Jantar Mantar of Delhi was built in 1725, during the reign of the later Mughal Emperor Muhammad Shah. He entrusted the most reliable astronomer of his time Maharaja Jai Singh II of Jaipur to construct an astronomical observatory to revise the calendar and to correct the astronomical tables used by contemporary priests.
It houses a massive sundial and other instruments intended to show the movements of the sun, moon, and other heavenly bodies. Generally, the constructions look a little strange to a visitor. But in fact, these structures are accurately and scientifically devised astronomical instruments for observing the movements and paths of celestial bodies in the absence of telescopes.
Jantar Mantar is the only observatory, out of the five built by Jai Singh II, which is functional. It consists of four main structures. The Samrat Yantra, the Ram Yantra, Jai Prakash Yantra and Misra Yantra. Jantar Mantar was declared a national monument in 1948 by India’s first Prime Minister, Jawaharlal Nehru.