In astronomy libration (from the Latin verb librare "to balance, to sway", cf. libra "scales") can refer to many different orbital conditions that show some oscillatory behavior; but it usually refers, more specifically, to the lunar librations, that is, to the various lunar orbital conditions which make it possible to see (from the Earth's surface) more than 50% of the moon's surface over time, even though the front of the Moon is tidally locked to always face towards the Earth. By extension, libration can also be used to describe the same phenomenon for other orbital bodies that are nominally locked to present the same face.
As orbital processes are repetitive, libration is manifested as a slow rocking back and forth (or up and down) of the face of the orbital body as viewed from the parent body, much like the rocking of a pair of scales about the point of balance. In the specific case of the Moon's librations, this motion permits 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 lunar libration:
- 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.
- 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. Also significant is the fact that the Moon's orbit is inclined to the plane of the ecliptic by a little more than 5°. As it is the Sun which illuminates the Moon - and both the Sun and the Earth are always located in the plane of the ecliptic - the Moon is sometimes illuminated from above and sometimes from below, allowing us to see some of the lunar surface beyond the poles.
- 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 center to the Moon's center, allowing the observer to look first around one side of the Moon and then around the other. This is because the observer is on the surface of the Earth, not at its centre.
- ↑ For an example of a general usage of "libration", not involving the Moon: "Libration of the close approaches of Pluto to Neptune", C J Cohen, E C Hubbard, Astronomical Journal, vol.70, pp.10-13.
- ↑ Spudis, Paul D.. "Moon." World Book Online Reference Center. 2004.". World Book, Inc. Retrieved on December 6 2009.
- J. D. Mulholland, E. C. Silverberg (1972). "Measurement of Physical Librations Using Laser Retroreflectors". Earth, Moon, and Planets 4: 155–159. doi:10.1007/BF00562923, http://adsabs.harvard.edu/abs/1972Moon....4..155M.
- Moore, Sir Patrick (2003). Philip's Atlas of the Universe, Philip's. ISBN 9780540087075. OCLC 51966591.
- Astronomy Picture of the Day: 2005 November 13 – time-lapse animation of moon through one complete cycle, hosted by NASA
- Libration: 2 years in 2 seconds – 24 full moon pictures taken over two years, compiled in an animation (linked on page) showing the Moon's libration and variations in diameter
- National Maritime Museum: The Moon's Movement – discussion of the components of the Moon's libration with diagrams
- The Antikythera Calculator (Italian and English versions)
- Observing the Lunar Libration Zonesca:Libració
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