The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

orrery
    n 1: 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