Jantar Mantar is an observatory built in Varanasi in the year 1737 by Maharaja Jai Singh II of kingdom of Amber (later called Jaipur). The observatory is one of the five built by Maharaja Jai Singh II.
Maharaja Jai Singh II was also an astronomer and had a deep interest in science and astronomy. In the early 18th century, he sent his scholars to several countries to study design, construction and technology of the observatories and also the prevalent technology. The scholars returned with their observations and many manuals on astronomy. Subsequently, between 1724 and 1737, Maharaja Jai Singh II had five observatories constructed in Jaipur, Mathura, New Delhi, Ujjain and in Varanasi. There is another evidence of the observatory being built at least two centuries before 1772. According to the Collected writing of Dharampal, Sir Robert Barker, in 1772, wrote that the observatory was built about 200 years before by Raja Mawnsing, son of Jaysing.