Jantar Mantar

"Jantar Mantar, an observatory built by King Jayasingh of Jaipur in 1737." Jai Singh was a great admirer of science and technology and had a special love for astronomy. In Varanasi, Jantar Mantar was built with the observatories of Delhi, Mathura, Ujjain and Jaipur. It was built to determine the local time, the inclination of the sun, height from sea level, inclination of planets and stars and eclipse. In Jantar Mantar of Varanasi, there are many tools for measuring the movements, the speed and the properties of the planets, and accurate study of astronomy, which can still be used efficiently today.
Jantar Mantar was at the top of a building near Man Singh Ghat. The Jantar Mantar is similar to the ones in Jaipur and Delhi. On the ground floor there is are a few instructions, videos etc provided by Archeological Survey of India. You can read them and use the yantras to tell the time as well as planetary positions. It's called the Man Mandir observatory and is managed by the Archaeological Survey of India.