Orreries

Orreries are mechanical models of the solar system designed to illustrate the motions of the planets around the Sun. They typically consist of a frame with a central Sun and a gear train or armature that carries miniature planets on orbits. The motion is produced by clockwork, a motor, or manual cranking, and the planetary speeds are scaled to approximate orbital periods. In many models the Moon is included as a satellite of the Earth.

A notable historical origin is the early 18th century, when a model was created for an English nobleman known as the Earl of Orrery, giving the device its name. The term orrery has since applied to a wide range of gear-driven astronomical models, from simple educational toys to highly detailed scientific instruments.

Designs vary from heliocentric configurations, where all planets orbit the Sun, to geocentric arrangements that depict Earth at the center with epicycles to reproduce observed motions. Some models also illustrate the relative sizes of orbits and inclinations, though most are stylized rather than to scale in three dimensions.

Today's orreries are common in science museums and classrooms as teaching tools, and some are built as precision clockwork instruments for enthusiasts. Modern versions may use electric motors, LEDs, and modular kits for customization.

Related devices include the armillary sphere, which represents the celestial coordinate system rather than planetary orbits, and both have a place in the history of astronomy and horology.