orrery

thumb|upright=1.2|A small orrery showing Earth and the inner planets An orrery () is a mechanical model of the Solar System that illustrates or predicts the relative positions and motions of the planets and moons, usually according to the heliocentric model. It may also represent the relative sizes of these bodies; however, since accurate scaling is often not practical due to the actual large ratio differences, it may use a scaled-down approximation. Mechanical planetary models are known to have existed since the Ancient Greeks, and are known by various names, but the term orrery is derived from a device produced by John Rowley and named for his patron Charles Boyle, 4th Earl of Orrery.

Orreries are typically driven by a clockwork mechanism with a globe representing the Sun at the centre, and with a planet at the end of each of a series of arms.