A breakthrough in three-dimensional representations of the sky came in 1664 with the construction of devices such as the Gottorp Globe. This was a hollow sphere, 10 feet (3 meters) in diameter, inside which images of the constellations and gilded stars were placed. Accommodating up to 10 people inside, this globe could rotate and thus demonstrate the daily motion of the constellations. In 1758, Charles Long improved on this idea by building a rotatable sphere 18 feet (5.5 meters) in diameter, accommodating 30 persons. Instead of painted constellations, Long's sphere, called the "Uranium" ("place of the heavens"), had tiny holes through which light from outside could shine, making it look to viewers inside as if they were actually looking at stars in the dark.

A similar rotatable sphere, built by Charles Atwood in 1913, was until recently on permanent display at the Chicago Academy of Sciences. Called the Atwood Sphere - and later re-christened the Globe Planetarium - it was the first realistic sky simulator of any kind in the United States.

In 1923, Walther Bauersfeld, chief engineer of the Zeiss optical works in Jena, Germany, developed a means of representing the sky by optically projecting images of the stars onto the interior of a large, fixed hemisphere. This first optical projection planetarium was housed in a 52.5 foot (16 meter) test dome in Jena, then moved to a 32 foot (9.8 meter) dome in Munich, both of which could accommodate far more spectators than the largest of the rotatable spheres.

Bauersfeld's projector, or the Zeiss Model I, as it was called, was so successful that it set the technological standard for planetaria for the next 60 years. The next step in planetarium technology branches off in two directions.

The Adler Planetarium and Astronomy Museum was the first major planetarium constructed in the Western Hemisphere, by Max Adler in 1930. With more than 2,000 astronomical and other scientific artifacts, it is also the Western Hemisphere's largest museum of astronomical history.

Photograph of the Zeiss II Planetarium Projector used from 1930 to 1971 at the Adler Planetarium in Chicago.

One, developed by the Evans & Sutherland Computer Corporation, is called Digistar. Digistar is a wide-angle video projector which projects a computer-generated image of the stars onto the dome, and has capabilities that traditional planetarium projectors lack. For example, it can show how the shapes of the constellations change over time and simulate a 3-D trip through the stars. Digistar also projects animated vector, or "wire-frame," graphics onto the dome, replacing many special effects projectors. The other direction for planetaria is being developed by Zeiss, and incorporates fiberoptic technology which produces a much brighter and far more realistic-looking sky than ever before. In the midst of this seeming technological battle of graphic versatility vs. realism, some new planetaria (with lots of money) are buying BOTH projectors.

Traditionally-designed theaters have horizontal domes and concentrically-arranged seats, while modern "Space Theaters" have tilted domes and unidirectional or epiconcentric seating plans, where everyone faces the same direction. The tilt of this type of dome effectively eliminates the horizon directly in front of the audience and thus is said to simulate a more spacebound view of the stars. Tilted domes also more favorably accommodate one of the several wide-angle 70mm film formats that are now available.

In addition to permanent planetaria, there are also many portable, inflatable domes in operation, which allow astronomy programs to be brought to areas where a permanent facility - or the funds to visit one - may not be available.