Jantar Mantar is one of the five observatories built by Maharaja Jai Singh II. (The others were built at Varanasi, Mathura, Ujjain, and Delhi.) This one, built in Jaipur, is the most well-preserved and extensive of the five. The observatory largely consists of sun-dials that use shadows to mark the time or describe the movement of the Earth relative to the Sun. It’s impressive how accurately one can tell the time by these sun-dials.
Apparently, the devices have some astrological significance / role as well. [I didn’t ask.] Our guide asked whether: “I believed in astronomy and astrology.” Which is a question that sounds a lot to my ear like, “Do you believe in gravity and ghosts?”
Jantar Mantar is one of Jaipur’s most impressive sights.