| Libration | n. [ L. libratio: cf. F. libration. ] 1. The act or state of librating. Jer. Taylor. [ 1913 Webster ] 2. (Astron.) A real or apparent libratory motion, like that of a balance before coming to rest. [ 1913 Webster ]
Libration of the moon, any one of those small periodical changes in the position of the moon's surface relatively to the earth, in consequence of which narrow portions at opposite limbs become visible or invisible alternately. It receives different names according to the manner in which it takes place; as: (a) Libration in longitude, that which, depending on the place of the moon in its elliptic orbit, causes small portions near the eastern and western borders alternately to appear and disappear each month. (b) Libration in latitude, that which depends on the varying position of the moon's axis in respect to the spectator, causing the alternate appearance and disappearance of either pole. (c) Diurnal or parallactic libration, that which brings into view on the upper limb, at rising and setting, some parts not in the average visible hemisphere. [ 1913 Webster ]
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| Libration point | n. any one of five points in the plane of a system of two large astronomical bodies orbiting each other, as the Earth-moon system, where the gravitational pull of the two bodies on an object are approximately equal, and in opposite directions. A solid object moving in the same velocity and direction as such a libration point will remain in gravitational equilibrium with the two bodies of the system and not fall toward either body. [ 1913 Webster ] |
