Over one lunar month more than half of the Moon's surface can be seen from the surface of the Earth.
Simulated views of the Moon over one month, demonstrating librations in latitude and longitude

In astronomy, libration is an oscillating motion of orbiting bodies relative to each other, notably including the motion of the Moon relative to Earth, or of Trojan asteroids relative to planets.


Lunar libration

The Moon generally has one hemisphere facing the Earth, due to tidal locking. Therefore, humans' first view of the far side of the Moon resulted from lunar exploration in the 1960s. However, this simple picture is only approximately true: over time, slightly more than half (about 59%) of the Moon's surface is seen from Earth due to libration.[1]

Libration is manifested as a slow rocking back and forth of the Moon as viewed from Earth, permitting an observer to see slightly different halves of the surface at different times.

There are three types of lunar libration:

  • Libration in longitude results from the eccentricity of the Moon's orbit around Earth; the Moon's rotation sometimes leads and sometimes lags its orbital position.
  • Libration in latitude results from a slight inclination between the Moon's axis of rotation and the normal to the plane of its orbit around Earth. Its origin is analogous to how the seasons arise from Earth's revolution about the Sun.
  • 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 and the Moon's centers, allowing the observer to look first around one side of the Moon and then around the other—because the observer is on the surface of the Earth, not at its center.

Trojan libration

In 1772 Lagrange's analyses determined that small bodies can stably share the same orbit as a planet if they remain near Lagrange points, which are 60° ahead of or behind the planet in its orbit. Such ‘Trojan asteroids’ have been found co-orbiting with Earth, Jupiter, Mars and Neptune. Trojan asteroids associated with Earth are difficult to observe in the visible spectrum, as their libration paths are such that they would be visible primarily in the daylight sky. In 2011, however, using infrared observation techniques, the asteroid 2010 TK7 was found to be a Trojan companion of the Earth; it librates around the leading Lagrange point, L4, in a stable orbit.[2]

See also


  1. ^ Spudis, Paul D. (2004). "Moon". World Book at NASA. Retrieved May 27, 2010. 
  2. ^ Connors, Martin; Paul Wiegert & Christian Veillet (28 July 2011). "Earth’s Trojan asteroid". Nature (Nature) 475: 481–483. Bibcode 2011Natur.475..481C. doi:10.1038/nature10233. 

External links

Wikimedia Foundation. 2010.

Look at other dictionaries:

  • libration — [ librasjɔ̃ ] n. f. • 1547; lat. libratio ♦ Astron. Balancement apparent (d un astre, et spécialement de la Lune). « un capteur optique de la position angulaire du Soleil pour étudier la libration de Phobos » (Sciences et Avenir, 1988). ●… …   Encyclopédie Universelle

  • Libration — Libration: Die animierte Simulation zeigt den Anblick des Mondes über den Zeitraum von 27 Tagen, was einem täglichen Foto des Erdtrabanten in allen Mondphasen von Neu über Vollmond bis Neumond entspricht. Daneben ist auch seine Winkel und… …   Deutsch Wikipedia

  • Libration — Li*bra tion (l[ i]*br[=a] sh[u^]n), n. [L. libratio: cf. F. libration.] 1. The act or state of librating. Jer. Taylor. [1913 Webster] 2. (Astron.) A real or apparent libratory motion, like that of a balance before coming to rest. [1913 Webster]… …   The Collaborative International Dictionary of English

  • Libration — (vom lat.), das Schwanken, namentlich das Schwanken des Mondes, s.d …   Pierer's Universal-Lexikon

  • Libration — (lat.), das scheinbare Schwanken des Mondes, wird erzeugt durch dessen ungleichförmige Bewegung in seiner elliptischen Bahn und durch deren Neigung zur Erdbahn; sie bewirkt, daß man statt der Hälfte nahezu 4/7 der Mondoberfläche beobachten kann …   Kleines Konversations-Lexikon

  • Libration — Libration, s. Mond …   Herders Conversations-Lexikon

  • libration — [lī brā′shən] n. [L libratio] 1. the act of librating 2. Astron. a slight rocking motion of the moon which allows 59% of the moon s surface to be visible from the earth libratory [lī′brə tôr΄ē] adj …   English World dictionary

  • Libration — Cette animation montre un ensemble de vues simulées de la Lune sur une période d un mois, comme si une photographie avait été prise chaque jour à la même heure. Elle permet de mettre en évidence le phénomène de libration lunaire. En astronomie,… …   Wikipédia en Français

  • Libration — Li|bra|ti|on auch: Lib|ra|ti|on 〈f. 20〉 Schwankung des Mondkörpers um die mittlere Lage [<lat. libratio „das Abwägen, Schwingen“; zu librare „wägen, schwingen“] * * * Li|b|ra|ti|on, die; , en [lat. libratio = das Wägen, zu: librare = wägen]… …   Universal-Lexikon

  • libration — librational, adj. /luy bray sheuhn/, n. Astron. a real or apparent oscillatory motion, esp. of the moon. [1595 1605; < L libration (s. of libratio) a balancing. See LIBRATE, ION] * * * ▪ astronomy       in astronomy, an oscillation, apparent or… …   Universalium