This observatory was built by MaharajaJai Singh II of Jaipur (A.D. 1699-1743) and was the first of the five such observatories built by him in India. There exists some uncertainty about its date of construction, as Pandit Gokal Chand places it in AD. 1710, while Sir Sayyid Ahmad Khan dates it to AD. 1724. The principal observations made at this observatory formed the basis of the new tables of Jai Singh called the Zij Muhammad Shahi. Jai Singh himself described that he constructed at Delhi brass instruments of the astrolabe type in accordance with the Muslim books, which were found unsatisfactory, and, therefore he constructed instruments of his own invention, such as Jai Prakash, Ram Yantraand Samrat Yantra, etc. of brick, rubble and lime plastered with lime. At present the observatory contains the following structures:
a) 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'.
b) The Jai-Prakash Yantra consists of two concave hemispherical structures to ascertain the position of the Sun and other heavenly bodies.
c) The Ram Yantra, consisting of two circular buildings to the south of the jai-Prakash, has a pillar at the centre, the walls and floor of which 'are graduated for reading horizontal (azimuth) and vertical (altitude)Angles'
d) The Misra Yantra ("Mixed Instrument"), 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 altitudes and Karka-rasi-valaya, which indicates the entry of the Sun in the Cancer.
e) Two pillars south-west of the Misra Yantra.
f) Measuring platform, south of the Misra Yantra.