Jantar Mantar, built between 1728 and 1734, literally means the ‘instruments for measuring the harmony of the heavens’. Jai Singh, the brain behind the grand project, chose stone with marble facing. This was the biggest of all his observatories and the only one built of stone. He used it daily, often with his astronomy gurus Pandit Jagannath and Kewal Ram. In all there are 17 instruments in the Jantar Mantar complex. The function of each instrument is rather complex but serves a particular function where time plays the main theme.


Instruments Used in Jantar Mantar
The instruments and their functions are given below in the order of their anti-clockwise position in the complex. The large Kranti Yantra was employed for the measurement of the longitude and latitude of the celestial bodies. The Diganta Yantra was deviced to measure the azimuth (arc of the celestial great circle from Zenith to horizon) of planetary bodies. Similarly, the Small Ram Yantra and the Large Ram Yantra are used to find the altitude and the azimuth. The Chakra Yantra gives the angular measurement of an object from the equator. The Jai Prakash Yantra determines the precise coordinates of celestial bodies and the small iron plate strung between the crosswires gives the sun’s longitude and latitude and the zodiacal sign that it is passing through. Its main function is to keep an eye on all the other instruments.


Adequacy of Specific Yantras
The Rashivalayas Yantra operates in the same manner as that of the Samrat Yantra and has one sundial for each of the zodiac signs. Five of them (Gemini, Taurus, Cancer, Virgo and Leo) are at the back from north to south. Aries and Libra face them followed by Aquarius, Pisces, Capricorn, Scorpio and Sagittarius from north to south. It helps in taking readings the moment each zodiacal sign crosses the meridian. The Large Samrat Yantra is based on the same principle as that of the small one but in size it is 10 times larger and more accurate by 2 secs. The sundial is 27.4m high and is still used on the Guru Purnima (full moon day in the month of Jul/Aug) to forecast the onset of monsoon. The Dakshina Yantra is a wall is aligned along the north-south meridian. It is mainly used to observe the position and movement of the celestial bodies when they pass over the meridian. 

The Disha Yantra has only one function and that is to point towards the north. The Unnathamsa Yantra is used to find the altitudes of the astral bodies. Observations can be made round the clock and the sunken steps help in taking readings from any portion of the dial. The Raj Yantra or the King of Instruments is used only once a year to calculate the Hindu calendar, the details of which are based on the Jaipur Standard. To do so a telescope is fixed over the central hole. A bar for sighting is then attached at the back of the instrument. The plain disk is used to record the sightings.The Observer’s Seat belonged to Maharaja Jai Singh and was used for observing the wonders of the sky and universe. The Narivalya Yantra is a sundial with two dials; one facing north when the sun is in the Northern Hemisphere (21 Mar-21 Sep) and the other facing south for the rest of the year.

The Dhruva Yantra helps in finding the position of the Pole Star at night and also those of the 12 zodiac signs. It is based on an entirely different system to what is used today for the same purpose. Traditionally, human breath, approximately of 6 secs was used as the standard unit of measurement. The Small Samrat Yantra is a triangular structure and has a large sundial with quadrants at the borders given in hours and minutes. The arc on the left displays the time from sunrise to midday and the one on the right from midday to sunset. The sundials have been constructed on Latitude 27°N and the reading can be adjusted to the Indian Standard Time (84°N) but that must be done according to the month and the solar position.