A planetarium is a theater where you can look at images of planets, stars, and other celestial bodies on the dome-shaped ceiling. Planetariums host school field trips by day, and can even host Pink Floyd laser light rock shows by night.
Use the noun planetarium for the projector that shines light onto the surface of the building's dome, recreating solar systems and constellations — and also for the building itself, where such a show is put on. Because planetarium is a Modern Latin word (from the roots planeta, or "planet," and -arium, "a place for,"), its plural can be either planetariums or planetaria.
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