Ujjain enjoyed a position of considerable importance in the field of astronomy. Great works on astronomy such as the Surya Siddhanta and the Panch Siddhanta were written in Ujjain. According to Indian astronomers, the Tropic of Cancer is supposed to pass through Ujjain. It is also the fist meridian of longitude of the Hindu geographers. From about the 4th century BC, Ujjain enjoyed the reputation of being India’s Greenwich.