The third planet from the Sun. At perihelion each January it is 147 099 590 km from the Sun, as against 152 096 150 km at aphelion in July. Its mean distance is 149 597 870.7 km. Seen from far off in space it has a strong blue coloration. The Earth is slightly ellipsoidal in shape (equatorial diameter 12 756 km, polar diameter 12 714 km). Its age is 4.57 × 10⁹ years.

The Earth’s atmosphere is composed (by volume) of 78% nitrogen, 21% oxygen, and 0.9% argon, plus carbon dioxide (0.04%) and other gases in much smaller quantities. Water vapour is also present in variable amounts, typically 1–2%. White clouds of condensed water vapour may obscure a quarter of the Earth’s surface at a time, belts of cloud being common around the equator and at temperate and polar latitudes. The pressure of the atmosphere at sea level varies around 1000 mbar. The average atmospheric temperature at the surface is 15°C, but ranges from −50°C average in winter in Siberia up to +40°C in the Sahara in summer. The Earth’s atmosphere has five distinct layers: the troposphere (up to 10 or 15 km, depending on latitude and season); the stratosphere (from the tropopause to 50 km); the mesosphere (50–85 km); the thermosphere (85–500 km), which includes the ionosphere; and the exosphere (500 km and above).

Liquid water covers 71% of the Earth’s surface. Volcanism and impact cratering occur; more than 500 volcanoes are presently known to have been active on land over the course of recorded human history. Impact cratering was an important process early in the Earth’s history, but the atmosphere now protects the surface from all but the largest impactors. About 190 impact crater sites have been found on the Earth, but most are old and heavily eroded. The dominant geological process on the Earth’s surface is erosion and deposition by water or ice. Liquid water is also responsible for the development of life, which has itself played an important role in transforming the landscape.

The Earth’s outer layer is the lithosphere, topped by the crust, which together vary in thickness between 70 km in parts of the oceans to 150 km in the thickest parts of the continents. Below this is the mantle, which stretches down to a depth of about 2900 km, where the iron–nickel core begins. This core has allowed the development of a magnetic field, which attains a strength of about 3 × 10^–5 tesla near the equator. Convection within the mantle, coupled with the thin crust, has given rise to plate tectonics and continental drift, creating vast ranges of mountains and the ocean deeps. The Earth has one natural satellite, the Moon.

http://earthobservatory.nasa.gov/Features/BlueMarble/BlueMarble_2002.php