The libration of the Moon

Who believes to see a staggering Moon at night, has probably had a drop too much, although a staggering movement of the Moon actually exists in astronomy. This is called »libration«,  but happens very slowly and is therefore only visible by the naked eye in time-lapse photography.

The Moon orbits around the Earth in a so called synchronous rotation. This means that it always faces one hemisphere towards the Earth, while the reverse side of the Moon is not visible from Earth. Due to certain physical conditions (relating to orbits, angular velocity, centres of mass, among others), what happens is that the visible surface of the Moon varies slightly during the course of the moon cycle, and there is a gentle change of inclination of the Moon’s axis or it appears to turn a little.

Based on this fact, one is able to see 59 % of the Moon’s surface during a moon cycle. Not actually a hemisphere of 50%, as one would normally imagine with a synchronous rotation.

By the way, the word libration has nothing to do with liberation, but with the scale. It derives from lat. libratio and librare = weigh, to keep in balance. It also refers to the star sign Libra.

However, we can still feel liberated when glancing at the nightly Moon and his wondrous appearance.

Animation: Wikipedia (Tomruen)