Earth is the third of nine planets in our solar system. Its surface is mostly water (about 70%), and it has a moderately dense nitrogen and oxygen atmosphere that supports life. Rich in iron and nickel, Earth is a dense, molten oblate sphere with a solid core and a thin outer crust. Earth rotates about its polar axis as it revolves around the Sun. Earth has one natural satellite, the Moon. A complete revolution of Earth around the Sun takes about one year, while a rotation on its axis takes one day. The surface of Earth is constantly changing, as tectonic plates slowly move about on the turbulent foundation of partially molten rock beneath them. Collisions between landmasses build mountains; erosion wears them down. Slow changes in the climate cause equally slow changes in vegetation and animals.

Earth orbits the Sun at a distance of about 93,000,000 mi...