Libration

noun

1. The act of librating.
2. 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. A vibrational degree of freedom whereby a group of atoms partly rotates back and forth, similar to the Moon's motion.