plan·e·tar·i·um / ˌplaniˈte(ə)rēəm/ • n. (pl. -tar·i·ums or -tar·i·a / -ˈte(ə)rēə/ ) 1. a building in which images of stars, planets, and constellations are projected on the inner surface of a dome for public entertainment or education. ∎  a device used to project such images.2. another term for orrery.

planetarium Domed building in which a projector displays an artificial sky in order to demonstrate the positions and motions of the Sun, Moon, planets and stars relative to the Earth. Walter Bauersfeld of the Zeiss Optical Company invented the planetarium in 1913.