In astronomy a libration (from libra, Latin for scales) is a very slow oscillation, real or apparent, of a satellite as viewed from the larger celestial body around which it revolves. Used alone, the term usually refers to the apparent movements of the Moon relative to 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 around 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 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 around 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 around 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 Earth's rotation. This carries an observer first to one side and then to the other of the straight line joining 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.

Although the moon keeps the same side towards the earth, careful observations will reveal you can actually see 59% of the moon's surface. These variations called librations are caused by the fact that the moon rotates at a constant rate, but travels around the earth at a variable rate, being in an elliptical orbit and moving faster when it is closer. The animation shows a set of simulated views of the moon over one month, like a picture taken at the same time each day.

External links

• APOD 11/8/99 - real photo sequence like this one http://antwrp.gsfc.nasa.gov/apod/ap991108.html