A Jantar Mantar is an assembly of stone-built astronomical instruments, designed to be used with the naked eye. There were five Jantar Mantars in India, all of them built at the command of the Rajah Jai Singh II, who had a keen interest in mathematics, architecture and astronomy, four remain as the Jantar Mantar at Mathura was torn down just before the revolt of 1857.
This is the most significant, most comprehensive, and the best preserved of India's historic observatories. It is an expression of the astronomical skills and cosmological concepts of the court of a scholarly prince at the end of the Mughal period.
The largest example is the equinoctial sundial belonging to Jaipur's assembly of instruments, consisting of a gigantic triangular gnomon with the hypotenuse parallel to the Earth's axis. On either side of the gnomon is a quadrant of a circle, parallel to the plane of the equator.
The instrument can be used with an accuracy of about 2 seconds by a "skilled observer" to measure the time of day, and the declination of the Sun and the other heavenly bodies. It is the world's largest stone sundial, known as the Vrihat Samrat Yantra.
By the close of the nineteenth century, it had fallen into ruin. In 1901, Maharaja Ram Singh embarked on the restoration and reconstruction of the observatory. The Jaipur Jantar Mantar is a UNESCO World Heritage Site.