noun, plural plan·e·tar·i·ums, plan·e·tar·i·a [plan-i-tair-ee-uh] /ˌplæn ɪˈtɛər i ə/.

  1. an apparatus or model representing the planetary system.
  2. a device that produces a representation of the heavens by the use of a number of moving projectors.
  3. the building or room in which such a device is housed.

noun plural -iums or -ia (-ɪə)

  1. an instrument for simulating the apparent motions of the sun, moon, and planets against a background of stars by projecting images of these bodies onto the inside of a domed ceiling
  2. a building in which such an instrument is housed
  3. a model of the solar system, sometimes mechanized to show the relative motions of the planets

n.1734, “orrery,” 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” is attested from 1929.

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