| Term: | Earth (planet) |
| Definition: |
The third planet from the Sun. At perihelion, it is 147,099,590 km from the Sun, and at aphelion it is 152,096.150 km, whereas its mean distance from the Sun (astronomical unit) is 149,598 x 10^6 km. Its orbital period is 365.2563 days (sidereal year) and its eccentricity 0.017. Other characteristics: axial inclination 23.44°; rotation period 23.934 h (sidereal day); mean density 5.52 g/cm^3; mass 5.974 x 10^24 kg; escape velocity 11.18 km/s; average albedo 0.37. The Earth's atmosphere consists of 78% nitrogen, 21% oxygen and 0.9% argon, plus carbon dioxide, hydrogen, and other gases in much smaller quantities. The atmospheric pressure at sea level is about 1,000 mbar. The surface average temperature is 15° C, but it varies, on the average, between -50° C (winter, Siberia) and up to + 40° C (summer, Sahara). Liquid water covers 71% of the surface. Over 5,000 active volcanoes have been registered throughout man's known history. The outer layer of the planet, the lithosphere, is covered with the crust. In the upper mantle and beneath the lithosphere, lies asthenosphere. Convection in the asthenosphere has caused plate tectonics motions and continent drifts. The densest layer of the Earth is its core, about 3000 km to 6400 km beneath the surface, consisting primarily of iron and nickel. This core is believed to be at the origin of the magnetic field, which reaches about 3 x 10^-5 tesla near the equator. It has only one natural satellite, the Moon. |