All orreries are "planetariums" or "planetaria" (alternative plural). The term orrery has only existed since 1714. A grand orrery is one that includes the outer planets known at the time of its construction. The word planetarium has been captured, and now usually refers to hemispherical theatres in which images of the night sky are projected onto an overhead surface. Planetariums (orreries) can range widely in size from hand-held to room-sized. An orrery is used to demonstrate the motion of the planets, while a mechanical device used to predict eclipses and transits is called an astrarium.