What is a planetarium?
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.