A planetarium is a theatre built for presenting shows about astronomy and the night sky.
Planetariums typically use a large dome shape for the projection screen, with inclined chairs for comfortable viewing "straight up". A large projector in the center of the dome creates the scene, using a number of movable projectors projecting the images of stars or planets onto the screen. The various projectors are geared to provide an accurate relative motion of the sky, and the entire system can be set to display the sky at any point in time.
Archimedes is attributed with possessing a primitive planetarium device that could predict the movements of the Sun, the Moon and the planets. The finding of the Antikythera mechanism proved that such devices existed already during antiquity.
The first modern planetarium projectors were designed and built by Carl Zeiss in 1924, and have grown more complex. Smaller projectors include a set of fixed stars, Sun, Moon, and planets, and various nebulae. Larger machines also include comets and a far greater selection of stars. Additional projectors can be added to show twilight around the outside of the screen (complete with city or country scenes) as well as the Milky Way. Still others add coordinate lines and constellations, photographic slides, laser displays, and other images.
The lasers are often used for non-astronomical "laser shows", frequently to musical accompaniment.
Today many planetariums are moving to a fully digital projection system, in which a single large projector with a fish eye lens, or a system of digital video projectors around the edge of the dome are used to create any scene provided to it from a computer. This gives the operator tremendous flexibility in showing not only the modern night sky as visible from Earth, but any other image they wish (including the night sky as visible from points far distant in space and time).
The term "planetarium" can also be used to describe the projector itself, or other devices to illustrate the solar system, like a computer simulation or an orrery.
A planetarium is unlike most theatres in that the best seats are at the rear, instead of the front.
- Adler Planetarium, Chicago, Illinois
- Artis Planetarium, Amsterdam 
- Eise Eisinga Planetarium, Franeker, 1774
- Fels Planetarium at the Franklin Institute (Philadelphia, Pennsylvania)
- Charles Hayden Planetarium at the Boston Museum of Science (Boston, Massachusetts)
- Hayden Planetarium, at the Rose Center for Earth and Space, American Museum of Natural History, New York, NY, James Stewart Polshek , architect, 2000.
- London Planetarium , Marylebone Road, London - part of Madame Tussaud's