\Li*bra"tion\ (l[-i]*br[=a]"sh[u^]n), n. [L. libratio:
cf. F. libration.]
1. The act or state of librating. --Jer. Taylor.
2. (Astron.) A real or apparent libratory motion, like that
   of a balance before coming to rest.
{Libration of the moon}, any one of those small periodical
   changes in the position of the moon's surface relatively
   to the earth, in consequence of which narrow portions at
   opposite limbs become visible or invisible alternately. It
   receives different names according to the manner in which
   it takes place; as: {(a)} Libration in longitude, that
   which, depending on the place of the moon in its elliptic
   orbit, causes small portions near the eastern and western
   borders alternately to appear and disappear each month.
   ({b}) Libration in latitude, that which depends on the
   varying position of the moon's axis in respect to the
   spectator, causing the alternate appearance and
   disappearance of either pole. ({c}) Diurnal or parallactic
   libration, that which brings into view on the upper limb,
   at rising and setting, some parts not in the average
   visible hemisphere.