Libration

In lunar astronomy, libration is the wagging or wavering of the Moon perceived by Earth-bound observers and caused by changes in their perspective. It permits an observer to see slightly different halves of the surface at different times. It is similar in both cause and effect to the changes in the Moon's apparent size due to changes in distance. It is caused by three mechanisms detailed below, two of which are causing a relatively tiny physical libration via tidal forces exerted by the Earth. Such true librations are known as well for other moons with locked rotation.

This article is about astronomical observations. For molecular motion, see Libration (molecule).
Not to be confused with liberation, libation, or vibration.
Simulated views of the Moon over one month, demonstrating librations in latitude and longitude. Also visible are the different phases, and the variation in visual size caused by the variable distance from Earth.
Theoretical extent of visible lunar surface (in green) due to libration, compared to the extent of the visible lunar surface without libration (in yellow). The projection is the Winkel Tripel projection.

The quite different phenomenon of a trojan asteroid's movement has been called Trojan libration; and Trojan libration point means Lagrangian point.

Lunar libration

The Moon keeps one hemisphere of itself facing the Earth, due to tidal locking. Therefore, the first view of the far side of the Moon was not possible until the Soviet probe Luna 3 reached the Moon on October 7th, 1959, and further lunar exploration by the United States and the Soviet Union. 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]

The following are the 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. The lunar libration in longitude was discovered by Johannes Hevelius in 1648.[2] It can reach 7°54′ in amplitude.[3]
  • Libration in latitude results from a slight inclination (about 6.7°) 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. Galileo Galilei is sometimes credited with the discovery of the lunar libration in latitude in 1632,[2] although Thomas Harriot or William Gilbert could have done so before.[4] Note Cassini's laws. It can reach 6°50′ in amplitude.[3]
  • Diurnal libration is a small daily oscillation due to 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—since the observer is on Earth's surface, not at its center. It reaches less than 1° in amplitude.[3]
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See also

References
  1. Spudis, Paul D. (2004). "Moon". World Book at NASA. Archived from the original on July 3, 2013. Retrieved May 27, 2010.
  2. Jacqueline Bergeron, ed. (2013). Highlights of Astronomy: As Presented at the XXIst General Assembly of the IAU, 1991. Springer Science & Business Media. p. 521. ISBN 9401128286.
  3. Ratkowski, Rob; Foster, Jim (May 31, 2014). "Libration of the Moon". Earth Science Picture of the Day.
  4. Stephen Pumfrey: Harriot’s Maps of the Moon: New Interpretations. Notes Rec. R. Soc. 63, 2009, doi:10.1098/rsnr.2008.0062.
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