the complex revolving projector used to simulate the past, present, or future motions or positions of the sun, moon, planets, and stars on the inside of a large dome
the room or building in which this is contained
See planetarium in American Heritage Dictionary 4
(plănˌĭ-târˈē-əm)
nounpl.plan·e·tar·i·ums or plan·e·tar·i·a(-ē-ə)
An apparatus or model representing the solar system.
a. An optical device for projecting images of celestial bodies and other astronomical phenomena onto the inner surface of a hemispherical dome.
b. A building or room containing a planetarium, with seats for an audience.