Jantar Mantar, Jaipur

The Jantar Mantar in Jaipur is the largest and best-preserved of the five astronomical observatories Maharaja Sawai Jai Singh II built between 1724 and 1735 (the others are at Delhi, Ujjain, Mathura, and Varanasi). Completed in 1734, it holds 19 large masonry instruments — sundials, equinoctial dials, hemispherical dials, declination instruments, and a celestial coordinate system — designed to be read with the naked eye to a precision matching contemporary European telescopic observations. The Samrat Yantra ('Supreme Instrument') is the showpiece — a 27-metre-tall right-triangular sundial whose gnomon casts a shadow that moves visibly across the calibrated arc at roughly 1 mm per second; it can tell the time to an accuracy of two seconds. Other notable instruments include the Jai Prakash Yantra (a hemispherical bowl-dial calibrated for solar/lunar position), the Rama Yantra (cylindrical instruments for measuring altitude/azimuth), and the Disha Yantra (a compass-and-meridian instrument). UNESCO inscribed the site in 2010. The instruments are functional — visitors can verify the local solar time against their watch. Allow 90 minutes; the included audio guide is excellent. The observatory is adjacent to the City Palace.