What is the connection between Ujjain and the Tropic of Cancer?

The Tropic of Cancer passes through the region of Ujjain, which gave the city special cosmological significance in ancient Hindu astronomy. The Vedha Shala (Jantar Mantar) observatory was established here partly because of this geographic position. Ancient Hindu astronomers used Ujjain as the prime meridian for calculating the Indian Standard Time meridian and compiling the Panchang (Hindu almanac).