libration

Etymology

From Latin lībrātiō (“a hurling, swinging”), from lībrō (“poise, cause to swing”).

noun

1. The act of librating.
2. (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.
3. (by extension) A similar rotational or orbital characteristic of some other celestial body.
4. (dynamics) The oscillation of an angle about a stable equilibrium point.
5. (chemistry) A vibrational degree of freedom whereby a group of atoms partly rotates back and forth, similar to the Moon's motion.