of heat. The wind transports the cold or the heat of the ocean to the land.[203] Atmospheric circulation also plays an important role: San Francisco and Washington DC are both coastal cities at about the same latitude. San Francisco's climate is significantly more moderate as the prevailing wind direction is from sea to land.[204] Finally, temperatures decrease with height causing mountainous areas to be colder than low-lying areas.[205]
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Water vapor generated through surface evaporation is transported by circulatory patterns in the atmosphere. When atmospheric conditions permit an uplift of warm, humid air, this water condenses and falls to the surface as precipitation.[197] Most of the water is then transported to lower elevations by river systems and usually returned to the oceans or deposited into lakes. This water cycle is a vital mechanism for supporting life on land and is a primary factor in the erosion of surface features over geological periods. Precipitation patterns vary widely, ranging from several meters of water per year to less than a millimeter. Atmospheric circulation, topographic features, and temperature differences determine the average precipitation that falls in each region.[206]
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This view from orbit shows the full moon partially obscured by Earth's atmosphere.
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Above the troposphere, the atmosphere is usually divided into the stratosphere, mesosphere, and thermosphere.[194] Each layer has a different lapse rate, defining the rate of change in temperature with height. Beyond these, the exosphere thins out into the magnetosphere, where the geomagnetic fields interact with the solar wind.[210] Within the stratosphere is the ozone layer, a component that partially shields the surface from ultraviolet light and thus is important for life on Earth. The Kármán line, defined as 100 km above Earth's surface, is a working definition for the boundary between the atmosphere and outer space.[211]
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Thermal energy causes some of the molecules at the outer edge of the atmosphere to increase their velocity to the point where they can escape from Earth's gravity. This causes a slow but steady loss of the atmosphere into space. Because unfixed hydrogen has a low molecular mass, it can achieve escape velocity more readily, and it leaks into outer space at a greater rate than other gases.[212] The leakage of hydrogen into space contributes to the shifting of Earth's atmosphere and surface from an initially reducing state to its current oxidizing one. Photosynthesis provided a source of free oxygen, but the loss of reducing agents such as hydrogen is thought to have been a necessary precondition for the widespread accumulation of oxygen in the atmosphere.[213] Hence the ability of hydrogen to escape from the atmosphere may have influenced the nature of life that developed on Earth.[214] In the current, oxygen-rich atmosphere most hydrogen is converted into water before it has an opportunity to escape. Instead, most of the hydrogen loss comes from the destruction of methane in the upper atmosphere.[215]
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A planet that can sustain life is termed habitable, even if life did not originate there. Earth provides liquid water—an environment where complex organic molecules can assemble and interact, and sufficient energy to sustain metabolism.[220] Plants can take up nutrients from the atmosphere, soils and water. These nutrients are constantly recycled between different species.[221] The distance of Earth from the Sun, as well as its orbital eccentricity, rate of rotation, axial tilt, geological history, sustaining atmosphere, and magnetic field all contribute to the current climatic conditions at the surface.[222]
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Earth's human population passed seven billion in the early 2010s,[229] and is projected to peak at around ten billion in the second half of the 21st century.[230] Most of the growth is expected to take place in sub-Saharan Africa.[230]Human population density varies widely around the world, but a majority live in Asia. By 2050, 68% of the world's population is expected to be living in urban, rather than rural, areas.[231] 68% of the land mass of the world is in the Northern Hemisphere.[232] Partly due to the predominance of land mass, 90% of humans live in the Northern Hemisphere.[233]
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It is estimated that one-eighth of Earth's surface is suitable for humans to live on – three-quarters of Earth's surface is covered by oceans, leaving one-quarter as land. Half of that land area is desert (14%),[234] high mountains (27%),[235] or other unsuitable terrains. States claim the planet's entire land surface, except for parts of Antarctica and a few other unclaimed areas. Earth has never had a planetwide government, but the United Nations is the leading worldwide intergovernmental organization.[236][237]
