Commissioned by the Second Maharaja Jai Singh from the years 1727-34. While there are actually other similar structures, most notably those at Delhi (which also happened to predate the Jaipur ones), the Jaipur structures are held to be the most worthwhile to see for tourists because they are so remarkably well-preserved and are also among the biggest.
The name “Jantar Mantar” translates fairly closely to “tool of calculation”, as indeed would be fitting for an instrument designed for the purpose of performing Jyotisha. Jyotisha is what we currently known as Hindu astrology, an ancient and much-revered art that ramifies into the schools of Siddhanta (astronomy), Samhita (mundane astrology), and Hora (prediction-inclined astrology). Even to this jyotisha is a very important part of Indian life in most of its aspects, and the same might be said of its status back in the days of Maharaja Jai Singh II.
A visit to the site is certain to stun you at first glance. The sheer size of the instruments (which are architectural marvels in themselves) is breathtaking. Take for instance the splendid Samrat Yantra, which is very aptly named indeed (its name translates to “the Supreme Instrument”). This huge sundial is in fact recorded as the largest in the world, and has been measured as having a 27-metre height. There is at least one other giant sundial here that would rob most of the other sundials in the world of their glory.
Yet what is most remarkable about the instruments spread throughout the Jantar Mantar—and it bears noting that there are no fewer than fourteen of them—is the precision of their measurements. Granted, there have been changes overtime and refinements in current astronomic technology that overshadow their precision nowadays, but for the time in which they were constructed, these tools were superb and far ahead of many others indeed. Shifts in the land and subsidence has put off the accuracy of several of the instruments at the present time, unfortunately, but several of them are still quite marvellously spot-on or close to it. The Samrat Yantra itself is a perfect example. This sundial actually has a shadow that moves by precisely a millimetre each second, so visitors may watch this huge clock, which in its own way is no less impressive than the English Big Ben clock tower, tell what time it is in Jaipur to within 2 seconds of accuracy.
Some conservationists and state experts have noted that there have been some signs of damage to the structures over time, largely due to the fact that they are exposed to the elements (which now include the haze of pollution that modernisation and commercialisation have brought to the city). There have been some renovation projects on the Jantar Mantar in the past, fortunately, and it is undeniably one of the better-preserved sights in the city. It is so well-known throughout the world, in fact, that it has been named a UNESCO World Heritage Site, as well as a national monument of India.