Why is the geographical surface of the moon rocking?
It isn’t rocking. You must be looking at a “fast-forward” rendition of the monthly phases of the moon, maybe with a program like Redshift. When you do that you will notice an “apparent” rocking of the moon, but this is an “apparent” motion, not a “true” motion. It happens because of something called east-west “libration” which is difficult to explain briefly. It comes down to this: the moon’s orbit around the earth is an ellipse, which means that the moon speeds up a little and slows down a little once each during every orbit of the earth. But while the orbital velocity changes, the rotational velocity of the moon is just about constant. So the rotation of the moon is either a little ahead or, or lagging behind, the orbital velocity. See link for more.
