The Moon is the only natural satellite of the Earth. It has a diameter of 2,158.9 mi (3,475.9 km), lies at a mean distance from the Earth of 238,760 mi (384,404 km), and completes one orbit around the Earth in 27.32166 days. The Moon maintains approximately the same side (near side) toward the Earth by synchronously rotating on its axis with respect to the stars in the same average period that it revolves about the Earth. Still, the Moon's orbit (often called the lunar orbit) is constantly changing because of perturbations, with the largest perturbation caused by the gravitational attraction of the Sun. In some ways, the Sun has as much of an influence on the Moon as the Earth does, and because of this some scientists refer to the Earth and Moon as a double-planet system rather than a planet-satellite system.

The mass of the Moon is about 81.3 times less than the...