The Jantra Mantra (literally the 'instrument and formula' and often called the Jantar Mantar), is located in the modern city of New Delhi, Delhi. It consists of 13 architectural astronomy instruments, built by Maharaja Jai Singh II of Jaipur, from 1724 onwards, and is one of five built by him, as he was given by Mughal emperor Muhammad Shah the task of revising the calendar and astronomical tables.
The primary purpose of the observatory was to compile astronomical tables, and to predict the times and movements of the sun, moon and planets. Some of these purposes nowadays would be classified as astronomy.
There are four distinct instruments within the observatory of Jantar Mantar in New Delhi: the Samrat Yantra, the Ram Yantra, the Jayaprakash, and the Mishra yantras.
The Samrat Yantra, or Supreme Instrument, is a giant triangle that is basically an equal hour sundial. It is 70 feet high, 114 feet long at the base, and 10 feet thick. It has a 128-foot-long (39 m) hypotenuse that is parallel to the Earth's axis and points toward the North Pole. On either side of the triangle is a quadrant with graduations indicating hours, minutes, and seconds. At the time of the Samrat Yantra's construction, sundials already existed, but the Samrat Yantra turned the basic sundial into a precision tool for measuring declination and other related coordinates of various heavenly bodies.
Ram Yantra consists of two large buildings with open top. Both these two buildings form a complete device. The device is used to measure the altitude of stars which is equivalent to the latitude and the longitude on the earth.
The Jayaprakash consists of hollowed out hemispheres with markings on their concave surfaces. Crosswires were stretched between points on their rim. "Jai Prakash Yantra consists of two concave hemispherical structures used for ascertaining the position of the sun and other heavenly bodies. This instrument is an efficient dial, showing at any instant the local time, the sun's declination and the sign of the zodiac." written on the plaque on the site.
The Mishra yantras were able to indicate when it was noon in various cities all over the world and was the only structure in the observatory not invented by Jai Singh II.
In all, between 1727 and 1734, Jai Singh II, built five similar observatories, Yantra Mantras, in west central India, all known by the same name, thus includes ones at Jaipur -Yantra Mantra (Jaipur), Ujjain, Mathura and Varanasi. Today the observatory is mainly a tourist attraction, and is significant in the history of astronomy.
Jantar Mantar sculptural structures inspired contemporary art and architecture. In 2008 Tatiana Bilbao designed the Observatory House in collaboration with the artist Gabriel Orozco, who visited Jantar Mantar astronomical observatory in 1996.