StarChild Question of the Month for April 2001

Question:

What is the orbital period of the Moon?

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Answer:

There are two periods involved with the orbit of the Moon around the Earth. This often leads to some confusion, but can be easily understood. Let's investigate!

Sidereal Period versus Synodic Period

So why are the sidereal and synodic lunar months not equal in length?

Remember that the Earth moves in its own orbit around the Sun. The synodic period is related to the lunar phases; it depends on the relative locations of the Sun-Earth-Moon. If we start measuring at Full Moon, then one sidereal month later we will not yet be back to a Full Moon, since the Moon must travel further in its orbit around the Earth to reach the same relative Sun-Earth-Moon alignment...all because during the 27.3 days of the sidereal month, the Earth moved along in its orbit around the Sun and now the Moon must "catch up" to this new position. It takes it just over 2 days to do so.

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