From Encyclopedica Indica
The Jantra Mantra in New Delhi was built by the Rajput maharaja Swai Jai Singh II and consist of 5 observatories. “Jantar” is derived from the Sanskrit word “yantra” which means instrument and “mantar” comes from “mantra” which means formula. A plate attached to an instrument in the Jantar mantar in 1910 says that it was built in 1710, but it could be finished in 1724. The observatory has 4 different instruments of stone. The most known is the Samrat yantra or upper instrument, an observatory with equal segments for each hour of the day. It consists of an enormous triangle of 21 m high and 3 m thick and a length at the base of 34 m. The 39 meter long oblique side of the triangle shows to the north pole. To the left and right of the triangle there is a quadrant with a scale distribution in hours, minutes and seconds. This fundamental time-measurement instrument was changed by Jai Singh into an instrument to measure the declination and other coordinates of the celestial bodies.
The 3 other building are the Ram yantra, the Yayaprakash yantra and the Mishra yantra. Their design was to measure the declination, the height and the azimuth of the sun and stars. T The Mishra-instrument indicates when in different citis on earth it was 12 o’clock noon. Except for the Mishra yantra all yantras were designed by Jai Singh.
Jayaprakash Yantra consists of hollow, half celestial spheres with marks on the inside. Between the points on the edge there were cross wires.
Mishra yantra indicates when it was noon in different cities.
To localise a star, the height ( at the sky) and the azimuth (how far to the east the star is from the geographical north), must be known.