This UNESCO world heritage site is the largest of five astronomical observatories build by Maharaja Jai Singh during the period 1727-1734 in north India. The observatory consists of fourteen major geometric devices (or yantras in Hindi) for measuring time, predicting eclipses, tracking stars in their orbits, ascertaining the declinations of planets and determining the celestial altitudes etc. There is signage providing elaborate explanations for the use of each device, and guides can be hired to provide much the same information in a more digestible format. The audio guide at the observatory isn't great and doesn't tell you very much more information than the already existent signage. The observatory, the water and the wind palace are free on the 18th of May.