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The first human to orbit Earth was Yuri Gagarin on 12 April 1961.[238] In total, about 550 people have visited outer space and reached orbit as of November 2018[update], and, of these, twelve have walked on the Moon.[239][240] Normally, the only humans in space are those on the International Space Station. The station's crew, made up of six people, is usually replaced every six months.[241] The farthest that humans have traveled from Earth is 400,171 km (248,655 mi), achieved during the Apollo 13 mission in 1970.[242]
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Land use in 2015 as a percentage of ice-free land surface[243]
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Land use
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Percentage
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Cropland
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12 – 14%
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Human-used forests
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16 – 27%
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Infrastructure
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1%
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Unused land
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24 – 31%
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Earth has resources that have been exploited by humans.[244] Those termed non-renewable resources, such as fossil fuels, only renew over geological timescales.[245] Large deposits of fossil fuels are obtained from Earth's crust, consisting of coal, petroleum, and natural gas.[246] These deposits are used by humans both for energy production and as feedstock for chemical production.[247] Mineral ore bodies have also been formed within the crust through a process of ore genesis, resulting from actions of magmatism, erosion, and plate tectonics.[248] These metals and other elements are extracted by mining, a process which often brings environmental and health damage.[249]
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Earth's biosphere produces many useful biological products for humans, including food, wood, pharmaceuticals, oxygen, and the recycling of organic waste. The land-based ecosystem depends upon topsoil and fresh water, and the oceanic ecosystem depends on dissolved nutrients washed down from the land.[250] In 2019, 39 million km2 (15 million sq mi) of Earth's land surface consisted of forest and woodlands, 12 million km2 (4.6 million sq mi) was shrub and grassland, 40 million km2 (15 million sq mi) were used for animal feed production and grazing, and 11 million km2 (4.2 million sq mi) were cultivated as croplands.[251] Of the 12–14% of ice-free land that is used for croplands, 2 percent point was irrigated in 2015.[243] Humans use building materials to construct shelters.[252]
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Human cultures have developed many views of the planet.[253] The standard astronomical symbol of Earth consists of a cross circumscribed by a circle, ,[254] representing the four corners of the world. Earth is sometimes personified as a deity. In many cultures it is a mother goddess that is also the primary fertility deity.[255]Creation myths in many religions involve the creation of Earth by a supernatural deity or deities.[255] The Gaia Principle, developed mid-20th century, compared Earth's environments and life as a single self-regulating organism leading to broad stabilization of the conditions of habitability.[256][257][258] Images of Earth taken from space, particularly during the Apollo program, have been credited with altering the way that people viewed the planet that they lived on, emphasising its beauty, uniqueness and apparent fragility.[259][260]
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Scientific investigation has resulted in several culturally transformative shifts in people's view of the planet. Initial belief in a flat Earth was gradually displaced in Ancient Greece by the idea of a spherical Earth, which was attributed to both the philosophers Pythagoras and Parmenides.[261][262] Earth was generally believed to be the center of the universe until the 16th century, when scientists first conclusively demonstrated that it was a moving object, comparable to the other planets in the Solar System.[263]
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It was only during the 19th century that geologists realized Earth's age was at least many millions of years.[264]Lord Kelvin used thermodynamics to estimate the age of Earth to be between 20 million and 400 million years in 1864, sparking a vigorous debate on the subject; it was only when radioactivity and radioactive dating were discovered in the late 19th and early 20th centuries that a reliable mechanism for determining Earth's age was established, proving the planet to be billions of years old.[265][266]
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^All astronomical quantities vary, both secularly and periodically. The quantities given are the values at the instant J2000.0 of the secular variation, ignoring all periodic variations.
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^ abaphelion = a × (1 + e); perihelion = a × (1 – e), where a is the semi-major axis and e is the eccentricity. The difference between Earth's perihelion and aphelion is 5 million kilometers.—Wilkinson, John (8 January 2009). Probing the New Solar System. CSIRO Publishing. p. 144. ISBN978-0-643-09949-4.
