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The moon's synodic period of orbit around the earth is 29.53 days, roughly. This is the length of time it takes for the moon to reach the same position relative to the sun as the starting position. So the synodic period reflects the cycle of moon phases. Each full cycle of moon phases takes about 29.53 days to complete. The synodic period covers more than 360 degrees of orbit, because the earth is orbiting the sun at the same time that the moon is orbiting the earth. This means that the moon has to do a little "catching up" each month to reach the same position relative to the sun. The moon's sidereal period of orbit, the true 360 degree orbit as it would be reckoned from the point of view of the distant stars, is 27.32 days. The strong apparent illogic in having 2 orbital periods (there are actually several other kinds of orbit) for the moon comes from the fact that we can clearly observe the moon's phases, and it is perfectly sensible to think of the moon's phases as resulting from the moon's orbit. The sidereal orbit on the other hand doesn't result in anything as dramatic as different phases, or anything else that can be easily observed and identified by casual observers.

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16y ago

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