The Jantar Mantar, an observatory consisting of masonry built astronomical instruments lies on the Parliament Street, south of Connaught Circus in New Delhi. These instruments were erected by Maharaja Jai Singh II of Jaipur (1699 – 1743 AD), who was keenly interested in astronomical observations and studied all systems, western and eastern, before embarking on his constructions. Initially he built metal instruments some of which are still preserved in Jaipur, but later discarded them.
The observatory at Delhi was the first to be built, and it was followed by construction of similar observatories at Jaipur, Ujjain, Varanasi and Mathura, the last of which no longer survives. According to tradition, Jai Singh built the Delhi observatory in 1710, while Sayyid Ahamd Khan, author of Athar-us-Sanadid, takes 1724 to be the date of its construction. Since Jai Singh himself mentions that he built the instruments by the order of the emperor Muhammad Shah, who ascended the throne only in 1719 and granted a governorship to him, Sayyid Ahmad Khan’s date would appear to be nearer the truth.
Built with brick rubble and plastered with lime, the instruments have been repaired and restored repeatedly, but without any large scale alteration. Among them, the Samrat Yantra (supreme instrument) is ‘an equinoctial dial, consisting of a triangular gnomon with the hypotenuse parallel to the earth’s axis, and on either side of the gnomon is a quadrant of a circle parallel to the plane of the equator’. The Jai Prakash to its south consists of two concave hemispherical structures to ascertain the position of the sun and other heavenly bodies.
Two circular buildings to the south of the Jai Prakash, with a pillar at the centre, constitute the Ram Yantra, the walls and floor of which are graduated for reading horizontal (azimuth) and vertical (altitude) angles. The Misra Yantra (mixed instrument) to its north west combines four instruments in one, and hence its name. these are Niyata Chakra which indicates the meridian at four places, two in Europe and one each in Japan and the Pacific Ocean; half on an equinoctial dial; Dakshinottara-bhitti-Yantra, used for obtaining meridian altitides and Karka-rasi-valaya, which indicates the entry of the sun in the cancer. To the east of the instruments, the small temple of Bhairava also appears to have been built by Maharaja Jai Singh.