Lunar Libration

The moon generally has one side facing the earth, due to tidal locking. The far side of the moon had never been seen by humans until the advent of space exploration in the 1950s. However, this simple picture is only approximately true: Over time, more than half (about 59%) of the moon's surface can be seen from the Earth's surface, even though the front of the Moon is tidally locked to always face towards the Earth. As orbital processes are repetitive, libration is manifested as a slow rocking back and forth of the face of the orbital body as viewed from the parent body, much like the rocking of a pair of scales about the point of balance, permitting an observer to see slightly differing faces of the surface at different times.

There are three types of lunar libration:
1- Libration in longitude is a consequence of the Moon's orbit around Earth being somewhat eccentric, so that the Moon's rotation sometimes leads and sometimes lags its orbital position.
2- Libration in latitude is a consequence of the Moon's axis of rotation being slightly inclined to the normal to the plane of its orbit around Earth. Its origin is analogous to the way in which the seasons arise from Earth's revolution about the Sun. Also significant is the fact that the Moon's orbit is inclined to the plane of the ecliptic by a little more than 5°. As it is the Sun which illuminates the Moon - and both the Sun and the Earth are always located in the plane of the ecliptic - the Moon is sometimes illuminated from above and sometimes from below, allowing us to see some of the lunar surface beyond the poles.
3- Diurnal libration is a small daily oscillation due to the Earth's rotation, which carries an observer first to one side and then to the other side of the straight line joining Earth's center to the Moon's center, allowing the observer to look first around one side of the Moon and then around the other. This is because the observer is on the surface of the Earth, not at its centre.

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