Literally known as 'Instruments for measuring the harmony of the heavens', the world-renowned astronomical observatory, Jantar Mantar is one of the five astronomical observatories built in the 18th century by Maharaja Jai Singh II. This open-air observatory houses a dozen-odd metal and masonry structures and 18 different Yantras used for astronomical calculations. Each instrument is used for a particular function and gives an accurate reading. Samrat yantra, Dhruva yantra, Narivalya yantra, Shasthansa yantra, Kranti yantra, Raj yantra, Unnsyhsmsa yantra, Disha yantra, Dakshina yantra, Large Samrat yantra, Rashivalayas yantra, Jai Prakash yantra, Chakra yantra are some of the yantras placed in the observatory. A guide is available from the main gate who will tell a detail description about these yantras. The observatory is opened from 9 am to 4.30 pm and there is a small entry fee.