li·bra·tion 
(lī-brāshən)
(lī-brāshən)n.
A very slow oscillation, real or apparent, of a satellite as viewed from the larger celestial body around which it revolves.
[Latin lībrātiō, lībrātiōn-, oscillation, from lībrātus, past participle of lībrāre, to balance, from lībra, balance.]
librate′ (lībrāt′) v.
li·bration·al adj.
libra·to′ry (-brə-tôr′ē) adj.