A planetarium is an instrument designed to represent the relative positions and motions of celestial objects. Originally a mechanical model of the Solar System (see orrery), the planetarium of today is a facility in intricate optical device projects disks and points of light representing the Sun, Moon, planets, and stars on the interior of a fixed hemispherical dome. The various cyclic motions of these bodies as seen from a given latitude on Earth can be simulated. Of great assistance to students of astronomy and celestial navigation, planetariums also attract large public audiences.

In 1913 the planetarium projector was invented by Walter Bauersfeld of the Zeiss Optical Company. In 1923 the first planetarium, built by the Zeiss company (see Carl Zeiss), began operating in Munich and is still in use at the Deutsches Museum there.