librationyoudaoicibaDictYouDict[libration 词源字典]
"An apparent or real oscillation of the moon, by which parts near the edge of the disc that are often not visible from the earth sometimes come into view", Early 17th century (denoting an oscillating motion, or equilibrium): from Latin libratio(n-), from the verb librare, from libra 'a balance'.[libration etymology, libration origin, 英语词源]