The Jantar-Mantar is a cluster of 18th century astronomical instruments that formed an observatory for observing the celestial skies in Delhi. These were built by the Maharajah of Jaipur Jai Singh II. Today these buildings sit as an ochre and green oasis in the centre of New Delhi.
The photo below is that of the Ram Yantra, a cylindrical building that is used to measure the movement of stars.

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