The Jantar Mantar, or “House of Instruments” at Jaipur is the largest of the observatories and contains eighteen instruments. It was Built by Sawi Jai Singh II the founder of Jaipur who was very fond of astronomy and sciences. He built such four astronomical observatories in India namely Delhi, Jaipur, Varanasi and Ujjain. He wanted to bring precision in establishing the celestial positions of the heavenly bodies. The observatories consisted of large masonry structures with protractors and marked grids to help in exact location of heavenly bodies.
The Observatory at Jaipur is called the Jantar Mantar, meaning “House of Instruments” This observatory at Jaipur is the largest of the observatories which houses eighteen instruments. The credit of their excellent condition today can be attributed to Chandra Dhar Sharma Guleri, who supervised its restoration in 1901.
The Sundial: or the Samrat Yantra is the largest of the instruments It consists of a straight 90 feet high ramp, aligned north-south and elevated at an angle of 27 degrees which is the latitude of Jaipur . On each side of the ramp there are two quarter circles, or quadrants, made in masonry. The quadrants are centered on the nearest edge of the ramp. The Sun casts a shadow of the ramp edge on one of the quadrants; before noon the shadow is cast on the quadrant to the west of the ramp, and during the afternoon the shadow is cast on the eastern quadrant. The local solar time can be read from markings on the quadrant at the edge of the shadow. With not much difficult measurements one can precisely measure the local solar time to as close as to a few seconds. There are stairs up the ramp and one can move a stick up and down the edge of the ramp until the shadow of the stick falls on the appropriate edge of the quadrant. Markings on the ramp can then be read, at the location of the stick, which give the celestial longitude of the Sun
Another instrument is the Jai Prakash Yantra. Which is said to have been invented by Sawai Jai Singh II, is one part a hemisphere, sunk in the ground, sliced with passages so that the instrument reader can be close to the remaining segments of the hemisphere. The second part of the instrument is another hemisphere, sunk in the ground, sliced with passages to compliment the first instrument. The parts of the hemisphere in the first instrument, which are missing because of the passageways, are present in the second and visa versa. The hemisphere surfaces are of marble and are scribed with celestial latitude and longitude lines. A small marker is suspended at the center of each hemisphere by wires. There are a number of sundials; one is a beautiful vertical sundial. There is also a smaller equatorial sundial.. There are also twelve smaller instruments similar in design to the equatorial sundial. They are unique in that they are constructed to help read the ecliptic coordinates of celestial objects. Because the ecliptic pole is not a fixed point relative to an observer on the rotating Earth, one cannot align a single gnomon to always point to the ecliptic pole. The angle of the gnomon would have change with time to point to the ecliptic pole. As an approximate solution to this problem Jai Singh built twelve “equatorial” sundials with differing gnomon slopes. Thus at any one time one could use the dial with the angle closest to the true angle of elevation of the ecliptic pole and use that dial to read the ecliptic coordinates of celestial objects. This would be particularly valuable for tracking the Sun and planets. There is one instrument for each zodiac sign.