Astronomers were invited to his court for study and discussions and the king read all the treatises he could find on the subject. The Jantar Mantar (observatories) which the king built to gain her love and admiration still stand in New Delhi, Jaipur, Varanasi and Ujjain. Unfortunately, the one at Mathura was destroyed by building contractors who wanted the stones.
Maharaja Sawai Jai Singh (November 3, 1688-September 21, 1743) was ruler of the kingdom of Amber (later called Jaipur). He was born at Amber, the capital of the Kachwahas. He became ruler of Amber in 1699 at the age of 11 when his father Maharaja Bishan Singh died. The Mughal emperor Aurangzeb bestowed upon him the title of "Sawai" which meant one and a quarter times superior to his contemporaries. This title adorns his descendants even to this date.
Sawai Jai Singh was the first Hindu ruler in centuries to perform the ancient Vedic ceremonies like the Ashwamedha (1716) sacrifices — and the Vajapeya (1734) on both occasions vast amounts were distributed in charity.
Jai Singh’s observatories were called ‘Jantar Mantars’, which in Sanskrit roughly translates to ‘The Formula of Instruments’. The first one was built at Delhi in 1724, the second at Jaipur in 1734 and the other smaller ones at Mathura, Ujjain and Varanasi between 1732 and 1734. These monumentally grand, surrealistic structures, with their remarkable geometric shapes, are themselves the astronomical instruments, outfitted with drafting devices and grid indicators, and are so highly sophisticated that they are capable of exactly measuring planetary positions and reading time precise to one second. They were built with the assistance of the Bengali Pandit Vidyadhar Bhattacharya (also the engineer of Jaipur City), and are based on Ulugh Beg’s large 15th century instruments at Samarkhand. Although smaller, futuristic instruments like the telescope and newer-type of observatories in Paris and Greenwich were revolutionizing contemporary Europe, Jai Singh had more faith in the accuracy of his huge masonry structures.
These days Jai Singh's observatories at Jaipur, Varanasi, and Ujjain are functional. Only the one at Delhi is not functional and that at Mathura disappeared long time ago.
He also himself designed some of the instruments, like the Samrat Yantra (a huge equinoctial dial), Ram Yantra (a cylindrical building with an open top and a pillar in its center), and Jai Prakash Yantra (a concave hemisphere), the Digamsha Yantra (a pillar surrounded by two circular walls), and the Narivalaya Yantra (a cylindrical dial).
Jai Singh’s greatest achievement was the construction of Jaipur city (known originally as Jainagara (in Sanskrit, as the 'city of victory' and later as the 'pink city' by the British by the early 20th century), the planned city, later became the capital as the Indian state of Rajasthan.
Jai Singh opened his observatories to the public in order to popularize astronomy. Today, Jai Singh’s Jantar Mantars are open to tourists and are well-worth a visit.
For these multiple achievements Sawai Jai Singh II is remembered even to this date, as the most enlightened king of 18th Century India.