Jantar Mantar, Jaipur/Delhi, North India, is an astronomical observatory built by Raja Jai Singh II between 1724 & 1733. This site contains a collection of architectural astronomical instruments built (of stone) to measure time, predict eclipses, predict onset, length & duration of local monsoons, track stars in orbits, measuring latitudes and longitudes of celestial bodies, etc. The Samrat Yantra located here is the world’s largest sundial and can measure time to an accuracy of 2 seconds – Its shadow moves 1mm per second and its movement can be seen! More info. Pics.