Earth

Earth
	A photograph of Earth taken by the crew of Apollo 17 in 1972. A processed version became widely known as The Blue Marble.
Designations
Alternative names	Gaia, Terra, Tellus, the world, the globe
Adjectives	Earthly, terrestrial, terran, tellurian
Orbital characteristics
	Epoch J2000
Aphelion	152100000 km (94500000 mi)
Perihelion	147095000 km (91401000 mi)
Semi-major axis	149598023 km (92955902 mi)
Eccentricity	0.0167086
Orbital period (sidereal)	365.256363004 d; (1.00001742096 aj)
Average orbital speed	29.78 km/s; (107200 km/h; 66600 mph)
Mean anomaly	358.617°
Inclination	7.155° to the Sun's equator;; 1.57869° to invariable plane;; 0.00005° to J2000 ecliptic;
Longitude of ascending node	−11.26064° to J2000 ecliptic
Time of perihelion	2022-Jan-04
Argument of perihelion	114.20783°
Satellites	1 natural satellite: the Moon; 5 quasi-satellites; >4 500 operational artificial satellites; >18 000 tracked space debris;
Physical characteristics
Mean radius	6371.0 km (3958.8 mi)
Equatorial radius	6378.137 km (3963.191 mi)
Polar radius	6356.752 km (3949.903 mi)
Flattening	1/298.257222101 (ETRS89)
Circumference	40075.017 km equatorial (24901.461 mi); 40007.86 km meridional (24859.73 mi);
Surface area	510072000 km2 (196940000 sq mi); 148940000 km2 land (57510000 sq mi); 361132000 km2 ocean (139434000 sq mi);
Volume	1.08321×1012 km3 (2.59876×1011 cu mi)
Mass	5.97237×1024 kg (1.31668×1025 lb); (3.0×10−6 M☉)
Mean density	5.514 g/cm3 (0.1992 lb/cu in)
Surface gravity	9.80665 m/s2 (1 g; 32.1740 ft/s2)
Moment of inertia factor	0.3307
Escape velocity	11.186 km/s (40270 km/h; 25020 mph)
Synodic rotation period	1.0 d; (24h 00m 00s)
Sidereal rotation period	0.99726968 d; (23h 56m 4.100s)
Equatorial rotation velocity	0.4651 km/s; (1674.4 km/h; 1040.4 mph)
Axial tilt	23.4392811°
Albedo	0.367 geometric; 0.306 Bond;
Surface equivalent dose rate	0.274 μSv/h
Atmosphere
Surface pressure	101.325 kPa (at MSL)
Composition by volume	78.08% nitrogen (N2; dry air); 20.95% oxygen (O2); ~ 1% water vapor (climate variable); 0.9340% argon; 0.0413% carbon dioxide; 0.00182% neon; 0.00052% helium; 0.00019% methane; 0.00011% krypton; 0.00006% hydrogen;

Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large amounts of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surface is made up of the ocean, dwarfing Earth's polar ice, lakes, and rivers. The remaining 29% of Earth's surface is land, consisting of continents and islands. Earth's surface layer is formed of several slowly moving tectonic plates, interacting to produce mountain ranges, volcanoes, and earthquakes. Earth's liquid outer core generates the magnetic field that shapes Earth's magnetosphere, deflecting destructive solar winds.

Earth's atmosphere consists mostly of nitrogen and oxygen. More solar energy is received by tropical regions than polar regions and is redistributed by atmospheric and ocean circulation. Water vapor is widely present in the atmosphere and forms clouds that cover most of the planet. Greenhouse gases in the atmosphere like carbon dioxide (CO₂) trap a part of the energy from the Sun close to the surface. A region's climate is governed by latitude, but also by elevation and proximity to moderating oceans. Severe weather, such as tropical cyclones, thunderstorms, and heatwaves, occurs in most areas and greatly impacts life.

