planetarium, 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.

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