Earth

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Earth  

Adjective Terrestrial, Terran, Telluric, Tellurian, Earthly

Epoch J2000.0[note 1]
Aphelion 152,097,701 km
1.0167103335 AU
Perihelion 147,098,074 km
0.9832898912 AU
Semi-major axis 149,597,887.5 km
1.0000001124 AU
Eccentricity 0.016710219
Orbital period 365.256366 days
1.0000175 yr
Average orbital speed 29.783 km/s
107,218 km/h
Inclination 1°34'43.3"[1]
to Invariable plane
Longitude of ascending node 348.73936°
Argument of perihelion 114.20783°
Satellites 1 (the Moon)

Mean radius 6,371.0 km[2]
Equatorial radius 6,378.1 km[3]
Polar radius 6,356.8 km[4]
Flattening 0.0033528[3]
Circumference 40,075.02 km (equatorial)
40,007.86 km (meridional)
40,041.47 km (mean)
Surface area 510,072,000 km²[5][6][note 2]

148,940,000 km² land  (29.2 %)

361,132,000 km² water (70.8 %)
Volume 1.0832073×10¹² km³
Mass 5.9736×10²⁴ kg[7]
Mean density 5.5153 g/cm³
Equatorial surface gravity 9.780327 m/s²[8]
0.99732 g
Escape velocity 11.186 km/s 

Sidereal rotation
period 0.99726968 d[9]
23^h 56^m 4.100^s
Equatorial rotation velocity 1,674.4 km/h (465.1 m/s)
Axial tilt 23.439281°
Albedo 0.367[7]
Surface temp.
   Kelvin
   Celsius min mean max
184 K 287 K 331 K
−89 °C 14 °C 57.7 °C

Surface pressure 101.3 kPa (MSL)
Composition 78.08% Nitrogen (N₂)
20.95% Oxygen (O₂)
0.93% Argon
0.038% Carbon dioxide
About 1% water vapor (varies with climate)[7]

Earth (pronounced /ɝːθ/ (help·info))[10] is the third planet from the Sun. Earth is the largest of the terrestrial planets in the Solar System in diameter, mass and density. It is also referred to as the World and Terra.[note 3]

Home to millions of species,[11] including humans, Earth is the only place in the universe where life is known to exist. The planet formed 4.54 billion years ago,[12][13][14][15] and life appeared on its surface within a billion years. Since then, Earth's biosphere has significantly altered the atmosphere and other abiotic conditions on the planet, enabling the proliferation of aerobic organisms as well as the formation of the ozone layer which, together with Earth's magnetic field, blocks harmful radiation, permitting life on land.[16] The physical properties of the Earth, as well as its geological history and orbit, allowed life to persist during this period. The world is expected to continue supporting life for another 1.5 billion years, after which the rising luminosity of the Sun will eliminate the biosphere.[17]

Earth's outer surface is divided into several rigid segments, or tectonic plates, that gradually migrate across the surface over periods of many millions of years. About 71% of the surface is covered with salt-water oceans, the remainder consisting of continents and islands; liquid water, necessary for all known life, is not known to exist on any other planet's surface.[note 4][note 5] Earth's interior remains active, with a thick layer of relatively solid mantle, a liquid outer core that generates a magnetic field, and a solid iron inner core.

Earth interacts with other objects in outer space, including the Sun and the Moon. At present, Earth orbits the Sun once for every roughly 366.26 times it rotates about its axis. This length of time is a sidereal year, which is equal to 365.26 solar days.[note 6] The Earth's axis of rotation is tilted 23.4° away from the perpendicular to its orbital plane,[18] producing seasonal variations on the planet's surface with a period of one tropical year (365.24 solar days). Earth's only known natural satellite, the Moon, which began orbiting it about 4.53 billion years ago, provides ocean tides, stabilizes the axial tilt and gradually slows the planet's rotation. A cometary bombardment during the early history of the planet played a role in the formation of the oceans.[19] Later, asteroid impacts caused significant changes to the surface environment.

Both the mineral resources of the planet, as well as the products of the biosphere, contribute resources that are used to support a global human population. The inhabitants are grouped into about 200 independent sovereign states, which interact through diplomacy, travel, trade and military action. Human cultures have developed many views of the planet, including personification as a deity, a belief in a flat Earth, and a modern perspective of the world as an integrated environment that requires stewardship. Humans first left the planet in 1961, when Yuri Gagarin reached outer space.

Chronology

See also: Geological history of Earth

Scientists have been able to reconstruct detailed information about the planet's past. About 4.54 billion years ago (within an uncertainty of 1%),[12][13][14][15] the Earth and the other planets in the Solar System formed out of the solar nebula—a disk-shaped mass of dust and gas left over from the formation of the Sun. This assembly of the Earth through accretion was largely completed within 10–20 million years.[20] Initially molten, the outer layer of the planet Earth cooled to form a solid crust when water began accumulating in the atmosphere. The Moon formed soon afterward, possibly as the result of a Mars-sized object (sometimes called Theia) with about 10% of the Earth's mass[21] impacting the Earth in a glancing blow.[22] Some of this object's mass would have merged with the Earth and a portion would have been ejected into space, but enough material would have been sent into orbit to form the Moon.

Outgassing and volcanic activity produced the primordial atmosphere. Condensing water vapor, augmented by ice and liquid water delivered by asteroids and the larger proto-planets, comets, and trans-Neptunian objects produced the oceans.[19] Beginning with almost no dry land, the total amount of surface lying above the oceans has steadily increased. During the past two billion years, for example, the total size of the continents has doubled.[23][24] On time scales lasting hundreds of millions of years, the surface continually reshaped itself as continents formed and broke up. The continents migrated across the surface, occasionally combining to form a supercontinent. Roughly 750 million years ago (mya), one of the earliest known supercontinents, Rodinia, began to break apart. The continents later recombined to form Pannotia, 600–540 mya, then finally Pangaea, which broke apart 180 mya.[25]

Evolution of life

Highly energetic chemistry is believed to have produced a self-replicating molecule around 4 billion years ago, and half a billion years later the last common ancestor of all life existed.[26] The development of photosynthesis allowed the Sun's energy to be harvested directly by life forms; the resultant oxygen accumulated in the atmosphere and formed in a layer of ozone (a form of molecular oxygen [O₃]) in the upper atmosphere. The incorporation of smaller cells within larger ones resulted in the development of complex cells called eukaryotes.[27] 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 the surface of Earth.[28]

Since the 1960s, it has been hypothesized that severe glacial action between 750 and 580 mya, during the Neoproterozoic, covered much of the planet in a sheet of ice. This hypothesis has been termed "Snowball Earth", and is of particular interest because it preceded the Cambrian explosion, when multicellular life forms began to proliferate.[29]