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Adjacent to the City Palace is Jantar Mantar, an observatory begun by Jai Singh II in 1728 that resembles a collection of bizarre giant sculptures. Built for measuring the heavens, the name is derived from the Sanskrit yanta mantr, meaning 'instrument of calculation', and in 2010 it was added to India’s list of Unesco World Heritage Sites. Paying for a local guide is highly recommended if you wish to learn how each fascinating instrument works.
Jai Singh liked astronomy even more than he liked war and town planning. Before constructing the observatory he sent scholars abroad to study foreign constructs. He built five observatories in total, and this is the largest and best preserved (it was restored in 1901). Others are in Delhi, Varanasi and Ujjain. No traces of the fifth, the Mathura observatory, remain.
A valid Amber Fort/Hawa Mahal composite ticket will also gain you entry.