Astronomy
Planetary Science
The Moon

How many times does the Earth's moon revolve around the Earth?

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September 13, 2011 3:07AM

Approximately 13.4 times per year. This is based on the moon's

sidereal orbital period of 27.32 days. The sidereal orbit is the

orbit around earth as it is observed from the distant stars, and

independent of the moon's apparent movement as observed from earth.

The synodic period of revolution is 29.53 days, and this is how

long it takes for the moon to go through one full cycle of moon

phases as observed from earth. There are roughly 12.4 lunar cycles

per year. Because the earth is orbiting the sun at the same time

that the moon is orbiting the earth, the moon has to revolve a

little extra (beyond a full sidereal orbit) in order to get to the

same phase as observed from earth as when the cycle began. In other

words, the moon has to "catch up" with the earth in a sense. This

is why the synodic period is a little longer than the sidereal

period.

Rotations occur about an axis, while revolutions occur when one

astronomical body orbits around another.

Every 29.5 days, for the past, oh, 3 billion years or so. We

don't know precisely when the Moon came into being, but 3 billion

years is a conservative estimate.


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