Jantar Mantar - Largest Stone Astronomical ObservatoryJantar Mantar was Founded by Maharaja Jai Singh in between 1727 and 1734. Meaning of the Name is Jantar (instrument) Mantra( formula or calculattion). Jantar Mantar is the Largest Stone Astronomical Observatory in the World. Initially Jantar Mantar was named as Yantra Mantra, which means instruments and formulae, but due to some reason it was changed.Jantar Mantar is wholly constructed with stone and marble. The observatory has fourteen statistical instruments for measuring time, predicting eclipses and to ascertain other astronomical events.