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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
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.