Expert: James Gort - 5/12/2007

Question
The same side of the moon always faces us, therefore as the moon orbits around the Earth, it must also revolve at the same constant rate to ensure that the same side faces us. If it was even slightly faster of slower then we would see the other side. What is the reason for this? Is it complete coincidence? How is it possible that it reolves at such a perfect rate so that we only ever see one side of it?

Answer
Hi Elliot,

The moon is "locked" into synchronous rotation with the earth, meaning that it always presents the same face towards earth.  That's not entirely true, however, since variations in the moon's orbit (it's not a perfect circle, nor is it directly over the equator) results in about 59% of the moon being seen from earth.  See http://en.wikipedia.org/wiki/Librations

This is not a coincidence, however.  Early in the moon's history, it was rotating faster, and one could see (if any humans were alive then!) the entire surface as it rotated.  However, it's strong gravity produces tidal forces both on the earth and on the moon, slightly distorting the bodies.  As they rotate, this distortion creates friction, which was enough to slow the moon to its current rotation (which actually minimizes the tidal frictional forces).

Hope that helps.

Prof. James Gort