Ujjain has been India’s Greenwich since the 4th century BC, and this simple-looking but surprisingly complicated observatory was built by Maharaja Jai Singh between 1725 and 1730. Each of its five structures (for tracking celestial bodies and recording time) has quite detailed explanations in English, but you'd need to be an astronomer to understand them!

Jai Singh also built observatories in Jaipur, Delhi, Varanasi and Mathura, but Ujjain’s is the only one still in use.