Jantar Mantar in Jaipur is constructed by Sawai Jai Singh , the Rajput ruler.This is very near to Sawai mansingh II City Palace.
The observatory consists of fourteen major geometric devices for measuring time, predicting eclipses, tracking star location as the earth orbits around the sun, ascertaining the declination of planets, and determining the celestial altitudes.
Really a lot to understand learn about this observatory. Of all the Jantar mantar constructed ( there are about 5) the one at jaipur is the largest and well preserved.
This is preserved under World heritage Site now.Check here for Jantar Mantar