Why does the moon appear to rock back and forth in the sky?

[thetexan]

Why does the moon always have the same side facing earth?

Can it be simply that the rotation of the moon just happens to coincide with it period around the earth. If so, how might that happen? Which requires the axis of rotation to be exactly 90 degrees from the plane of orbit.

tex

 

[jedishrfu]

Its called tidal lock:

https://en.wikipedia.org/wiki/Tidal_locking

 

[Janus]

The Moon does not always have the same side facing the Earth even though its period of rotation matches that of it orbital period due to the tidal locking mentioned. For one thing, its axis of rotation is not at 90 degrees to the plane of its orbit but is 6 degrees from being so. As result, at one parts of its orbit we see a little more of its South Pole while at another we see more of its North pole. The Moon also travels in a slightly elliptical orbit around the Earth and thus its orbital speed slows down and speeds up while Its rotation speed stays constant. So for parts of its orbit the rotation lags behind the orbital motion and for parts it speeds ahead. This causes the Moon to appear rock back a forth a little to side to side over the course of a month. These apparent motions of the Moon as seen from the Earth are called "librations".
This video clip shows how this would appear if we sped things up.
https://en.wikipedia.org/wiki/Libration#/media/File:Lunar_libration_with_phase_Oct_2007_450px.gif