A planetarium is a special type of building where a dumbbell shaped planetarium projector is used to recreate the night sky on the inside of the dome-shaped ceiling. It is a very complex instrument fitted with lenses, prisms and mirrors. It can not only show the stars almost exactly as they appear in the real sky, but can also show their movements from season to season or as they appear from different places. It can also duplicate the movements of the Sun, Moon, planets and other heavenly bodies in the sky. Modern planetariums are also equipped with slide projectors that can simulate events like space travel and landing on other planets. The planetarium is used to educate not only astronomers and navigators but also students about the Universe. It also provides spell bounding spectacle to a lay person.
The first planetarium was built by Walter Bauersfeld of the Zeiss Optical Company of Jena (Germany) in 1923. The apparatus was kept at the centre of the dome. It could reproduce the real sky. The motions of the heavenly bodies taking place in one year could be reduced to an interval varying from several seconds to half an hour.
Planetariums are useful because it is very difficult to learn about heavenly bodies just by looking at the sky since it is a very time consuming process to track the heavenly bodies due to pollution, clouds, etc. In a planetarium the motions of heavenly bodies taking place in a year can be watched in an interval of a few seconds.
Today all big cities of the world have these dome-shaped buildings equipped with planetarium projectors to educate people. The biggest planetarium is in Moscow. It is located in a building with a diameter of 25.15 meters. The planetarium in San Francisco, USA, contains about 25,000 mechanical parts and weighs 2.5 metric tons. In our country there are planetariums in cities like Bombay, Calcutta, Jaipur, Delhi and Bangalore and thousands of people visit them everyday.
