Jantar Mantar is an observatory built in Varanasi in the year 1737 by Maharaja Jai Singh II of kingdom of Amber (later called Jaipur).

[1][2] Maharaja Jai Singh II was also an astronomer and had a deep interest in science and astronomy.

Subsequently, between 1724 and 1737, Maharaja Jai Singh II had five observatories constructed in Jaipur, Mathura, New Delhi, Ujjain and in Varanasi.

According to the Collected writing of Dharampal,[3] Sir Robert Barker, in 1772, wrote that the observatory was built about 200 years before by Raja Mawnsing, son of Jaysing.

He states, "The principal curiosity here is the observatory, built by Mawnsing,the son of Jysing, about 200 years ago; there is an exceedingly good mural arch cut upon a fine plaister of chunam, so fine and smooth, that it has the appearance of marble; and though it is certainly very old, it still is perfect, but the index is wanting; that is a loss which could very easily be supplied by a person who has a taste for these studies; for the centres are left in the wall."