Jantar Mantar
A Jantar Mantar (Hindustani pronunciation: ]) 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. 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. The Jaipur Jantar Mantar is a UNESCO World Heritage Site. History. In the early 18th century, Maharaja Jai Singh II of Jaipur constructed five Jantar Mantar in total, in New Delhi, Jaipur, Ujjain, Mathura and  Varanasi; they were completed between 1724 and 1735. The Jantar have like Samrat Yantra, Jai Prakash, Ram Yantra and Niyati Chakra; each of which are used to for various astronomical calculations. The primary purpose of the observatory was to compile astronomical tables and to predict the times and movements of the sun, moon and planets. List of instruments: ● Samrat Yantra ● Jai Prakash Yantra ● Disha Yantra ● Rama Yantra ● Chakra Yantra ● Rashiwalya Yantra ● Dingash Yantra ● Utaansh Yantra Name. The name "Jantar Mantar" is at least 200 years old, finding a mention in an account  from 1803. However, the archives of Jaipur State, such as accounts from 1735 and 1737–1738, do not use this as "Jantra", which in the spoken language is corrupted to "Jantar". The word "Jantra" is derived from "yantra", instrument, while the suffix "Mantar" is derived from mantrana meaning consult or calculate. The words "jantar" and "mantar" (or "yantra" and "mantra") mean calculation instrument.