The animation shows a set of simulated views of the Moon over one month.

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. Look up Liberation in Wiktionary, the free dictionary. ... Image File history File links Lunar_libration_with_phase2. ... A giant Hubble mosaic of the Crab Nebula, a supernova remnant Astronomy (also frequently referred to as astrophysics) is the scientific study of celestial objects (such as stars, planets, comets, and galaxies) and phenomena that originate outside the Earths atmosphere (such as the cosmic background radiation). ... Latin is an ancient Indo-European language originally spoken in Latium, the region immediately surrounding Rome. ... cf. ... The ancient Roman units of measurement were built on the Greek system with Egyptian influences. ... This article is about Earths moon. ... This article is about Earth as a planet. ...

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. Tidal locking makes one side of an astronomical body always face another, like the Moon facing 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 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. A surface normal, or just normal to a flat surface is a three-dimensional vector which is perpendicular to that surface. ... Two intersecting planes in three-dimensional space In mathematics, a plane is a two-dimensional manifold or surface that is perfectly flat. ... Two bodies with a slight difference in mass orbiting around a common barycenter. ... This article or section is in need of attention from an expert on the subject. ... In astrodynamics, under standard assumptions any orbit must be of conic section shape. ...

References

• J. D. Mulhollan, E. C. Silverberg (1972). "Measurement of Physical Librations Using Laser Retroreflectors". Earth, Moon, and Planets 4: 155-159. 

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)