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tufailA planetarium is an optical device used to project a representation of the heavens onto a domed ceiling; the term also designates the building that houses such a device. A modern planetarium consists of as many as 150 motor-driven projectors mounted on an axis. As the axis moves, beams of light are emitted through lenses and travel in predetermined paths on the ceiling. The juxtaposition of lights reproduces a panorama of the sky at a particular time as it might be seen under optimum conditions. The motions of the celestial bodies are accurately represented, although they can be compressed into much shorter time periods, allowing spectators to see in minutes the motions that may actually take the celestial bodies days or years to complete. A typical planetarium projects the fixed stars, the sun, moon, and planets, and various nebulae. A larger planetarium can reproduce the Milky Way, comets, and more than 9,000 fixed stars. It may also project a set of coordinate lines for locating objects, in addition to pictures of animals and other forms associated with the constellations of the zodiac. Some projectors can take into account the apparent motions of the stars, thereby depicting the sky as it will look thousands of years in the future or as it looked thousands of years in the past, and the most recent planetariums are capable of showing the heavens as they would appear from the moon and from other locations in space. The first of the modern planetariums was constructed in 1924 by the Carl Zeiss Optical Works in Jena, Germany for the Deutsches Museum in Munich. The Adler Planetarium (1930) in Chicago was the first in the United States, and in 1934 the Fels Planetarium was added to the Franklin Institute in Philadelphia. The Hayden Planetarium, a part of the American Museum of Natural History in New York City, was established the following year; in 2000 a new, much more sophisticated Hayden Planetarium was installed in the museum’s Rose Center for Earth and Space. By the end of the 20th cent. there were more than 100 major planetariums worldwide and, mainly at schools and colleges, about 1,000 smaller ones.
A mechanical device known as an orrery (for Charles Boyle, earl of Orrery) was a forerunner of the planetarium. It is a framework supporting globes that represent the sun, planets, and natural satellites in their approximate sizes and spatial relations and in their revolutions and rotations. Several orreries were built in the 16th and 17th cent. to explain the Copernican (heliocentric) model of the solar system. Today the orrery finds considerable use as an aid in the teaching of celestial mechanics.
A planetarium (plural planetaria or planetariums) is a theatre built primarily for presenting educational and entertaining shows about astronomy and the night sky, or for training in celestial navigation. A dominant feature of most planetaria is the large dome-shaped projection screen onto which scenes of stars, planets and other celestial objects can be made to appear and move realistically to simulate the complex ‘motions of the heavens’. The celestial scenes can be created using a wide variety of technologies, for example precision-engineered ‘star balls’ that combine optical and electro-mechanical technology, slide projector, video and fulldome projector systems, and lasers. Whatever technologies are used, the objective is normally to link them together to provide an accurate relative motion of the sky. Typical systems can be set to display the sky at any point in time, past or present, and often to show the night sky as it would appear from any point of latitude on Earth.
Planetaria range in size from the Hayden Planetarium’s 21-meter dome seating 423 people, to three-meter inflatable portable domes where children sit on the floor. Such portable planetaria serve education programs outside of the permanent installations of museums and science centers.
The term planetarium is sometimes used generically to describe other devices which illustrate the solar system, such as a computer simulation or an orrery. Planetarium software refers to a software application that renders a three dimensional image of the sky onto a two dimensional computer screen. The term planetarian is used to describe a member of the professional staff of a planetarium.

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