The Earth's only natural satellite. Information obtained by the Voyager missions to Jupiter, Saturn and Uranus has shown that the Moon is typical of the natural satellites in the solar system. It is a barren, heavily cratered world, lacking water or an atmosphere. It was explored at first hand by the American astronauts who travelled there during the Apollo programme landings of 1969-72 and has been extensively mapped from orbiting craft and from the Earth.
Tidal forces have ensured that the same side of the Moon now always faces the Earth, apart from the minor effects of libration. As the Moon travels round the Earth in the course of a month, it undergoes the familiar cycle of phases. The Moon shines only by reflected sunlight; the proportion of the sunlit side visible from Earth depends on the relative alignment of the Sun, Earth and Moon, which changes continuously over the Moon's orbital period.
The terrain on the nearside falls into two basic types: the heavily cratered, light-coloured highlands (or ╘terrae' meaning ╘land'), and the darker, more sparsely cratered maria (╘seas'). The maria have roughly circular outlines, a relic of their formation in the early history of the Moon by the impact of large meteorites. A further type of surface terrain is formed by ejecta. Significant areas are marked by material ejected from the large Imbrium and Orientale basins.
The way the Moon formed is uncertain, but it has existed as a separate body for around 4,500 million years. Early in its life it became hot and molten. As it cooled, the crust formed but it was heavily cratered by the impact of large numbers of meteorites, the largest of which created the mare basins. These subsequently filled with dark basaltic lavas. Significant volcanic activity then ceased, at least 2,000 million years ago.
The farside of the Moon differs from the nearside in that it lacks any large lava-flooded mare areas.
