Definition
An Earth-bound observer looking at the Moon would always see approximately the same sight, as the face shown to the Earth is locked by tidal locking. However, various effects cause that face to apparently wobble and slightly change over time. This effect is called libration (from Latin libration, to oscillate) and allows to observe about 59% of the Moon’s surface from Earth.
Theory and Application: General Lunar Motion and Librations
General Lunar Motion
The laws for lunar motion were first written down by Cassini in 1693 and are thus referred to as Cassini’s laws. They can be formulated as (Eckhardt 1981):
- 1.
For every revolution around the Earth, the Moon rotates uniformly once about its own axis.
- 2.
The equator precesses around the ecliptic at the same rate as its orbit does.
Observations have confirmed the first law. The Moon orbits around the Earth with a period of 27.322 days. It rotates around its axis with the same period of 27.322 days (Taylor 2007). Its orbit is...
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References
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Viatteau, J. (2022). Lunar Libration. In: Cudnik, B. (eds) Encyclopedia of Lunar Science. Springer, Cham. https://doi.org/10.1007/978-3-319-05546-6_156-1
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DOI: https://doi.org/10.1007/978-3-319-05546-6_156-1
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