'Planetarium' was first a mechanical model of the solar system — then a domed theater for projected stars.
A domed building in which images of stars, planets, and constellations are projected onto a ceiling for public education and entertainment; the optical projector used for such displays.
From Modern Latin 'planetarium' (an orrery, a model of the planetary system), from Late Latin 'planetarius' (of or pertaining to planets), from Latin 'planeta,' from Greek 'planētēs' (a wanderer), from 'planan' or 'planasthai' (to wander, to go astray), from PIE *pele- (flat, to spread — via the sense of wandering over a flat surface). The ancient Greeks called the planets 'asteres planētai' (wandering stars) because they moved against the fixed background of the constellations — they were stars that refused to stay put. Key roots: planētēs (Greek: "a wanderer"), *pleh₂- (Proto-Indo-European: "to wander, to go astray (
A 'planet' is literally a wanderer — the Greeks called planets 'wandering stars' because they moved across the sky while the fixed stars stayed put. The same root may connect to 'plane' and 'plain' (flat surfaces you can wander across). A 'planetarium' is thus a theater for watching the wanderers.