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diurnal librationDaily geometrical libration brought about by the change in the position of the observer with respect to the Earth–Moon line as the Earth rotates on its axis. This libration is most significant near the equator, where it amounts to 57′2″.6; this is equal to the Moon's mean equatorial horizontal parallax (see diurnal parallax). An observer closer to the poles will however experience an equivalent geometrical libration because of the displacement from the ecliptic. See also augmentation; physical libration.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006