The Jantar Mantar Observatory was built by Jai Singh II (1688-1743). This Maharajah was very found of astronomy. He had read all about the works of Euclide, Ptolémée, Copernic, Galilei, Newton... He made build five Observatories in Delhi, Jaipur, Ujjain, Varanasi and Matura. The Jantar Mantar is the greatest one.