Etymology
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planetarium (n.)

1734, "orrery, astronomical machine which by the movements of its parts represents the motions and orbits of the planets," Modern Latin, from Late Latin planeta (see planet) + Latin -arium "a place for." Sense of "device for projecting the night sky onto the interior of a dome," developed by Zeiss in Germany, debuted in Munich in 1923 and is attested by the name in English from 1929.

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Definitions of planetarium

planetarium (n.)
a building housing an instrument for projecting the positions of the planets onto a domed ceiling;
planetarium (n.)
an optical device for projecting images of celestial bodies and other astronomical phenomena onto the inner surface of a hemispherical dome;
planetarium (n.)
an apparatus or model for representing the solar systems;
From wordnet.princeton.edu