The Jantar Mantar are a series of large observatories in India. Built by the Maharajah Jai Singh II (1688-1743) they were intended to predict eclipses and other astronomical events. There are four surviving Jantar Mantar and I saw three of these on my holiday, in Delhi, Jaipur and Varanasi.
The large scale of the instruments allows them to take accurate measurements. In Delhi it's possible to walk on and around the structures, which feature some vertigo-inducing stairways. It's impressive to walk through buildings designed to measure time.
I'd been warned that the guides at the Jantar Mantar were not particularly reliable. One man followed me around, giving explanations of the instruments in the hope I would hire him. It's been a long time since I studied astrophysics, but I remembered enough to know he was making it up.
The photographs show some of the instruments. The first four photos show the Jantar Mantar in Delhi. Below that are two photos of the cream-coloured structures in Jaipur. At the bottom is a photograph of the instruments in Varanasi. Significantly smaller, these are housed on a single rooftop.