libration

In astronomy, slight apparent wobble in the rotation of the Moon due to its variable speed of rotation and the tilt of its axis.

Generally, the Moon rotates on its axis in the same time as it takes to complete one orbit, causing it to keep one face turned permanently towards the Earth (see captured rotation). Its speed in orbit varies, however, because its orbit is not circular but elliptical; hence at times the Moon's axial rotation appears to get either slightly ahead of or slightly behind its orbital motion, so that part of the ‘dark side’ of the Moon is visible around the east and west edges. This is known as libration in longitude.

Libration in latitude occurs because the Moon's axis is slightly tilted with respect to its orbital plane, so we can see over the north and south poles. In combination, these effects mean that a total of 59% of the Moon's surface is visible, rather than just 50%, as would be the case if libration did not occur.