The Moon is the Earth only natural satellite. It is a barren, heavily cratered world, lacking water or an atmosphere. Tidel forces have ensured that the same side of the Moon now always faces the Earth. 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, 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.
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 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.