Planetarium, is an educational device for showing the locations and movements of the planets and other objects in the universe. A modern planetarium is a complex optical instrument. It projects images of the planets, moon, and stars onto a domed ceiling, creating an accurate representation of the nighttime sky. The room or building in which such an instrument is housed is also called a planetarium.

A typical planetarium forms images of the stars by focusing light from one or more bright lamps through thousands of tiny holes drilled through metal plates. The plates are positioned around two spherical structures, one for Northern Hemisphere stars and the other for Southern Hemisphere stars. The images of the moon and planets are produced by separate projection devices mounted on a frame between the two star spheres. By means of various sets of gears and electric motors, the planetarium can show the rising and setting of the stars and the motion of the moon and planets along the ecliptic. The planetarium can also show the appearance of the heavens from any given place on Earth at any given time far into the past or future. Additional projection devices are used for depicting such phenomena as eclipses, auroras, or meteors, and for showing such aids to instruction as the system of celestial coordinates or outlines of the constellations.

Another type of planetarium uses a computer-controlled cathode-ray tube similar to the picture tube of a television set. The images of the stars and planets are formed on the screen of the tube and projected by a fish-eye lens onto a domed ceiling.

Early Planetariums

Early planetariums were either portable pictures of the starry sky painted on the inside of a sphere or dome, or mechanical models of the solar system. In late 17th century Europe, small model planetariums imitating the movement of planets around the sun were built inside clocks. Some of them even showed the revolution of moons around their planets.

During this time, one of the first planetariums, known as the Gottorp Globe, with a portable painting of the starry sky, was made in what now is Germany. The principal part of the planetarium was a hollow copper sphere 10.2 feet (3.1 m) in diameter with a table and a curved bench for 12 people inside it. The sphere's inner surface had the pictures of the constellations. The stars were gold-coated copper nail heads which shone by the light from a central oil lamp. A copper globe representing Earth lay on the table.

In the early 18th century, a model planetarium known as an orrerynamed for the Earl of Orrery, an Irish nobleman who had one made in 1712was built. To this day, small orreries are used by science teachers, because they help students understand the movements of the planets.

After the invention of electric lights and motors in the late 19th century, it became feasible to build large orreries. The first of these was installed in the Deutsches Museum in Munich, Germany, in the early 1920s.

At the center of a circular room was a large lighted globe, which represented the sun. Smaller lighted globes represented the planets and the smaller globes were suspended from the ceiling by rods. The rods were fixed to motor-driven cars that traveled along "orbital" tracks around the globe. Below the globe representing Earth was a small, motor-driven platform that an individual could ride. As the orrery ran, a rider could see a simulation of the planets' revolution around the sun from Earth's perspective. Similar kinds of orreries were later built at the Hayden Planetarium in New York City and at the University of North Carolina at Chapel Hill.

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.

Digital Projectors

Computers use a code of numbers or digits to process information. Thus a planetarium with computerized projectors is known as a digital planetarium. The mechanical projectors are able to show the stars as seen from only one region of spaceour solar system. This is because the holes in their star plates cannot change position relative to one another. However, a digital projector system does not have this limitation, as it does not use star plates. Instead, a computer creates images on video screens. A lens then projects each image onto the dome. To show the stars as seen from a place other than the solar system, the computer merely changes the images on the screens.

In the early 1980s, the Evans and Sutherland Corporation of the United States set up the first digital planetarium in the Science Museum in Richmond, Virginia. Since then digital planetariums have developed further and can now produce far more realistic images of planets. These advanced planetariums can show images of the objects that are significant in fields other than astronomy. For instance, they can simulate a flight through an extremely enlarged image of a living cell. Astronomy-based computer programs, which can simulate sky conditions at any time of the day, can also be considered as digital planetariums.

Digital planetariums also present shows for entertainment. For instance, they can be used for a display of animated, computer-generated art, accompanied by a musical sound track.

Portable Planetariums

Technologically advanced planetariums are too expensive for most schools and communities. But many of them have purchased inexpensive portable planetariums. For instance, one model has an inflatable dome that is designed to hold 35 children. The projector has a small light bulb enclosed by a plastic cylinder and a black cylinder with clear spots for projecting the stars. There are other affordable cylinders, which show galaxies, constellations, and even the inside of a living cell.