Earth is an ellipsoid with a circumference of about 40,000 km. It is the densest planet in the Solar System. Of the four rocky planets, it is the largest and most massive. Earth is about eight light minutes away from the Sun and orbits it, taking a year (about 365.25 days) to complete one revolution. Earth rotates around its own axis in just less than a day (in about 23 hours and 56 minutes). Earth's axis of rotation is tilted with respect to the perpendicular to its orbital plane around the Sun, producing seasons. Earth is orbited by one permanent natural satellite, the Moon, which orbits Earth at 380,000 km (1.3 light seconds) and is roughly a quarter as wide as Earth. The Moon always faces the Earth with the same side through tidal locking and causes tides, stabilizes Earth's axis, and gradually slows its rotation.

Earth formed over 4.5 billion years ago. During the first billion years of Earth's history, the ocean formed and then life developed within it. Life spread globally and began to affect Earth's atmosphere and surface, leading to Earth's Great Oxidation Event two billion years ago. Humans emerged 300,000 years ago, and have reached a population of almost 8 billion today. Humans depend on Earth's biosphere and natural resources for their survival, but have increasingly impacted Earth's environment. Today, humanity's impact on Earth's climate, soils, waters, and ecosystems is unsustainable, threatening people's lives and causing widespread extinction of other life.

Etymology

The modern English word Earth developed, via Middle English, from an Old English noun most often spelled eorðe.^[26] It has cognates in every Germanic language, and their ancestral root has been reconstructed as *erþō. In its earliest attestation, the word eorðe was already being used to translate the many senses of Latin terra and Greek γῆ gē: the ground, its soil, dry land, the human world, the surface of the world (including the sea), and the globe itself. As with Roman Terra/Tellūs and Greek Gaia, Earth may have been a personified goddess in Germanic paganism: late Norse mythology included Jörð ('Earth'), a giantess often given as the mother of Thor.^[27]

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.^[26] 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 (for example, "Earth's atmosphere") but writes it in lowercase when preceded by the (for example, "the atmosphere of the earth"). It almost always appears in lowercase in colloquial expressions such as "what on earth are you doing?"^[28]

Occasionally, the name Terra /ˈtɛrə/ is used in scientific writing and especially in science fiction to distinguish humanity's inhabited planet from others,^[29] while in poetry Tellus /ˈtɛləs/ has been used to denote personification of the Earth.^[30] Terra is also the name of the planet in some Romance languages (languages that evolved from Latin) like Italian and Portuguese, while in other Romance languages the word gave rise to names with slightly altered spellings (like the Spanish Tierra and the French Terre). The Latinate form Gæa or Gaea (English: /ˈdʒiːə/) of the Greek poetic name Gaia (Γαῖα; Ancient Greek: [ɡâi̯.a] or [ɡâj.ja]) 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 English /ˈɡeɪə/.^[31]

There are a number of adjectives for the planet Earth. From Earth itself comes earthly. From the Latin Terra comes terran /ˈtɛrən/,^[32] terrestrial /təˈrɛstriəl/,^[33] and (via French) terrene /təˈriːn/,^[34] and from the Latin Tellus comes tellurian /tɛˈlʊəriən/^[35] and telluric.^[36]

Chronology

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Earliest multicellular life

Earliest apes / humans

Chemical reactions led to the first self-replicating molecules about four billion years ago. A half billion years later, the last common ancestor of all current life arose.^[61] The evolution of photosynthesis allowed the Sun's energy to be harvested directly by life forms. The resultant molecular oxygen (O2) accumulated in the atmosphere and due to interaction with ultraviolet solar radiation, formed a protective ozone layer (O3) in the upper atmosphere.^[62] The incorporation of smaller cells within larger ones resulted in the development of complex cells called eukaryotes.^[63] True multicellular organisms formed as cells within colonies became increasingly specialized. Aided by the absorption of harmful ultraviolet radiation by the ozone layer, life colonized Earth's surface.^[64] Among the earliest fossil evidence for life is microbial mat fossils found in 3.48 billion-year-old sandstone in Western Australia,^[65] biogenic graphite found in 3.7 billion-year-old metasedimentary rocks in Western Greenland,^[66] and remains of biotic material found in 4.1 billion-year-old rocks in Western Australia.^[67]^[68] The earliest direct evidence of life on Earth is contained in 3.45 billion-year-old Australian rocks showing fossils of microorganisms.^[69]^[70]

