The earth’s only natural satellite, which orbits the earth at a mean distance of 384 400 km. It has a diameter of 3476 km. It has no atmosphere or surface water. Its surface temperature varies between 80 K (night minimum) and 400 K (noon at the equator). It is the only celestial body outside the earth to have been reached by man (1969). Several theories have been proposed for the origin of the moon. The theory that is currently most widely accepted is that it was formed about 4.5 billion years ago by a collision between the earth and a body about the size of Mars. The rotation of the moon has become locked with its orbital motion around the earth, meaning that only one side of the moon is visible from earth. Other planets in the solar system also have moons.