The Moon's Orbit

The Moon moves around the Earth in an elliptical orbit of small eccentricity, inclined by 5 deg 8' 43''.4 to the plane in which the Earth revolves around the Sun. Its distance from the Earth varies between 356,000 and 407,000 km (221,000 and 253,000 miles) in the course of each month; the average distance is 384,400 km (238,900 miles), less than 1% of the distance to Venus and Mars, even at the time of their closest approach. The lunar globe appears in the sky as a disc of a little over half a degree (31' 5''.2) in apparent diameter.

The period in which the Moon completes an orbit around the Earth and returns to the same position in the sky--the sidereal month--is 27 days, 7 h, 43 min, and 11.6 sec. Because the Earth is moving in its orbit around the Sun in the same direction as the Moon, the time needed to return to the same phase--the synodic month--is longer: 29 days, 12 hours, 44 minutes, and 2.8 seconds. This period is the time interval that, for example, elapses between two successive full moons, a period that was known within a second even in ancient times. The Moon's average velocity is 1.023 km/sec (0.635 mi/sec), corresponding to a mean angular velocity in the sky of about 33 minutes of arc per hour, a little greater than the apparent diameter of the Moon.

If you want to see phases, longitudinal libration, latitudinal libration, perigee and apogee all in one film, just open this animation created by John Kielkopf at the University of Louisville. It runs slowly through the first lunation, and then rapidly so you can see the moon rock and roll. It is also available at the site:

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