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.^[71]^[72] Following the Cambrian explosion, 535 Ma, there have been at least five major mass extinctions and many minor ones.^[73]^[74] 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.^[75] 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.^[76]

Future

Because carbon dioxide (CO2) has a long lifespan in the atmosphere, moderate human CO2 emissions may postpone the next glacial inception by 100,000 years.^[77]

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%.^[78] Earth's increasing surface temperature will accelerate the inorganic carbon cycle, reducing CO2 concentration to levels lethally low for plants (10 ppm for C4 photosynthesis) in approximately 100–900 million years.^[79]^[80] The lack of vegetation will result in the loss of oxygen in the atmosphere, making animal life impossible.^[81] Due to the increased luminosity, Earth's mean temperature may reach 100 °C (212 °F) in 1.5 billion years, and all ocean water will evaporate and be lost to space, which may trigger a runaway greenhouse effect, within an estimated 1.6 to 3 billion years.^[82] 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.^[82]^[83]

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.^[78]^[84] 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, otherwise, with tidal effects, it may enter the Sun's atmosphere and be vaporized.^[78]

Physical characteristics

Size and shape

Earth topological map, the area is redder if it is raised higher in real-life

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.^[85] Therefore, 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.^[86]

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),^[87] whereas Mount Everest (8,848 metres or 29,029 feet above local sea level) represents a deviation of 0.14%.^{[n 6]}^[89] 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).^[90]^[91]^[92]

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 (MSL).^[93] Sea surface topography are water deviations from MSL, analogous to land topography.

Internal structure

Geologic layers of Earth^[94]
Illustration of Earth's cutaway, not to scale
Depth^[95] (km)	Component layer name	Density (g/cm³)
0–60	Lithosphere^{[n 7]}	—
0–35	Crust^{[n 8]}	2.2–2.9
35–660	Upper mantle	3.4–4.4
660-2890	Lower mantle	3.4–5.6
100–700	Asthenosphere	—
2890–5100	Outer core	9.9–12.2
5100–6378	Inner core	12.8–13.1

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.^[96] 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.^[97]

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.^[98] 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.^[99] 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.

Chemical composition

Earth's mass is approximately 5.97×10²⁴ kg (5,970 Yg). It is composed mostly of iron (32.1%), oxygen (30.1%), silicon (15.1%), magnesium (13.9%), sulfur (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%), sulfur (4.5%), and less than 1% trace elements.^[100]

The most common rock constituents of the crust are nearly all oxides: chlorine, sulfur, 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.^[101]^[100]

Heat

Major heat-producing isotopes in present-day Earth^[102]
Isotope	Concentration inside mantle	Heat release in gigawatts
²³²Th	0.0000124%	13113
²³⁸U	0.00000308%	11669
⁴⁰K	0.00000369%	4331
²³⁵U	0.000000022%	501

The major heat-producing isotopes within Earth are potassium-40, uranium-238, and thorium-232.^[103] At the center, the temperature may be up to 6,000 °C (10,830 °F),^[104] and the pressure could reach 360 GPa (52 million psi).^[105] 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.^[102]^[106]

The mean heat loss from Earth is 87 mW m⁻², for a global heat loss of 4.42×10¹³ W.^[107] 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.^[108] 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.^[109]

Tectonic plates

Earth's major plates, which are:^[110]

Pacific Plate

African Plate^{[n 9]}

North American Plate

Eurasian Plate

Antarctic Plate

Indo-Australian Plate

South American Plate

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.^[111] 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.^[112]

