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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.
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