In astronomy the term libration (derived from libra the Latin for scales) describes apparent movements of the Moon relative to the Earth which can be compared to the rocking of a pair of scales about the point of balance.
Although the Moon's rotation on its axis is synchronously locked with its revolution about the Earth, these librations permit a terrestrial observer to see slightly differing halves of the Moon's surface at different times. This means that a total of 59% of the Moon's surface can be observed from the Earth.
There are three types of libration. 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 about the Earth. Its origin is analogous to way in which the seasons arise from Earth's revolution about the sun. Libration in longitude is a consequence of the Moon's orbit about the Earth being somewhat eccentric, so that the Moon's rotation sometimes leads and sometimes lags its orbital position. Finally, there is a small effect called diurnal libration. This is a consequence of the rotation of the Earth. This carries an observer first to one side and then to the other of the straight line joining the Earth's center to the Moon's center so allowing the observer to look first around one side of the Moon and then around the other.
