Fascination for the unknown elements of Universe enticed the astronomers of India since long. Aryabhata of fifth century is the first of the astronomer of the classical age of Indian mathematics. And in the list of other eminent astronomers of India is Sawai Jai Singh II of Jaipur.
Jai Singh II, the illustrious ruler studied several books on astronomy with the sole intention of understanding the planetary objects and their movements. He sent scholars to several foreign countries like Greece, Britain, Arabia and Portugal to study the science of astronomy. And with the help of scholars like Pt. Jagannath Samrat and Pt. Keval Ramji, he built not just one but five observatories between 1727 and 1734.
Narivalya Dakshin Gola used to find whether heavenly bodies are in Northern or Southern hemisphere. It can be used to read time also.
The first experimental observatory was built in Delhi and then he built four more in Jaipur, Ujjain, Varanasi and Mathura. Below is 'Jai Prakash Yantra' claimed to have been invented by Sawai Jai Singh II.The hemisphere surfaces are of marble and are scribed with celestial latitude and longitude lines, the location of the shadow of the sun, and thus the celestial coordinates of the sun, can be read from the markings on the hemisphere segments.
The observatory has fourteen statistical instruments for measuring time, predicting eclipses and to ascertain other astronomical events. The Sundial there tells the time to an accuracy of about two seconds in local time of Jaipur and is a major crowd puller.
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Update: Jantar Mantar was carefully renovated in 1901 and was declared a national monument in 1948. They are all freshly painted and therefore look new. (Answer to Reader Wil's query.)
(Once you are there, you will find a maze of structures of different shapes and sizes. You will be totally at a loss to understand what each of the structure means. Guides are available, costly affair but worth it.)