Earth

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This article is about the planet. For other uses, see Earth (disambiguation).

Earth 

A composite image of Earth produced by NASA.
Orbital characteristics
Epoch J2000.0 [n 1]
Aphelion	152098232 km (1.01671388 AU) [n 2]
Perihelion	147098290 km (0.98329134 AU) [n 2]
Semi-major axis	149598261 km (1.00000261 AU) [1]
Eccentricity	0.01671123[1]
Orbital period	365.256363004 d [2] (1.000017421 yr)
Average orbital speed	29.78 km/s[3] (107200 km/h)
Mean anomaly	269.05 deg[4]
Inclination	7.155 deg to Sun's equator; 1.57869 deg[5] to invariable plane.
Longitude of ascending node	348.73936 deg[3][n 3]
Argument of perihelion	114.20783 deg[3][n 4]
Satellites	One natural satellite; 1070 operational artificial satellites; 21000 pieces of debris over 10 cm  in size (as of 24 October 2013).[6]
Physical characteristics
Mean radius	6371.0 km[7]
Equatorialradius	6378.1 km[8][9]
Polar radius	6356.8 km[10]
Flattening	0.0033528[11] 1/298.257222101 (ETRS89)
Circumference	40075.017 km (equatorial) [9] 40007.86 km (meridional) [12][13]
Surface area	510072000 km2[14][15][n 5]  (148940000 km2 (29.2%) land   361132000 km2 (70.8%) water)
Volume	1.08321×1012 km3[3]
Mass	5.97219×1024 kg[16] (3.0×10-6 Suns)
Mean density	5.514 g/cm3[3]
Surface gravity	9.807 m/s2[17] (1 g)
Moment of inertia factor	0.3307[18]
Escape velocity	11.186 km/s[3]
Sidereal rotation period	0.99726968 d[19] (23h 56m 4.100s)
Equatorial rotation velocity	1,674.4 km/h (465.1 m/s)[20]
Axial tilt	23 deg 26 min 21.4119 s [2]
Albedo	0.367 geometric[3] 0.306 Bond[3]
Surface temp. min mean max Kelvin 184 K[21] 288 K[22] 330 K[23] Celsius −89.2 °C 15 °C 56.7 °C
Atmosphere
Surfacepressure	101.325 kPa (at MSL)
Composition	78.08% nitrogen (N2)[3] (dry air) 20.95% oxygen (O2) 0.930% argon 0.039% carbon dioxide[24] ~ 1% water vapor (climate-variable)

Earth, also known as the world,^[26] Terra,^[28] or Gaia,^[30] is the third planet from the Sun, the densest planet in theSolar System, the largest of the Solar System's four terrestrial planets, and the only celestial body known to accommodate life. Earth's biodiversity has evolved over hundreds of million years, expanding continually except when punctuated by mass extinctions.^[31] It is home to over eight million species.^[32][33] There are over 7.2 billionhumans^[34] who depend upon its biosphere and minerals. Earth's human population is divided among about two hundred independent states that interact through diplomacy, conflict, travel, trade, and media.

According to evidence from sources such as radiometric dating, Earth was formed around four and a half billion years ago. Within its first billion years,^[35] life appeared in its oceans and began to affect its atmosphere and surface, promoting the proliferation of aerobic as well as anaerobic organisms and causing the formation of the atmosphere's ozone layer. This layer and Earth's magnetic field block the most life-threatening parts of the Sun's radiation, so life was able to flourish on land as well as in water.^[36] Since then, Earth's position in the Solar System, its physical properties and its geological history have allowed life to persist.

Earth's lithosphere is divided into several rigid segments, or tectonic plates, that migrate across the surface over periods of many millions of years. 71 percent of Earth's surface is covered with water,^[37] with the remainder consisting of continents and islands that together have many lakes and other sources of water that contribute to the hydrosphere. Earth's poles are mostly covered with ice that includes the solid ice of the Antarctic ice sheet and the sea ice of the polar ice packs. Earth's interior remains active, with a solid iron inner core, a liquid outer corethat generates the magnetic field, and a thick layer of relatively solid mantle.

Earth gravitationally interacts with other objects in space, especially the Sun and the Moon. During one orbit around the Sun, Earth rotates about its own axis 366.26 times, creating 365.26 solar days, or one sidereal year.^{[n 6]} Earth's axis of rotation is tilted 23.4° away from the perpendicular of its orbital plane, producing seasonal variations on the planet's surface with a period of one tropical year (365.24 solar days).^[38] The Moon is Earth's only natural satellite. It began orbiting Earth about 4.53 billion years ago (bya). The Moon's gravitational interaction with Earth stimulates ocean tides, stabilizes the axial tilt, and gradually slows the planet's rotation.