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^ abAs of 4 January 2018, the United States Strategic Command tracked a total of 18,835 artificial objects, mostly debris. See: Anz-Meador, Phillip; Shoots, Debi, eds. (February 2018). "Satellite Box Score"(PDF). Orbital Debris Quarterly News. 22 (1): 12. Retrieved 18 April 2018.
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^Earth's circumference is almost exactly 40,000 km because the metre was calibrated on this measurement—more specifically, 1/10-millionth of the distance between the poles and the equator.
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^Due to natural fluctuations, ambiguities surrounding ice shelves, and mapping conventions for vertical datums, exact values for land and ocean coverage are not meaningful. Based on data from the Vector Map and Global LandcoverArchived 26 March 2015 at the Wayback Machine datasets, extreme values for coverage of lakes and streams are 0.6% and 1.0% of Earth's surface. The ice sheets of Antarctica and Greenland are counted as land, even though much of the rock that supports them lies below sea level.
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^ If Earth were shrunk to the size of a billiard ball, some areas of Earth such as large mountain ranges and oceanic trenches would feel like tiny imperfections, whereas much of the planet, including the Great Plains and the abyssal plains, would feel smoother.[93]
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^The ultimate source of these figures, uses the term "seconds of UT1" instead of "seconds of mean solar time".—Aoki, S.; Kinoshita, H.; Guinot, B.; Kaplan, G. H.; McCarthy, D. D.; Seidelmann, P. K. (1982). "The new definition of universal time". Astronomy and Astrophysics. 105 (2): 359–61. Bibcode:1982A&A...105..359A.
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^For Earth, the Hill radius is , where m is the mass of Earth, a is an astronomical unit, and M is the mass of the Sun. So the radius in AU is about .
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^Aphelion is 103.4% of the distance to perihelion. Due to the inverse square law, the radiation at perihelion is about 106.9% the energy at aphelion.
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^ abSimon, J.L.; Bretagnon, P.; Chapront, J.; Chapront-Touzé, M.; Francou, G.; Laskar, J. (February 1994). "Numerical expressions for precession formulae and mean elements for the Moon and planets". Astronomy and Astrophysics. 282 (2): 663–83. Bibcode:1994A&A...282..663S.
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^International Earth Rotation and Reference Systems Service (IERS) Working Group (2004). "General Definitions and Numerical Standards"(PDF). In McCarthy, Dennis D.; Petit, Gérard (eds.). IERS Conventions (2003)(PDF). IERS Technical Note No. 32. Frankfurt am Main: Verlag des Bundesamts für Kartographie und Geodäsie. p. 12. ISBN978-3-89888-884-4. Retrieved 29 April 2016.
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^Bouvier, Audrey; Wadhwa, Meenakshi (September 2010). "The age of the Solar System redefined by the oldest Pb–Pb age of a meteoritic inclusion". Nature Geoscience. 3 (9): 637–641. Bibcode:2010NatGe...3..637B. doi:10.1038/ngeo941.
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^Claeys, Philippe; Morbidelli, Alessandro (1 January 2011). "Late Heavy Bombardment". In Gargaud, Muriel; Amils, Prof Ricardo; Quintanilla, José Cernicharo; Cleaves II, Henderson James (Jim); Irvine, William M.; Pinti, Prof Daniele L.; Viso, Michel (eds.). Encyclopedia of Astrobiology. Springer Berlin Heidelberg. pp. 909–912. doi:10.1007/978-3-642-11274-4_869. ISBN978-3-642-11271-3.
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^Guinan, E. F.; Ribas, I. (2002). Benjamin Montesinos, Alvaro Gimenez and Edward F. Guinan (ed.). Our Changing Sun: The Role of Solar Nuclear Evolution and Magnetic Activity on Earth's Atmosphere and Climate. ASP Conference Proceedings: The Evolving Sun and its Influence on Planetary Environments. San Francisco: Astronomical Society of the Pacific. Bibcode:2002ASPC..269...85G. ISBN978-1-58381-109-2.
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^Wilkinson, B. H.; McElroy, B. J. (2007). "The impact of humans on continental erosion and sedimentation". Bulletin of the Geological Society of America. 119 (1–2): 140–56. Bibcode:2007GSAB..119..140W. doi:10.1130/B25899.1. S2CID128776283.
