Libration

n. vibration, swaying movement, back and forth motion

In astronomy  libration (from the Latin verb libro -are "to balance, to sway", cf. libra "scales") refers to the various orbital conditions which make it possible to see more than 50% of the moon's surface over time, even though the front of the Moon is tidally locked to always face towards the earth. By extension, libration can also be used to describe the same phenomenon for other orbital bodies that are nominally locked to present the same face. As the orbital processes are repetitive, libration is manifested as a slow rocking back and forth (or up and down) of the face of the orbital body as viewed from the parent body, much like the rocking of a pair of scales about the point of balance.
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Noun
1. (astronomy) a real or apparent slow oscillation of a moon or satellite; "the libration of the moon"
(hypernym) oscillation
(derivation) librate
(classification) astronomy, uranology

n. libration, vibration; trembling

n. libration, vibration, swaying movement, back and forth motion