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Planet Earth - The third planet from the sun, at an average distance of 149.6 million km (93 million mi). Its diameter at the equator is 12 756 km (7926 mi), slightly less at the Poles. It orbits the sun in 365.26 days, and makes one rotation on its axis every 23.93 hours.

The earth is believed to be about 4,600 million years old. It consists of an inner core of solid iron, surrounded by an outer core of molten iron. Surrounding this is the solid mantle which is separated from the crust by the Mohorovicic discontinuity. The crust consists of basaltic oceanic crust surmounted by less dense granitic continental crust (see basalt; granite). The crust varies in thickness from 5 km under the oceans to 60 km under mountain ranges. The most abundant elements in it are oxygen (47%) and silicon (28%). 70.8% of the earth’s surface is ocean. The greatest ocean depth is over 11 000 m (36 000 ft), in the Marianas Trench; the greatest height of land is 8848 m (29 028 ft), at Mount Everest.