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^ abcSackmann, I.-J.; Boothroyd, A. I.; Kraemer, K. E. (1993). "Our Sun. III. Present and Future". Astrophysical Journal. 418: 457–68. Bibcode:1993ApJ...418..457S. doi:10.1086/173407.
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^Tanimoto, Toshiro (1995). "Crustal Structure of the Earth"(PDF). In Thomas J. Ahrens (ed.). Global Earth Physics: A Handbook of Physical Constants. Global Earth Physics: A Handbook of Physical Constants. Washington, DC: American Geophysical Union. Bibcode:1995geph.conf.....A. ISBN978-0-87590-851-9. Archived from the original(PDF) on 16 October 2006. Retrieved 3 February 2007.
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^Pollack, Henry N.; Hurter, Suzanne J.; Johnson, Jeffrey R. (August 1993). "Heat flow from the Earth's interior: Analysis of the global data set". Reviews of Geophysics. 31 (3): 267–80. Bibcode:1993RvGeo..31..267P. doi:10.1029/93RG01249.
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^Sclater, John G; Parsons, Barry; Jaupart, Claude (1981). "Oceans and Continents: Similarities and Differences in the Mechanisms of Heat Loss". Journal of Geophysical Research. 86 (B12): 11535. Bibcode:1981JGR....8611535S. doi:10.1029/JB086iB12p11535.
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^de Pater, Imke; Lissauer, Jack J. (2010). Planetary Sciences (2nd ed.). Cambridge University Press. p. 154. ISBN978-0-521-85371-2.
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^Wenk, Hans-Rudolf; Bulakh, Andreĭ Glebovich (2004). Minerals: their constitution and origin. Cambridge University Press. p. 359. ISBN978-0-521-52958-7.
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^Seidelmann, P. Kenneth (1992). Explanatory Supplement to the Astronomical Almanac. Mill Valley, CA: University Science Books. p. 48. ISBN978-0-935702-68-2.
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^Zeilik, M.; Gregory, S. A. (1998). Introductory Astronomy & Astrophysics (4th ed.). Saunders College Publishing. p. 56. ISBN978-0-03-006228-5.
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^ abWilliams, David R. (10 February 2006). "Planetary Fact Sheets". NASA. Retrieved 28 September 2008.—See the apparent diameters on the Sun and Moon pages.
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^Williams, G.E. (2000). "Geological constraints on the Precambrian history of Earth's rotation and the Moon's orbit". Reviews of Geophysics. 38 (1): 37–59. Bibcode:2000RvGeo..38...37W. doi:10.1029/1999RG900016.
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^Perlman, Howard (17 March 2014). "The World's Water". USGS Water-Science School. Retrieved 12 April 2015.
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^Hendrix, Mark (2019). Earth Science: An Introduction. Bosten: Cengage. p. 330. ISBN978-0-357-11656-2.
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^Hendrix, Mark (2019). Earth Science: An Introduction. Bosten: Cengage. p. 329. ISBN978-0-357-11656-2.
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^Kennish, Michael J. (2001). Practical handbook of marine science. Marine science series (3rd ed.). CRC Press. p. 35. ISBN978-0-8493-2391-1.
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^Mullen, Leslie (11 June 2002). "Salt of the Early Earth". NASA Astrobiology Magazine. Archived from the original on 30 June 2007. Retrieved 14 March 2007.
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^Morris, Ron M. "Oceanic Processes". NASA Astrobiology Magazine. Archived from the original on 15 April 2009. Retrieved 14 March 2007.
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^Scott, Michon (24 April 2006). "Earth's Big heat Bucket". NASA Earth Observatory. Retrieved 14 March 2007.
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^Geerts, B.; Linacre, E. (November 1997). "The height of the tropopause". Resources in Atmospheric Sciences. University of Wyoming. Retrieved 10 August 2006.
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^Harrison, Roy M.; Hester, Ronald E. (2002). Causes and Environmental Implications of Increased UV-B Radiation. Royal Society of Chemistry. ISBN978-0-85404-265-4.
