Billy Jackson, age 9, of Dothan, Ala., for his question:
WHAT IS A PLANETARIUM?
A planetarium is an apparatus or model representing the star and planetary system. It is often a device that produces a representation of the heavens by the use of a number of moving projectors. The word is also used to denote the structure in which a planetarium is housed.
A planetarium is not an observatory, although often it is operated in conjunction with an astronomical museum or observatory.
The simplest form of planetarium is called an orrery. It is made up of a central ball that represents the sun, and a series of gears. The gears have arms that carry lesser globes, which represent the motions of earth, moon and planets. When the device is set in motion by clockwork, on orrery gives a rough idea of how the earth, moon and planets revolve about the sun.
The first modern planetarium was built at the Deutsches Museum in Munich, Germany, in 1923. Today planetariums add much to the educational resources of many of the world's major cities. Major United States planetariums include those in Boston; Chapel Hill, N.C.; Chicago; Denver; Flint, Mich.; Los Angeles; New York City; Philadelphia; St. Louis and San Francisco.
A modern planetarium consists of a dome with an inside that is whitened like a motion picture screen. It has a machine in the center of the dome that contains a great many cog wheels, projectors, lanterns and electrical devices.
When the room is darkened, and the projectors are illuminated and set in motion, a representation of the sky in action appears. It takes 119 optical projectors to illuminate each motion of a heavenly object.
At a large planetarium, a lecturer stands in a control booth in a large aluminum domed circular room. The audience sits before him, while overhead is a representation of the heavenly bodies. The projectors can make this picture of the sky go slow or fast, cutting years into minutes and days into seconds.
With the projectors of a planetarium moving the heavenly bodies fast, the moon flits through its phases and the complicated motions of the planets can be clearly seen. The stars can be turned off or on at will. The whole dome of the sky moves to represent the appearance of the heavens at any place.
Mathematical computations of the motions of the heavenly bodies can be made with great accuracy. Such computations are generally presented in the form of mathematical tables which the ordinary person finds difficult to understand.
These tables, however, can be translated into geometrical shapes that look like wavy lines, and these shapes can be further translated into the action of trains of gear wheels.
The point in the gear train that represents the actual position of a heavenly body moves a little searchlight or lantern. This projects light and portrays that body's motion in the sky.
Projectors in a planetarium can show objects in the sky as they appear today, or they can show how the stars appeared thousands of years ago. They can also show how the heavenly bodies will look in the future.