In the images of the Moon as viewed from Earth, the surface
markings all look the same. Does this mean the Moon does not rotate on
its axis (the way the Earth does)? No, it means that the Moon's
rotation is synchronized to its orbit, so that it rotates exactly once
each orbit. If you look closely at the diagram, you will notice the
markings on the Moons surrounding the Earth (the ones viewed from above
the north pole) do shift, as they must in order to keep the same face
towards the Earth at all times. This synchronization is a consequence
of tidally-induced friction early in the history of the
However, since the spin of the Moon proceeds at a uniform rate
while the orbit is at a slightly nonuniform rate (see below),
the spin can get a little bit ahead or behind the orbit.
Hence the face of the Moon presented to the Earth rocks
back and forth by some 6° each month,
a phenomenon known as longitudinal libration.