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^ abMoran, Joseph M. (2005). "Weather". World Book Online Reference Center. NASA/World Book, Inc. Archived from the original on 13 December 2010. Retrieved 17 March 2007.
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^ abBerger, Wolfgang H. (2002). "The Earth's Climate System". University of California, San Diego. Retrieved 24 March 2007.
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^Staff. "Climate Zones". UK Department for Environment, Food and Rural Affairs. Archived from the original on 8 August 2010. Retrieved 24 March 2007.
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^Rohli, Robert. V.; Vega, Anthony J. (2018). Climatology (fourth ed.). Jones & Bartlett Learning. p. 49. ISBN978-1-284-12656-3.
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^Rohli, Robert. V.; Vega, Anthony J. (2018). Climatology (fourth ed.). Jones & Bartlett Learning. p. 32. ISBN978-1-284-12656-3.
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^Rohli, Robert. V.; Vega, Anthony J. (2018). Climatology (fourth ed.). Jones & Bartlett Learning. p. 34. ISBN978-1-284-12656-3.
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^Rohli, Robert. V.; Vega, Anthony J. (2018). Climatology (fourth ed.). Jones & Bartlett Learning. p. 46. ISBN978-1-284-12656-3.
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^Various (21 July 1997). "The Hydrologic Cycle". University of Illinois. Retrieved 24 March 2007.
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^Rohli, Robert. V.; Vega, Anthony J. (2018). Climatology (fourth ed.). Jones & Bartlett Learning. p. 159. ISBN978-1-284-12656-3.
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^Abedon, Stephen T. (31 March 1997). "History of Earth". Ohio State University. Archived from the original on 29 November 2012. Retrieved 19 March 2007.
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^Smith, Sharon; Fleming, Lora; Solo-Gabriele, Helena; Gerwick, William H. (2 September 2011). Oceans and Human Health. Elsevier Science. p. 212. ISBN978-0-08-087782-2.
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^Staff. "Themes & Issues". Secretariat of the Convention on Biological Diversity. Archived from the original on 7 April 2007. Retrieved 29 March 2007.
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^Lloyd, John; Mitchinson, John (2010). The Discretely Plumper Second QI Book of General Ignorance. Faber & Faber. p. 116-117. ISBN978-0-571-29072-7.
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^Kuhn, Betsy (2006). The race for space: the United States and the Soviet Union compete for the new frontier. Twenty-First Century Books. p. 34. ISBN978-0-8225-5984-9.
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^Shayler, David; Vis, Bert (2005). Russia's Cosmonauts: Inside the Yuri Gagarin Training Center. Birkhäuser. ISBN978-0-387-21894-6.
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^Liungman, Carl G. (2004). "Group 29: Multi-axes symmetric, both soft and straight-lined, closed signs with crossing lines". Symbols – Encyclopedia of Western Signs and Ideograms. New York: Ionfox AB. pp. 281–82. ISBN978-91-972705-0-2.
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^Arnett, Bill (16 July 2006). "Earth". The Nine Planets, A Multimedia Tour of the Solar System: one star, eight planets, and more. Retrieved 9 March 2010.
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^Monroe, James; Wicander, Reed; Hazlett, Richard (2007). Physical Geology: Exploring the Earth. Thomson Brooks/Cole. pp. 263–65. ISBN978-0-495-01148-4.
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^Henshaw, John M. (2014). An Equation for Every Occasion: Fifty-Two Formulas and Why They Matter. Johns Hopkins University Press. pp. 117–18. ISBN978-1-4214-1491-1.
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^Burchfield, Joe D. (1990). Lord Kelvin and the Age of the Earth. University of Chicago Press. pp. 13–18. ISBN978-0-226-08043-7.
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+ Stack Exchange network consists of 176 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.
+
Twice today I have come across questions requesting assistance scraping data from copyrighted web pages. Both questions listed the site in question which, after a quick visit, shows the site content to be protected under copyright.
