Famous "Blue Marble" photograph of Earth, taken from Apollo 17. Orbital characteristics Epoch J2000 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 Avg. orbital speed: 29.783 km/s 107,218 km/h Inclination: Reference (0) 7.25° to Sun's equator Longitude of ascending node: 348.73936° Argument of perihelion: 114.20783° Satellites: 1 (the Moon) Physical characteristics Mean radius: 6,371.0 km [2] Equatorial radius: 6,378.1 km [1] Polar radius: 6,356.8 km [1] Flattening: 0.0033528 [1] Circumference: 40,075.02 km (equatorial) 40,007.86 km (meridional) 40,041.47 km (mean) Surface area: 510,072,000 km?[3] 148,940,000 km? land  (29.2 %) 361,132,000 km? water (70.8 %) Volume: 1.0832073?1012 km? Mass: 5.9736?1024 kg Mean density: 5,515.3 kg/m? Equatorial surface gravity: 9.780327 m/s?[4] 0.99732 g Escape velocity: 11.186 km/s  40,270 km/h Sidereal rotation period: 0.997258 d 23h 56m 04.09054s[4] Rotation velocity at equator: 465.11 m/s Axial tilt: 23.439281° Albedo: 0.367 Surface temp.:    Kelvin    Celsius min mean max 185 K 287 K 331 K -88.3 °C 14 °C 57.7 °C Adjectives: Terrestrial, Terran, Telluric, Tellurian, Earthly Atmosphere Surface pressure: 101.3 kPa (MSL) Composition: 78.08% Nitrogen (N2) 20.95% Oxygen (O2) 0.93% Argon 0.038% Carbon dioxide Trace water vapor (varies with climate) Earth (pronounced /'?r?/) is the third planet from the Sun and is the largest of the terrestrial planets in the Solar System, in both diameter and mass. It is also referred to as the Earth, Planet Earth, Gaia, Terra,[5] and "the World". Home to millions of species[6] including humans, Earth is the only place in the universe where life is known to exist. Scientific evidence indicates that the planet formed 4.54 billion years[7][8][9][10] ago, 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. 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.[11][12] 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.[13] The Earth's axis of rotation is tilted 23.4°[14] away from the perpendicular to its orbital plane, 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.[15] Later, asteroid impacts caused significant changes to the surface environment. Long term periodic changes in the Earth's orbit, caused by the gravitational influence of other planets, are believed to have given rise to the ice ages that have intermittently covered significant portions of Earth's surface in glacial sheets.