Definition from Wiktionary, the free dictionary
libration (plural librations)
- The act of librating.
- (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.
- (by extension) A similar rotational or orbital characteristic of some other celestial body.
act of libration
astronomy: apparent wobble of the moon