A planetary model of our Earth/Moon system around the Sun. This usually includes some movements in a third dimension, such as the inclination of the Moon’ s orbit or the Earth’ s axis fixed in space. A model that shows the planets revolving round the Sun, normally without any planetary rotation or axial tilt, is called a Planetarium.

In 1712 an instrument maker called John Rowley built an Earth/Moon/Sun model for Charles Boyle, who was the fourth earl of Orrery. John Rowley, most probably in an attempt to flatter his patron, named the device “an Orrery” and the name stuck!

They're primarily driven by a series of cogs and gears, and when you flip a switch, the individual pieces turn.

They are a teaching aid for showing students how the various planets revolve and rotate. With an Earth/Moon/Sun system model you can answer all these questions by just turning the handle…

They can be surprisingly accurate. Using a carefully chosen set of gears a model Earth can rotate around the sun to an accuracy of 5 minutes a year. It’ s all down to the designer, and how much time is spent in looking at different gear combinations.