The Jantar Mantar is an equinoctial sundial, consisting a gigantic triangular gnomon with the hypotenuse parallel to the Earth's axis. On either side of the gnomon is a quadrant of a circle, parallel to the plane of the equator. The instrument is intended to measure the time of day, correct to half a second and declination of the Sun and the other heavenly bodies.
The jantars have evocative names like, Samrat Yantra, Jai Prakash, Ram Yantra and Niyati Chakra; each of which are used to for various astronomical calculations. The primary purpose of the observatory was to compile astronomical tables, and to predict the times and movements of the sun, moon and planets.
The name "Jantar Mantar" is at least 200 years old, finding a mention in an account from 1803. However, the archives of Jaipur State, such as accounts from 1735 and 1737–1738, do not use this name, referring to it simply as Jantra, which in the spoken language is corrupted to Jantar. The word Jantra is derived from yantra, instrument, while the suffix Mantar is from the practice of adding a (usually meaningless) rhyming word for emphasis. The words jantar and 'mantar (or yantra and mantra) in their colloquial meanings are also related, referring to magical diagrams and magical words respectively. It has also been suggested that Jantar Mantar is derived from Yantra Mandira, but no evidence for this has been found.
- Anisha Shekhar Mukherji (2010), Jantar Mantar: Maharaja Sawai Jai Singh's Observatory in Delhi, Ambi Knowledge Resource, ISBN 978-81-903591-1-5, retrieved 23 July 2013
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- Jantar Mantar - The Astronomical Observatories of Jai Singh II, "a project initiated by Cornell University Professor of Art, Barry Perlus"
- Pictures with French text
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