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.^[113]^[114] By comparison, the oldest dated continental crust is 4,030 Ma,^[115] 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.^[50]

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)^[116] 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).^[117]

Surface

Satellite picture of Upsala Glacier, showing mountains, icebergs, lakes, and clouds

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.^[118] Below the ocean's surface are much of the continental shelf, mountains, volcanoes,^[86] 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).^[119]

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.^[120] 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.^[121] 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.^[122] Common carbonate minerals include calcite (found in limestone) and dolomite.^[123]

Erosion and tectonics, volcanic eruptions, flooding, weathering, glaciation, the growth of coral reefs, and meteorite impacts are among the processes that constantly reshape Earth's surface over geological time.^[124]^[125] 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.^[126]^[127] 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.^[128]

Gravitational field

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.^[129]

Magnetic field

Schematic of Earth's magnetosphere, with the solar wind flows from left to right

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 3.05×10⁻⁵ T, with a magnetic dipole moment of 7.79×10²² Am² at epoch 2000, decreasing nearly 6% per century.^[130] 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.^[131]^[132]

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.^[133] 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.^[134] Charged particles are contained within the magnetosphere; the plasmasphere is defined by low-energy particles that essentially follow magnetic field lines as Earth rotates.^[135]^[136] 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,^[137] and the Van Allen radiation belts are formed by high-energy particles whose motion is essentially random, but contained in the magnetosphere.^[138]^[139]

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.^[140]

Orbit and rotation

Rotation

Earth's rotation imaged by Deep Space Climate Observatory, showing axis tilt

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).^[141] 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.^[142]^[143]

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 23^h 56^m 4.0989^s.^[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) (23^h 56^m 4.0905^s).^[4] Thus the sidereal day is shorter than the stellar day by about 8.4 ms.^[144]

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.^[145]^[146]

Orbit

Illustration of the Earth, Earth's orbit, the Sun and the four seasons

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 Sun and Earth's north poles, 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]^[147]

The Hill sphere, or the sphere of gravitational influence, of Earth is about 1.5 million km (930,000 mi) in radius.^[148]^{[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.^[148] 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.^[149]

Axial tilt and seasons

Earth's axial tilt and its relation to the rotation axis and planes of orbit

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 in the Southern Hemisphere when the Tropic of Capricorn faces the Sun. In each instance, winter occurs simultaneously in the opposite hemisphere. 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.^[150] 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.^[151]^[152]

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.^[153]

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.^[154] 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.^[155]

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.^[156]^{[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.^[157]

Earth-Moon system

Moon

Characteristics

Diameter	3,474.8 km
Mass	7.349×10²² kg
Semi-major axis	384,400 km
Orbital period	27^d 7^h 43.7^m

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 planet Pluto.^[158]^[159] The natural satellites of other planets are also referred to as "moons", after Earth's.^[160] 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.^[42]

The gravitational attraction between Earth and the Moon causes tides on Earth.^[161] 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.^[162] As the Moon orbits Earth, different parts of its face are illuminated by the Sun, leading to the lunar phases.^[163] 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.^[164] 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.^[165]

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.^[166] 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.^[167]^[168]

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.^[146] This allows total and annular solar eclipses to occur on Earth.^[169]

Asteroids and artificial satellites

A model of Vanguard 1, the oldest human-made object in Earth orbit

Earth's co-orbital asteroids population consists of quasi-satellites, objects with a horseshoe orbit and trojans. There are at least five quasi-satellites, including 469219 Kamoʻoalewa.^[170]^[171] A trojan asteroid companion, 2010 TK7, is librating around the leading Lagrange triangular point, L4, in Earth's orbit around the Sun.^[172]^[173] The tiny near-Earth asteroid 2006 RH120 makes close approaches to the Earth–Moon system roughly every twenty years. During these approaches, it can orbit Earth for brief periods of time.^[174]

As of September 2021, there are 4,550 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.^[175]

