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Moon Motion Page
(Several of the links on this page go off the Science Rocks! web site. Use your browser "back" button to return here) |
| The orbit of the Moon is nearly circular and is about 384,000 km from the Earth. The plane of the orbit is tilted about 5 degrees with respect to the plane of the Earth's orbit around the Sun (the plane of the Ecliptic). |
Motion of the MoonThe Moon appears to move to the same location compared to a background of stars once in about 27.3 days as observed from the Earth (around the celestial sphere). This is called a sidereal month, and has an orbital period of 27.3 days. This is the time it takes the moon to make one "trip" around the Earth compared to a background of stars. |
| The Moon has a rotational period of 27.3 days that exactly matches the time it takes for the moon to complete one "trip" around the Earth (sidereal period for revolution). Gravitational attraction between the Moon and the Earth has slowed the orbit of the moon and the rotation of both the Moon and the Earth. This is called Tidal Locking. Because of this tidal locking of the periods for revolution and rotation, the Moon always keeps essentially the same face turned toward the Earth (small fluctuation mean that over a period of time we can actually see about 55% of the Lunar surface from the Earth). |
| The moon takes 29.5 days to return to the same point compared to the Sun. Since the Earth moves around the Sun, the time it takes the moon to return to the same position compared to the sun is a little longer than it takes to just make one "trip" around the Earth; this is called a synodic month (Lunar phases as observed from the Earth occur in one synodic month - the time from one full moon to the next). Many cultures have used the phase cycle of the moon as a basis for time keeping, called a Lunar Calendar. It is in fact the source of the word "MONTH" or "MOONTH"! The Islamic Calendar and the Chinese Calendar still use the Lunar Cycle. |