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        Science | Astronomy Terms

Mechanical Projectors

The first modern planetarium was built at the Zeiss optical works in Jena, Germany, about 1924. This device known as Mark I, installed at the Deutsches Museum in Munich by the German firm Carl Zeiss, was mounted inside a dome 32 feet (10 m) in diameter. The Adler Planetarium, built in Chicago in 1930, was the first major planetarium in the United States.

A concave metal sphere known as a star ball used 31 lenses to show images of 4,500 stars on the dome. Seven additional projectors attached to the ball created images of the sun, the moon, Mercury, Venus, Mars, Jupiter, and Saturn. The movement of these projectors replicated the movement of the solar bodies relative to the stars. The illumination of the images came from a bright electric lamp in the center of the ball, surrounded by the 31 lenses. Behind each lens was mounted a disk called a star plate which served as a photographic slide. Light from the lamp passed through holes in the plate, each of which represented a star. With each lens focusing light on the dome through holes in its star plate the 31projectors together produced an image of the entire sky.

However, the Munich Planetarium had some limitations. The view of the planetarium was confined to Munich and other places that have the same northern latitude, which means the planetarium could only show stars that rose above the horizon at the latitude of Munich. But with the technical advancements the improved versions of Munich planetariums could show the sky from any place on Earth and at any time up to 26,000 years into the past or future. In the improved planetariums, which use two large star balls and a planet projector in between, stars appear similar from any place in the solar system but the planets do not. This is because the solar system is much smaller than the distances to the stars.

The success of the Zeiss projectors led to the establishment of thousands of planetariums in the 20th century. In the United States, the first Zeiss projectors were installed in the 1930s at the Adler Planetarium in Chicago, the Hayden Planetarium in New York City, the Fels Planetarium at the Franklin Institute Science Museum in Philadelphia, and the Griffith Observatory in Los Angeles. The Japanese firms Goto Optical Manufacturing Company and Minolta Company Limited and the U.S. company Spitz Incorporated also became leading makers of planetarium projectors during the late 20th century.

Today the technically advanced mechanical projectors show images as clear and bright as the actual stars. One such device known as Zeiss Mark IX at the Hayden Planetarium, New York City, projects images of more than 9,000 stars. It uses a hair-thin strand of glass called optical fiber to throw light on the dome. The fiber is so tiny that the image on the dome is pointlike and looks like a real star in the sky. Images of the sun, the moon, and the planets are created by separate projectors steered by computer-controlled motors.