Constructed by Sawai Raja Jai Singh, the ruler of Amber, between 1725-1730 AD, Jantar Mantar was built to help Hindu astrologers and scholars with their studies and research; particularly the collation of astronomical tables and the prediction of movements of the sun, the moon and the planets. It is one of the five observatories that were constructed in India during the 18th century by the raja; the other four are in Delhi, Mathura, Jaipur and Varanasi. The original instruments installed here with the help of Raja Jai Singh include the Sundial, Nadi Valay Yantra, Transit Instrument, Digyansa Yantra and Shanku Yantra. There are plaques and notice boards all over the place carrying information in both Hindi and English to explain the workings of several instruments. Also known as Ved Shala, the building houses masonic instruments that are still being used to carry out research. The observatory is an architectural marvel and is a must-visit in the city.