PLANETARIUM

institution devoted to popular education in astronomy and related fields, especially space science, and in which the principal teaching arrangement is a hemispherical screen onto which images of celestial objects as seen from Earth are projected from an instrument also known as a planetarium, or planetarium projector. The term was originally used to describe teaching or exhibit devices designed to portray the orbital motions of the planets and their satellites. Such devices consisted of small globes, representing the planets, that were mounted on wire rods supported and geared at a central pedestal, with a globe for the Sun at the top. Many included the major satellites known at the time of construction. Also called orreries (after the English sponsor of one built in 1712), these devices could be quite elaborate and accurate (see orrery). Planetarium installations vary greatly. Some have extensive exhibit space, museum collections, and sizable professional and support staffs. The projection dome may have a diameter of 25 m (82 feet) or more and a seating capacity in excess of 600. Others are large enough to accommodate only a small group of people and might better be described as teaching aids than as institutions. At the heart of every planetarium is the projection instrument. Here, too, the range is great. Perhaps the best-known name in the field is Zeiss; since World War II, projectors have been constructed in a new Zeiss factory in Oberkochen, Ger., as well as in the original Zeiss plant at Jena, Ger., where the first projector was built in 1923. Modern Zeiss instruments are large, technically advanced combinations of lenses, lights, gears, and motors designed to place the planets, Sun, and Moon in their correct locations among the stars for thousands of years past and future. Star images are projected from two spherical units, one for the northern sky and one for the southern sky, while images of the other celestial objects are projected from separate devices supported in a latticework cage between the star balls. The goal is to create a highly realistic artificial sky. Auxiliary projectors produce horizon scenes, the Milky Way, comets, meteors, and the reference lines used for teaching descriptive astronomy and celestial navigation. Other planetarium projectors are less complex but accomplish the same purpose of creating an artificial sky that can be used for teaching purposes. Manufacturers in Japan and the United States compete with Zeiss, producing an array of instruments appropriate for a variety of dome sizes and for different purposes. In a typical planetarium, demonstrationsor sky shows, as they are commonly calledare offered to the public on a regular schedule. The astronomy theme is often embellished by music, special effects of all kinds, and a narration that is usually prepared in advance and taped by a professional skilled in this type of dramatic interpretation. There is a trend toward automation of the program to assure that all visitors experience essentially similar presentations on the current theme. When the first planetarium was opened at the Deutsches Museum in Munich in 1923, it was described as a schoolroom under the vault of the heavens. Special sky shows for schoolchildren remain an essential part of the program in nearly every installation, with the content often integrated with the science curriculum of the local schools. Lectures in the basics of descriptive astronomy are given to college classes, and the facilities are commonly used for courses or lectures in a continuing-education program, sometimes for credit by arrangement with a local college or university. Although concentric seating around the central projector in the domed theatre remains the most common arrangement, there have been experiments with unidirectional seating. Horizons have been tilted in the construction of some theatres to give audiences a more objective view of the artificial sky. A recent trend in space theatres places emphasis on wide-angle motion pictures in a tilted or distorted hemisphere, with projected star fields in a secondary role. Cathode-ray projection systems may eventually replace the optical-mechanical planetarium projector. Such electronically controlled systems are readily integrated with computer data banks and can create many variations on traditional projected patterns. By taking into account star distances, for example, the system can create the illusion of flying through space among the stars while it maintains an accurate representation of the surroundings from any new perspective.