Jantar Mantar is located in Parliament Street on the left as one goes from Connaught Place to Parliament House in New Delhi. It was built by Maharaja Sawai Jai Singh II of Jaipur (1699-1743). Jai Singh was a keen astronomer. His scientific researches added luster to this name.
In the Jantar Mantar there are six masonry instruments. Herein lay the Chief contribution of Jai Singh to the development of Astronomical science; he discarded brass instruments and built massive masonry ones in their place. These instruments vary in size from a few meters to 27.4 meters in height.
The Samrat Yantra (the supreme instrument) is the largest of all these instruments. It is in principle one of the simplest ‘equal hour’ sun dials. The second instrument – the Jai Prakash – consisting of two complementary concave hemispheres – is located south of the Samarat Yansra. The Ram Yantra, south of Jai Prakash, consist of two large circular buildings, complementary to each other. North-west of the Samrat Yantra is the Misra Yantra or mixed instruments, so called because the building contains four separate instruments. One of these is Niyat Chakra Yantra, a sun dial like Samrat Yantra. On each side of it are two graduated semi-circles, used for obtaining meridian altitudes.
The two pillars located south-west of the Misra Yantra help in determining the shortest and longest day of the year, for in December, one pillar casts its shadow over the other pillar while in June it does not. The Observatory at Delhi was the first one built by Jai Singh.