Maharaja Jai Singh II of Jaipur constructed this Jantar Mantar in Ujjain in 18th century along with four more at Delhi, Jaipur, Mathura and Varanasi. Also known as Vedh Shala, this observatory has various instruments like Shanku Yantra, Digyansha Yantra and Sundial used to for many astronomical calculations such as time of day, positions of Sun, stars and planets in the solar system.
Jantar Mantar in Ujjain was renovated by Scindias in the early 20th century. Compared the Jantar Mantras of Jaipur and Delhi, the one in Ujjain is built on smaller scale but it is also supposed to be the oldest of them.
It is the only observatory among all the Jantar mantars that is still in use. Readings using various instruments are still taken to conduct astronomical studies of movement of heavenly bodies.