Source : Webster's Revised Unabridged Dictionary (1913)

Orrery \Or"re*ry\, n.; pl. {Orreries}. [So named in honor of the
   Earl of Orrery.]
   An apparatus which illustrates, by the revolution of balls
   moved by wheelwork, the relative size, periodic motions,
   positions, orbits, etc., of bodies in the solar system.

Source : WordNet®

orrery
     n : planetarium consisting of an apparatus that illustrates the
         relative positions and motions of bodies in the solar
         system by rotation and revolution of balls moved by
         wheelwork; sometimes incorporated in a clock