Hydrosphere

Water is transported to various parts of the hydrosphere via the water cycle

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 1.35×10¹⁸ metric 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).^[176] 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).^[177] About 97.5% of the water is saline; the remaining 2.5% is fresh water.^[178]^[179] Most fresh water, about 68.7%, is present as ice in ice caps and glaciers.^[180]

In Earth's coldest regions, snow survives over the summer and changes into ice. This accumulated snow and ice 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.^[181] 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.^[182]

The average salinity of Earth's oceans is about 35 grams of salt per kilogram of seawater (3.5% salt).^[183] Most of this salt was released from volcanic activity or extracted from cool igneous rocks.^[184] The oceans are also a reservoir of dissolved atmospheric gases, which are essential for the survival of many aquatic life forms.^[185] Sea water has an important influence on the world's climate, with the oceans acting as a large heat reservoir.^[186] Shifts in the oceanic temperature distribution can cause significant weather shifts, such as the El Niño–Southern Oscillation.^[187]

The abundance of water on Earth's surface is a unique feature that distinguishes it from other planets in the Solar System. Solar System planets with considerable atmospheres do partly host atmospheric water vapor, but they lack surface conditions for stable surface water.^[188] Despite some moons showing signs of large reservoirs of extraterrestrial liquid water, with possibly even more volume than Earth's ocean, all of them are large bodies of water under a kilometers thick frozen surface layer.^[189]

Atmosphere

The atmospheric pressure at Earth's sea level averages 101.325 kPa (14.696 psi),^[190] 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.^[190] Water vapor content varies between 0.01% and 4%^[190] 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.^[191]

Earth's biosphere has significantly altered its atmosphere. Oxygenic photosynthesis evolved 2.7 Gya, forming the primarily nitrogen–oxygen atmosphere of today.^[62] This change enabled the proliferation of aerobic organisms and, indirectly, the formation of the ozone layer due to the subsequent conversion of atmospheric O2 into O3. The ozone layer blocks ultraviolet solar radiation, permitting life on land.^[192] 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.^[193] 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),^[194] and life on Earth probably would not exist in its current form.^[195]

Weather and climate

Hurricane Felix seen from low Earth orbit, September 2007

Massive clouds above the Mojave Desert, February 2016

Earth's atmosphere has no definite boundary, gradually becoming thinner and fading into outer space.^[196] 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.^[197] 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.^[198]

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°.^[199] Ocean heat content and currents are also important factors in determining climate, particularly the thermohaline circulation that distributes thermal energy from the equatorial oceans to the polar regions.^[200]

Earth receives 1361 W/m² of solar irradiance.^[201]^[202] The amount of solar energy that reaches the 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.^[203] 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.^[204]

Further factors that affect a location's climates are its proximity to oceans, the oceanic and atmospheric circulation, and topology.^[205] Places close to oceans typically have colder summers and warmer winters, due to the fact that oceans can store large amounts of heat. The wind transports the cold or the heat of the ocean to the land.^[206] 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.^[207] Finally, temperatures decrease with height causing mountainous areas to be colder than low-lying areas.^[208]

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.^[198] 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.^[209]

The commonly used Köppen climate classification system has five broad groups (humid tropics, arid, humid middle latitudes, continental and cold polar), which are further divided into more specific subtypes.^[199] The Köppen system rates regions based on observed temperature and precipitation.^[210] Surface air temperature can rise to around 55 °C (131 °F) in hot deserts, such as Death Valley, and can fall as low as −89 °C (−128 °F) in Antarctica.^[211]^[212]

Upper atmosphere

Top of Earth's blue-tinted atmosphere, with the Moon at the background

Above the troposphere, the atmosphere is usually divided into the stratosphere, mesosphere, and thermosphere.^[193] 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.^[213] 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 (62 mi) above Earth's surface, is a working definition for the boundary between the atmosphere and outer space.^[214]

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.^[215] 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.^[216] Hence the ability of hydrogen to escape from the atmosphere may have influenced the nature of life that developed on Earth.^[217] 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.^[218]

Life on Earth

Fungi are one of the kingdoms of life on Earth.