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In each case I added a comment cautioning respondents to consider the ethical issues of assisting in the theft of intellectual property. In one case, a previous respondent replied he hadn't considered this when answering the OP's question.
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Ignoring for the moment the possible legal culpability of respondents and of SO itself...
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How should questions like these be handled?
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Should respondents consider ethical issues like these before answering? And if so, how can we assure they do?
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+ If it is clear that the OP is being abusive, I close these kinds of questions as too localized.
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+– user102937
+ Jun 3 '11 at 17:33
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+ Knowledge should be free... It's up to the asker to determine legality of use.
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+– Soumya
+ Jun 3 '11 at 17:39
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+ Note that the Terms of Use for the StackExchange network is pretty clear about this. It specifically prohibits posting of content that violates the rights of others. See stackexchange.com/legal under the Subscriber Content section.
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+– user102937
+ Jun 3 '11 at 17:48
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+ Does web scraping always violate copyright? I would assume that's only an issue if it's actually being copied somewhere, whereas a tool that you write to display data for your own use is more or less a stripped-down web browser. For example, I have a small python script that does a GET on one website and tells me what the headline is. I don't think I've violated anyone's copyright, at least that I can see. Would it be OK for me to ask for help constructing something like my tool?
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+– dsolimano
+ Jun 3 '11 at 19:01
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+ @dsolimano Good question! I suppose it's a question of degree. In the questions that triggered this discussion, the OPs stated they wanted to extract all the terms and definitions from an online dictionary or all of the records from a multi-page list of contacts. This kind of scraping isn't the same as, for example, creating an RSS feed of articles from a news site for personal use.
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+– Rob Raisch
+ Jun 3 '11 at 19:06
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+ @dsolimano: Check the website's Terms of Use. Scraping is often prohibited. If you're in doubt, just ask the website's owner. Your particular usage scenario is probably OK. There's a vast difference between obtaining the title of individual pages for personal use, and ripping an entire website for commercial purposes.
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+– user102937
+ Jun 3 '11 at 22:10
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+ @RobertHarvey what is the legal definition of scraping? Are you breaking any law because you visit the site - your browser must scrap the content before it can show it to you!
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+– Danubian Sailor
+ Sep 4 '13 at 11:08
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+ @ŁukaszLech check with the site's Terms of Service. If the TOS allows you to scrape or crawl the site with robots, then it's OK. Most sites do not allow this, however. Get permission from the site owner.
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+– user102937
+ Sep 4 '13 at 14:19
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+ @RobertHarvey what legal meaning have the Terms of Service? Everyone can write anything, but it doesn't mean that everything will have legal power. If someone writes you are not allowed to view the site barefoot, such statement is meaningless.
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+– Danubian Sailor
+ Sep 4 '13 at 18:30
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+ @ŁukaszLech: You're just trying to get me to say that it's somehow legal, so that you can do whatever you want with the site, against the site owner's wishes. I'm not going to say that. It doesn't have to be illegal to be something that we don't want to promote on Stack Overflow, so stop asking for legal justifications.
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+– user102937
+ Sep 4 '13 at 18:35
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I'm hard pressed to find a solution better than the one you did, which is to provide technical guidance for the process in general and add a disclaimer (I'd prefer at the beginning of the answer vs. at the end) warning the developer to consider the legal and ethical ramifications of what they're doing.
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At a previous job of mine this was actually a bit of an issue. We were aggregating public criminal records and in many cases that involved screen scraping on various jurisdiction websites. Some of the websites had statements indicating that the information was their property and that scraping was against the terms of service.
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In that particular case, our company had a liability attorney on staff and he assured us that the statements are of no concern. The company made the decision to go forward with it against the advice of the developers. (Makes sense, he is after all the company's attorney. And the data is public records.) In one case it caused our scraper to be IP-banned by a server, but I don't know of any other problems that came about as a result.
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Long story short, we as a community (both employees of and users of Stack Exchange) are in no position to offer legal advice of any kind. We aren't experts in law, we have no knowledge of the specific case, etc. Even if the site in question explicitly states that scraping is entirely illegal and violators will be killed to the fullest extent of the law, etc. that doesn't mean that the statement actually has any legal value.
