The Moon

The Moon is the only natural satellite of the Earth. It is a spherical rocky body, possibly with a small metal core, orbiting the Earth in a slightly eccentric orbit at a mean distance of near 400,000 kilometres. Its radius is about 1,738 kilometres, and its shape is slightly flattened, with its longest axis along the radial direction from the Earth. The Moon has no global magnetic field like that of the Earth, but some of its surface rocks have remnant magnetism, indicating one or more magnetic episodes in the past. The Moon presently has very slight seismic activity and little heat flow, indications that most internal activities ceased long ago. It is now known that billions of years ago the Moon was subject to violent heating- resulting in a differentiated crust- followed later by volcanic outpourings of lava. Because of the Moon's small size and mass, its surface gravity is only about one-sixth of the Earth's and it retains little atmosphere.