[W/K] :: Libration of the moon

  Libration \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.
        [1913 Webster]
  
     2. (Astron.) A real or apparent libratory motion, like that
        of a balance before coming to rest.
        [1913 Webster]
  
     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.
        [1913 Webster]