An orrery is a mechanical device that illustrates the relative positions and motions of the planets and moonss in the solar system. They are typically built as a large clockwork with a globe representing the Sun at the center, and with a planet at the end of each of the arms.
For instance a normal clock could be used to produce an orrery with the Sun in the center and Mercury and Venus on the minute and hour hands. However a real orrery would make the arms move at the proper relative speeds, and would perhaps make the planets turn as well.
Orrerys are sometimes referred to as planetariums.
See also Armillary sphere.