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In astronomy a 'libration' (from the Latin verb ''libro -are'' "to balance, to sway", cf. ''libra'' "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 the 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, 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.

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References

★ Measurement of Physical Librations Using Laser Retroreflectors, J. D. Mulhollan, E. C. Silverberg, , , Earth, Moon, and Planets, 1972

External links

★ APOD 11/8/99 - real photo sequence like this one

★ PixHeaven.net 04/30/07 - 24 Full Moon pictures (2 years in real time) compiled in an animation (click on the links in the page) showing the Moon's libration and variations in diameter

★ National Maritime Museum - A discussion the components of libration of the Moon with diagrams (on page 2)

★ The Antikythera Calculator (Italian and English versions)