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The best we can do is offer technical advice and strongly urge the reader to seek legal counsel for non-technical concerns.
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Edit: In re-reading your question, you make a very interesting point at the end. The idea of ensuring that respondents make such disclaimers. For that, I see two options for Stack Exchange:
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Have site-wide disclaimers (surely they already do, but maybe re-think the visibility of them) indicating that this isn't legal advice, etc., etc.
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Encourage (or at least not frown upon) users to edit other users' answers to include such disclaimers.
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The former solution seems like the best. I'd hate to see the long-term results of the latter solution. Users would get upset that their answers are being filled with legal jargon when they don't see why it should. Answer meanings could be slightly changed by the edits, which does a disservice to the original respondent. Etc.
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+ Thanks much for your considered reply. But as I understand Intellectual Property law (barely), providing assistance in this case opens the respondent to legal liability should the property owner decide to protect her rights. It's not hard to imagine that an argument could also be made that SE facilitated that assistance and is thus also legally culpable. I'm not interested in providing legal advice to anyone, but I'm concerned respondents are unaware of the possibility of legal exposure here.
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+– Rob Raisch
+ Jun 3 '11 at 17:40
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+ @Rob Raisch: Good point. As that same company lawyer once told me, "In court 'illegal' is defined as how well they can out-debate me. It's entirely subjective." Going that route, anything Stack Exchange and its userbase provides can be liable for any wrongdoings provided that an argument can be made at all, whether it makes sense or not. (Kind of scary, really.) We live in a culture (at least I do) where potential liability seems to be a way of life. Giving an injured motorist first aid can land you in court for touching them inappropriately. (This slope sure looks slippery...)
+
+– David
+ Jun 3 '11 at 17:48
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+ Indeed, but I call out this specific example (where a poster specifically requests help to commit theft) because it is so blatant. Lots of things carry the potential to create legal liability, these questions realize that potential immediately.
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+– Rob Raisch
+ Jun 3 '11 at 17:56
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+ 4
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+ @Rob Raisch: Given that, I really like Robert Harvey's comment on the question above. It's up to the voters on a case-by-case basis of course, but it's definitely a viable approach. In that case I guess it really comes down to the difference between "How do I scrape data from a website?" vs. "How do I scrape data from this specific website?" If the question is asked as the latter, then I agree with you that there's a potential problem. Close as too localized. If the question is asked as the former, then it's just a technical question with no legal context.
+
+– David
+ Jun 3 '11 at 18:00
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+ I also like Robert Harvey's comment and will point others to the SO Terms when appropriate. Thanks again.
+
+– Rob Raisch
+ Jun 3 '11 at 18:03
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+ Good answer, the owner of the site can state that the web site can be viewed only by white males wearing black socks (or opposite) and any other use is the violation of terms of use, but such statements have no legal validity.
+
+– Danubian Sailor
+ Sep 4 '13 at 11:10
+
The question as stated is almost useless without specific examples. Here is why:
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+The [legal] loophole in copyright is fair use. Under the banner of fair use, you could legally upload a video without the copyright holder's permission. Anyone who contributes anything to the web should have the four factors of fair use commited to memory by now:
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the purpose of the use
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the nature of the copyrighted work
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the relative amount of the portion used
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the market effect of the use on the copyrighted work
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+These are the four factors courts use to determine if something is fair use.
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No way to judge if it's fair use or not until we have specifics.
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+
We regularly smack down anything that copies wholesale; we want a contextual quote with the most relevant bit, and a link to the rest.
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+ In the examples I originally referenced, the posters requested assistance scraping the entire contents of a contact database and all listings from a dictionary of medical terms, neither of which fall under the Doctrine of Fair Use as I understand it. But even if a use could be defended as "fair", I think responders should be made aware of the legal risk they run, especially given the potential financial disparity they might experience if drawn into litigation. (I'll provide relevant quotes and links in a bit.)
