libration
/laɪˈbɹeɪ.ʃən/ noun
Forms librations
- noun The act of librating.
- noun (astronomy) The apparent wobble or variation in the visible side of the Moon that permanently faces the Earth, allowing observers on Earth to see, over a period of time, slightly more than half of the lunar surface.
- noun (by extension) A similar rotational or orbital characteristic of some other celestial body.
- noun (dynamics) The oscillation of an angle about a stable equilibrium point.
- noun (chemistry) A vibrational degree of freedom whereby a group of atoms partly rotates back and forth, similar to the Moon's motion.
- noun (figuratively) A wavering between two opposing thoughts or choices.
Etymology
From Latin lībrātiō (“a hurling, swinging”), from lībrō (“poise, cause to swing”).
Webster’s 1913
- noun The act or state of librating. Jer. Taylor.
- noun A real or apparent libratory motion, like that of a balance before coming to rest. 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.
Etymology [L. libratio: cf. F. libration.]
Sources: Wiktionary · Webster’s 1913 — from the Wiktionary entry “libration”, CC BY-SA 4.0. Details