Jantar Mantar or Solar Observatory of New Delhi is one of the 5 observatories built by Maharana Jai Singh II of Jaipur. Constructed in the year 1972, its basic reason was to see Calendar dates and astronomical tables. The instruments in Jantar Mantar were built to tell exact time and movements of Sun, Moon and other Solar Planets. It was also used to give astronomical predictions as it is said that Maharajah Jai Singh II himself was a great and renowned astrologer of his time.
Jantar Mantar of New Delhi has basically 3 instruments: Samrat Yantra, Jayaprakash Yantra and Misra Yantra.
Samrat Yantra or the Supreme Instrument: basically is a giant triangle representing a Sundial.
Jayprakash Yantra : This was used to align the position of stars with various markings.
Misra Yantra : It was designed to determine the shortest and longest days of the year. It was a great instrument as it could easily determine the longest and shortest days of the year of various cities and locations regardless of their distance from Delhi! Interestingly, this was the only instrument which was not invented by Maharajah Jai Singh II.