libration


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libration

(lÿ-bray -shŏn) The means by which 59% of the Moon's surface is made visible, over a 30-year period, to the terrestrial observer despite the lunar day being of the same length as the sidereal month (see synchronous rotation). Physical libration is a real irregularity in the Moon's rotation, arising from minor distortions in shape. Geometrical librations, including diurnal librations, are apparent oscillations due to the Moon being observed from slightly different directions at different times. A geometrical libration in longitude of 6°9′ (mean value) results from the inconstancy of the Moon's orbital velocity, which follows from the eccentricity of the Moon's orbit and Kepler's second law. This libration reveals part of the eastern or western farside limbs at different times during one month. In addition the 5°9′ inclination of the Moon's orbital plane to the ecliptic, plus the 1°32′ tilt of the Moon's equator to its orbital plane, produces a monthly geometrical libration in latitude of 6°41′. This makes the Moon's north and south polar regions more readily observable during one month.

libration

[lī′brā·shən]
(physics)
Any oscillatory rotational motion, such as that of the moon, or of a molecule in a solid which does not have enough energy to make full rotations.
References in periodicals archive ?
re de l'Arme de libration nationale (ALN), et des diffrents corps de scurit.
The two angles are introduced to investigate the libration of the tethered satellite system based on different coordinate systems.
Antoniadi mentions and illustrates the idea of libration zones in his classic work, The Planet Mercury, (2) showing that the considerable orbital eccentricity of Mercury would lead to a total libration in longitude of some 47[degrees].
Des photographies relatant les faits d'armes des chouhada, des archives de la presse crite franaise ou nationaliste, des ouvrages traitant des vnements les plus marquants de la guerre de libration nationale ont t exposs dans le hall de l'difice.
9[degrees], shows broadly comparable shadow shapes within Plato, but without the hook shown by Collins, again emphasising just how critically linked its appearance is to both lighting and libration.
conjoncture qui prvalait avant le dclenchement de la guerre de libration tait pourtant difficile.
Le ministre des Moudjahidine, Tayeb Zitouni, a reu, jeudi dernier, une dlgation d'anciens rsistants et d'anciens membres de l'Arme de libration marocaine.
Caption: Yellow dots indicate which part of the Moon's limb is tipped the most toward Earth by libration.
Le ministre de l'Interieur et des Collectivites locales, Daho Ould Kablia est revenu, hier, sur la reunion du Comite central du Front de libration nationale (FLN) qui s'est deroulee, le week-end dernier, a Alger.
Even when seen with favorable lunar libration, the extreme foreshortening created by looking sideways at the lunar poles compresses circular crater rims into apparently straight ridges.
The Moon's rocking motion is known as libration and it changes our view of features along the limb, alternately revealing farside features and hiding some nearside ones.
For key dates, yellow dots indicate which part of the Moon's limb is tipped the most toward Earth by libration under favorable illumination.