From Wikipedia, the free encyclopedia

Simulated views of the Moon over one month, demonstrating librations in latitude and longitude.

In science (particularly astronomy), libration (from the Latin verb librare "to balance, to sway", cf. libra "scales") is a certain type of motion where an object has approximately a fixed orientation, but instead of staying exactly fixed in that orientation, rotates slightly back and forth in an oscillatory manner. (Everyday examples of this motion include the motion of an empty rocking chair, or the swaying of a balance scale.)

The term is most commonly applied to astronomy, describing the motion of orbital bodies, particularly the moon.^[1] It is also used in physics and chemistry to describe molecular motions in solids and liquids.

Lunar libration

The moon generally has one side facing the earth, due to tidal locking. The far side of the moon was never seen by humans until the advent of space exploration in the 1950s. However, this simple picture is only approximately true: Over time, more than half (about 59%^[2]) of the moon's surface can be seen from the Earth's surface, even though the front of the Moon is tidally locked to always face towards the Earth.

As orbital processes are repetitive, libration is manifested as a slow rocking back and forth 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, permitting an observer to see slightly differing faces of the surface at different times.

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.

In physics and chemistry

In physics and chemistry, there are some situations where a molecule (or other group of atoms) within a larger system can undergo libration. For example, in liquid water, any given water molecule is attracted to neighboring molecules, so that it has a preferred orientation and cannot freely rotate. (Of course, over time, the neighboring molecules move around and the preferred orientation changes.) However, it can undergo librational motions, which are measureable in an infrared absorption spectrum.^[3]

Another example is a molecular crystal: Each molecular unit has a preferred orientation due to interactions with the nearby molecules, but they have librational modes corresponding to small rotations about this preferred orientation.^[4]

References

1. ^ 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.
2. ^ Spudis, Paul D.. "Moon." World Book Online Reference Center. 2004.". World Book, Inc. http://www.nasa.gov/worldbook/moon_worldbook.html. Retrieved December 6 2009. 
3. ^ For example: Structure of Aqueous Solutions: Infrared Spectra of the Water Librational Mode in Solutions of Monovalent Halides, David W. James and Richard F. Armishaw, Aust. J. Chem., 1975,28, 1179-86.
4. ^ For example: Librational Motion in Diatomic Molecular Crystals, Hideaki Chihara and Takako Shinoda, weblink

See Also

Parallactic angle

External links

• 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 Zones

From LoveToKnow 1911