The Moon's obirt has an ecentricity of .055, which is over three times eccentircity of the Earth's orbit, and this causes large differences in the distance the Moon is from the Earth during the course of a month. The Moon's average distance is 384,000 km. However, due to the elliptical nature of the orbit the minimu distance is 363,000 km and is called perigee. The maximum distance is called apogee and is 405,000 km. The true diameter of the moon is 3,500 km (1/4 that of Earth's) however as viewed from Earth the size of the Moon appears to change by as much as 11% from perigee and apogee.

In addition to the Earth's gravity the Sun is also exerting a gravitational force on the Moon as well. The Sun's gravity sometimes causes the Moon to speed up or slow down slightly in its orbit. Thus, the Moon's orbital path is constantly being slightly altered by the Sun's gravit. The magnitude of these variations obviously depend on the relative postion of the Earth, Sun and Moon. The end result of these interactions is that both the sideral and synodic months are variable in length. The sideral month (average 27d7h43m11s) can vary up to seven hours. The synodic month (average length = 29d12h44m3s) can vary by as much as 12 hours.