In astronomy libration (from the Latin verb librare "to balance, to sway", cf. libra "scales") can refer to many different orbital conditions that show some oscillatory behavior; but it usually refers, more specifically, to the lunar librations, that is, to the various lunar orbital conditions which make it possible to see (from the Earth's surface) 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.