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.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006

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.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
Huang, "Librational characteristics of a dumbbell modeled tethered satellite under small, continuous, constant thrust," Proceedings of the Institution of Mechanical Engineers, Part G: Journal of Aerospace Engineering, vol.
(28) Hill H., 'Observing in the Librational Regions', BAA Lunar Section Circular 5(11), 103 (1970 November)
5, five second derivative [[nu].sub.4] P[O.sub.4] bands were detected; the absorbance band and second derivative band at 633 c[m.sup.-1] derive from the O[H.sup.-] librational mode.
Bands of HA [26-29]: (a) The 3573 c[m.sup.-1] and 329 c[m.sup.-1] bands arise from stretching and translational modes of the O[H.sup.-] ions, respectively; the O[H.sup.-] librational bands expected in the 630 c[m.sup.-1] region are not clearly detected although two bands are predicted by [C.sub.6] factor group symmetry analysis [21].
Librational, translational and diffusional motions have been observed in all liquids studied and, remarkably, molecular vibrational modes have been observed in some cases.
In a previous article, the broadband dielectric relaxation of several polyfether imides) containing different ether segments were investigated [11], Dielectric relaxation is a useful probe of the local dynamics of the polymer structure and this study indicated that librational motions occur at low temperatures associated with the imide block.
The molecular librational modes are found to soften as the temperature increases, eventually overlapping with the molecular translational modes, leading to the 394 K order-disorder transition.