A planet's life forms inhabit ecosystems, whose total forms the biosphere.^[219] The biosphere is divided into a number of biomes, inhabited by broadly similar plants and animals.^[220] On land, biomes are separated primarily by differences in latitude, height above sea level and humidity. Terrestrial biomes lying within the Arctic or Antarctic Circles, at high altitudes or in extremely arid areas are relatively barren of plant and animal life; species diversity reaches a peak in humid lowlands at equatorial latitudes.^[221] Estimates of the number of species on Earth today vary; most species have not been described.^[222] Over 99% of all species that ever lived on Earth are extinct.^[223]^[224]

A planet that can sustain life is termed habitable, even if life did not originate there. 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.^[225] Earth provides liquid water—an environment where complex organic molecules can assemble and interact, and sufficient energy to sustain metabolism.^[226] Plants can take up nutrients from the atmosphere, soils and water. These nutrients are constantly recycled between different species.^[227]

Extreme weather, such as tropical cyclones (including hurricanes and typhoons), occurs over most of Earth's surface and has a large impact on life in those areas. From 1980 to 2000, these events caused an average of 11,800 human deaths per year.^[228] Many places are subject to earthquakes, landslides, tsunamis, volcanic eruptions, tornadoes, blizzards, floods, droughts, wildfires, and other calamities and disasters.^[229] Human impact is felt in many areas due to pollution of the air and water, acid rain, loss of vegetation (overgrazing, deforestation, desertification), loss of wildlife, species extinction, soil degradation, soil depletion and erosion.^[230] Human activities release greenhouse gases into the atmosphere which cause global warming.^[231] This is driving changes such as the melting of glaciers and ice sheets, a global rise in average sea levels, increased risk of drought and wildfires, and migration of species to colder areas.^[232]

Human geography

The seven continents of Earth:^[233]

Earth's human population passed seven billion in the early 2010s,^[234] and is projected to peak at around ten billion in the second half of the 21st century.^[235] Most of the growth is expected to take place in sub-Saharan Africa.^[235] 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.^[236] The Northern Hemisphere contains 68% of the world's land mass.^[237] Partly due to the predominance of land mass, 90% of humans live in the Northern Hemisphere.^[238]

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%),^[239] high mountains (27%),^[240] or other unsuitable terrains. Humans have developed diverse societies and cultures; politically, the world has about 206 sovereign states.^[241] States claim the planet's entire land surface, except for parts of Antarctica and a few other unclaimed areas.^[242] Earth has never had a planetwide government, but the United Nations is the leading worldwide intergovernmental organization.^[243]

The first human to orbit Earth was Yuri Gagarin on 12 April 1961.^[244] In total, about 550 people have visited outer space and reached orbit as of November 2018, and, of these, twelve have walked on the Moon.^[245]^[246] 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.^[247] The farthest that humans have traveled from Earth is 400,171 km (248,655 mi), achieved during the Apollo 13 mission in 1970.^[248]

Natural resources and land use

Land use in 2015 as a percentage of ice-free land surface^[249]
Land use	Percentage
Cropland	12–14%
Pastures	30–47%
Human-used forests	16–27%
Infrastructure	1%
Unused land	24–31%

Earth has resources that have been exploited by humans.^[250] Those termed non-renewable resources, such as fossil fuels, are only replenished over geological timescales.^[251] Large deposits of fossil fuels are obtained from Earth's crust, consisting of coal, petroleum, and natural gas.^[252] These deposits are used by humans both for energy production and as feedstock for chemical production.^[253] 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.^[254] These metals and other elements are extracted by mining, a process which often brings environmental and health damage.^[255]

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.^[256] 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.^[257] Of the 12–14% of ice-free land that is used for croplands, 2 percentage points were irrigated in 2015.^[249] Humans use building materials to construct shelters.^[258]

