SAWAI JAI SINGH IIof Jaipur, a keen astronomer, built this observatory in 1724 because he wanted to calculate planetary positions and alignments accurately, in order to perform sacred rituals and pujas at propitious moments. One of the five observatories he built (see pp358-9), Jantar Mantar’s instruments are large and fixed, making them resistant to vibration and therefore exact. The Saturat Yantra, a right-angled triangle whose hypotenuse is parallel to the earth’s axis, is a gigantic sundial, with two brick quadrants on either side of it to measure the sun’s shadow. The Ram Yantra, reads the altitude of the sun, and the Jai Prakash Yantra (invented by Jai Singh II himself) verifies the time of the spring equinox. Now obsolete, the observatory lies in the centre of a pleasant park surrounded by high-rises.