A device that projects images of celestial bodies onto a dome. Often used interchangeably with プラネタリウム, though 天象儀 may sound slightly more technical or formal.
See also: プラネタリウム
I listened to an explanation of the starry sky at the science museum's planetarium.
Compound of 天象 (celestial phenomena) and 儀 (instrument). The exact historical derivation is uncertain, but the word was coined to describe a device that models the heavens.