Humans and environment

Human activities have impacted Earth's environments. Through activities such as the burning of fossil fuels, humans have been increasing the amount of greenhouse gases in the atmosphere, altering Earth's energy budget and climate.^[259]^[260] It is estimated that global temperatures in the year 2020 were 1.2 °C (2.2 °F) warmer than the preindustrial baseline.^[261] This increase in temperature, known as global warming, has contributed to the melting of glaciers, rising sea levels, increased risk of drought and wildfires, and migration of species to colder areas.^[232]

The concept of planetary boundaries was introduced to quantify humanity's impact on Earth. Of the nine identified boundaries, five have been crossed: Biosphere integrity, climate change, chemical pollution, destruction of wild habitats and the nitrogen cycle are thought to have passed the safe threshold.^[262]^[263] As of 2018, no country meets the basic needs of its population without transgressing planetary boundaries. It is though possible to provide all basic physical needs globally within sustainable levels of resource use.^[264]

Cultural and historical viewpoint

Earthrise, taken in 1968 by William Anders, an astronaut on board Apollo 8

Human cultures have developed many views of the planet.^[265] The standard astronomical symbol of Earth consists of a cross circumscribed by a circle, ,^[266] representing the four corners of the world. (See also Earth symbol.) Earth is sometimes personified as a deity. In many cultures it is a mother goddess that is also the primary fertility deity.^[267] Creation myths in many religions involve the creation of Earth by a supernatural deity or deities.^[267] The Gaia hypothesis, developed in the mid-20th century, compared Earth's environments and life as a single self-regulating organism leading to broad stabilization of the conditions of habitability.^[268]^[269]^[270] 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, called the overview effect, emphasizing its beauty, uniqueness and apparent fragility.^[271]^[272] In particular, this caused a realization of the scope of effects from human activity on Earth's environment. Enabled by science, particularly Earth observation,^[273] humans have started to take action on environmental issues globally,^[274] acknowledging the impact of humans and the interconnectedness of Earth's environments.

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.^[275]^[276] Earth was generally believed to be the center of the universe until the 16th century, when scientists first concluded that it was a moving object, one of the planets of the Solar System.^[277]

It was only during the 19th century that geologists realized Earth's age was at least many millions of years.^[278] 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.^[279]^[280]

Notes

References

External links

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[epoch-3] 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.

[apsis-4] Jump up to:^a ^b aphelion = 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 (2009). Probing the New Solar System. CSIRO Publishing. p. 144. ISBN 978-0-643-09949-4.

[space_debris-11] Jump up to:^a ^b As 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.

[circ-18] Earth's circumference is almost exactly 40,000 km because the meter was calibrated on this measurement—more specifically, 1/10-millionth of the distance between the poles and the equator.

[surfacecover-20] 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 Landcover Archived 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.

[94] 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.^[88]

[102] Locally varies between 5 and 200 km.

[103] Locally varies between 5 and 70 km.

[jaes41_3_379-119] Including the Somali Plate, which is being formed out of the African Plate. See: Chorowicz, Jean (October 2005). "The East African rift system". Journal of African Earth Sciences. 43 (1–3): 379–410. Bibcode:2005JAfES..43..379C. doi:10.1016/j.jafrearsci.2005.07.019.

[Aoki-153] 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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[Allen296-24] Allen, Clabon Walter; Cox, Arthur N. (2000). Allen's Astrophysical Quantities. Springer. p. 296. ISBN 978-0-387-98746-0. Retrieved 17 August 2010.

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[27] Jones, P. D.; Harpham, C. (2013). "Estimation of the absolute surface air temperature of the Earth". Journal of Geophysical Research: Atmospheres. 118 (8): 3213–3217. Bibcode:2013JGRD..118.3213J. doi:10.1002/jgrd.50359. ISSN 2169-8996.

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[35] "Tellus". Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)

[36] "Gaia". Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)

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[age_earth1-43] See:
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