The Yantra Mantra (commonly known as the Jantar Mantar) is an equinoctial dial, 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.
In the early 18th century, Maharaja Jai Singh II of Jaipur constructed five Yantra Mantras in total, in Delhi, Jaipur, Ujjain, Mathura and Varanasi; they were completed between 1724 and 1735. The most famous Yantra Mantras are:
The name is derived from Yantra, instrument, and Mantra, for formula or, in this context, calculation. Therefore jantar mantar means literally calculation instrument. Jantar Mantar is colloquially equated with the magical practices where "Mantar" refers to the incantation (or spell) and Jantar Mantar refers to questionable magical arts (astrology falling into such arts).The yantras 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.
- Sankalp India Foundation. Jantart Mantar: Get lost in space! (2008-07-22).
- Sharma, Virendra Nath (1995). Sawai Jai Singh and his astronomy. Motilal Banarsidass Publishers Pvt. Ltd. ISBN 81-208-1256-5.
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