MOON

Internal structure of the moon

The Moon is a natural satellite of the Earth, and takes 27.3 days to travel round its orbit. As the Moon takes the same time to rotate on its own axis, the surface seen from the Earth is always the same. The amount of the surface visible, however, varies according to how much of it is in sun light; this produces the familiar lunar phases. The Moon is barren, with no atmosphere or liquid water, but polar craters are believed to contain small quantities of ice. Its dusty surface has been cratered by meteorites; some of the larger craters have filled with lave to form dark areas known as maria or mares('seas).