Earth

the Earth is one of a group of nine planets which orbit a star that we call the sun. It occurs in one small part of the galaxy (also called the milky way) which is referred to as the solar system. The Earth is the fifth largest of the planets and the third nearest to the Sun. Most of the Earth's surface is covered by the oceans -- about two-thirds compared to one third which is land. A layer of air, the atmosphere, surrounds the Earth which is sub-divided into a number of different regions according to height above the surface. The Earth, as viewed from space, is like a blue ball or sphere which is flattened at both the poles. At the equator, the radius of the Earth is 6378 km or 3963 miles.

The Earth itself is composed of four main layers; an outer crust overlying a mantle, followed by an outer core and inner core. The rocky crust varies in thickness, being about 40 km thick under the continents and 8 km under the oceans. It 'floats' on the mantle which is made up of extremely hot rock and is about 2900 km thick. The core is so hot that it is part liquid, with a temperature in the region of 6500 km. The outer core is believed to be about 2250 km thick and is thought to be more liquid than the inner core which is solid and composed mainly of iron and nickel.

Taken from Dictionary of Science