+
+– Rob Raisch
+ Jun 4 '11 at 15:56
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+ @RobRaisch is the scrapping of the whole site violating the doctrine of fair use? How it differs from clicking through the whole site? All depends, what you want to do with that data. Is there any specific law against downloading sites for offline usage? But that's exactly how browser cache works! And if there are such laws, are they in power for the whole world or only for a small part of it?
+
+– Danubian Sailor
+ Sep 4 '13 at 11:13
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diff --git a/World Population by Year - Worldometer.html b/World Population by Year - Worldometer.html
new file mode 100644
index 0000000..a1bd25b
--- /dev/null
+++ b/World Population by Year - Worldometer.html
@@ -0,0 +1,14 @@
+ World Population by Year - Worldometer
From 1950 to current year: elaboration of data by United Nations, Department of Economic and Social Affairs, Population Division. World Population Prospects: The 2019 Revision. (Medium-fertility variant).
.svg)
.gif)
+
+ THE HOT 100
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diff --git a/World Population by Year - Worldometer.html b/World Population by Year - Worldometer.html
new file mode 100644
index 0000000..a1bd25b
--- /dev/null
+++ b/World Population by Year - Worldometer.html
@@ -0,0 +1,14 @@
+ 
\ No newline at end of file
+ +-
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+ If it is clear that the OP is being abusive, I close these kinds of questions as too localized.
+
+– user102937
+ Jun 3 '11 at 17:33
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+ -
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+ Knowledge should be free... It's up to the asker to determine legality of use.
+
+– Soumya
+ Jun 3 '11 at 17:39
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+ -
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+ Note that the Terms of Use for the StackExchange network is pretty clear about this. It specifically prohibits posting of content that violates the rights of others. See stackexchange.com/legal under the Subscriber Content section.
+
+– user102937
+ Jun 3 '11 at 17:48
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+ -
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+ Does web scraping always violate copyright? I would assume that's only an issue if it's actually being copied somewhere, whereas a tool that you write to display data for your own use is more or less a stripped-down web browser. For example, I have a small python script that does a GET on one website and tells me what the headline is. I don't think I've violated anyone's copyright, at least that I can see. Would it be OK for me to ask for help constructing something like my tool?
+
+– dsolimano
+ Jun 3 '11 at 19:01
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+ @dsolimano Good question! I suppose it's a question of degree. In the questions that triggered this discussion, the OPs stated they wanted to extract all the terms and definitions from an online dictionary or all of the records from a multi-page list of contacts. This kind of scraping isn't the same as, for example, creating an RSS feed of articles from a news site for personal use.
+
+– Rob Raisch
+ Jun 3 '11 at 19:06
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+ -
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+ @dsolimano: Check the website's Terms of Use. Scraping is often prohibited. If you're in doubt, just ask the website's owner. Your particular usage scenario is probably OK. There's a vast difference between obtaining the title of individual pages for personal use, and ripping an entire website for commercial purposes.
+
+– user102937
+ Jun 3 '11 at 22:10
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+ -
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+ @RobertHarvey what is the legal definition of scraping? Are you breaking any law because you visit the site - your browser must scrap the content before it can show it to you!
+
+– Danubian Sailor
+ Sep 4 '13 at 11:08
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+ -
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+ @ŁukaszLech check with the site's Terms of Service. If the TOS allows you to scrape or crawl the site with robots, then it's OK. Most sites do not allow this, however. Get permission from the site owner.
+
+– user102937
+ Sep 4 '13 at 14:19
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+ -
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+ @RobertHarvey what legal meaning have the Terms of Service? Everyone can write anything, but it doesn't mean that everything will have legal power. If someone writes you are not allowed to view the site barefoot, such statement is meaningless.
+
+– Danubian Sailor
+ Sep 4 '13 at 18:30
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+ @ŁukaszLech: You're just trying to get me to say that it's somehow legal, so that you can do whatever you want with the site, against the site owner's wishes. I'm not going to say that. It doesn't have to be illegal to be something that we don't want to promote on Stack Overflow, so stop asking for legal justifications.
+
+– user102937
+ Sep 4 '13 at 18:35
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