What am I looking at?
The Jantar Mantar is a collection of 19 astronomical instruments built by the Rajput king Sawai Jai Singh, the founder of Jaipur, Rajasthan, India.
The quick answer
The Jantar Mantar is a collection of 19 astronomical instruments built by the Rajput king Sawai Jai Singh, the founder of Jaipur, Rajasthan, India. The monument was completed in 1734. It features the world's largest stone sundial, and is a UNESCO World Heritage Site. It is near City Palace and Hawa Mahal. The instruments allow the observation of astronomical positions with the naked eye. The observatory is an example of the Ptolemaic positional astronomy which was shared by many civilizations.
How it came to be
The monument features instruments operating in each of the three main classical celestial coordinate systems: the horizon-zenith local system, the equatorial system, and the ecliptic system. The Kanmala Yantraprakara is one that works in two systems and allows the transformation of the coordinates directly from one system to the other. It has the biggest sundial in the world.
The detail most people miss
The monument was damaged in the 19th century. Early restoration work was undertaken under the supervision of Major Arthur Garrett, a keen amateur astronomer, during his appointment as Assistant State Engineer for the Jaipur District. UNESCO describes the observatory as the most significant and best-preserved historic observatory in India, representing the astronomical and cosmological knowledge of ancient India.
Why it still matters
The name jantar is derived from yantra, a Sanskrit word meaning "instrument, machine", and mantar from mantrana, also a Sanskrit word meaning "consult, calculate". Therefore, Jantar Mantar literally means 'calculating instrument'.
Look closer
Jai Singh noticed that the Zij, which was used for determining the position of celestial objects, did not match the positions calculated on the table. He constructed five new observatories in different cities in order to create a more accurate Zij. The astronomical tables Jai Singh created, known as the Zij-i Muhammad Shahi, were continuously used in India for a century. (However, the table had little significance outside of India.) Also, it was used to measure time.