...As an enthusiast of astronomy, navigation, physics, and architecture (among many others), I had been eagerly awaiting a visit to Jantar Mantar in Jaipur. Literally translated as 'calculation instrument,' Jantar Mantar was built by Maharaja (King) Jai Singh II between 1728 and 1734 to improve scientific understanding of celestial bodies and provide more exact astrological predictions. He built five such complexes throughout Northern India but the one in Jaipur is the most impressive and best preserved.
There are 16 different sets of large instruments here that each serve different purposes or provide different methods of taking the same measurements. Walking through Jantar Mantar is like walking through a sculpture park that also happens to be one of the most incredible science projects on Earth.
Located at 27 degrees North latitude, each of the instruments (except for the instruments dedicated to specific zodiac signs) is inclined at an angle of 27 degrees from level and points directly toward true north. Therefore, if you looked up any of these instruments from the base of the inclination, you would see Polaris (the North Star) seemingly resting on the top of the device.
The Laghu Samrat Yantra is essentially a large sundial that is capable of determining local time within a margin of error of 20 seconds. When we visited, the local time's deviation from India Standard Time was 33 min back. In other words, when the sun was directly overhead in Jaipur and the devices that point to true north cast no discernible shadow to the east or west, a watch would read 12:33 pm India Standard Time.
Unsatisfied with an accuracy of 20 seconds, Jai Singh II had the Samat Yantra ("The Supreme Instrument") constructed. At 27 m (90 ft) tall, it is the largest sundial in the world and is capable of measuring local time to within 2 seconds. The shadow from the Samat Yantra moves at a speed of 1mm per second (6 cm per min) along what is essentially a massive ruler shaped into a half ring and tilted at the same 27 degree angle.
Other devices are used to track stars and constellations, determine the declinations of celestial bodies, and predict events or horoscopes.
The Rashivalaya Yantra is comprised of 12 different devices each at a different angle and orientation. Each of the 12 devices is specifically used to make measurements for one zodiac sign, such as Sagittarius or Taurus, and is oriented and inclined at the appropriate angle for its respective constellation. In combination with other instruments, astrologers could make highly specific (they call it 'accurate') horoscopes using these devices.
The Ram Yantra is an instrument comprised of two cylindrical devices made up of thin wedges that complement each other. The instrument would normally be one large cylinder, but then no one could enter the device to make accurate readings. Thus, the cylinder is cut into wedges and adjoining wedges are separated so that people can walk between every other wedge. The missing wedges are then built into a separate, complementary cylinder just beside the first. Thus, only one of the instruments works at a time. About every 15 minutes or so, the center post's shadow will move off of the measurable surface into one of the spaces where a wedge is missing. Then, the astronomer would have to go to the other cylinder where the center post's shadow had just entered the measurable surface area. It's quite hard to explain, but maybe you can figure it out looking at the photos.
The Jai Prakash Yantra is another device comprised of two complementary hemispheres. If overlaid with one another, the two fractional hemispheres would make one solid hemisphere without any gaps. On the hemispheres is a map of the heavens. It is used to determine the accuracy of other instruments in the complex and determine which zodiac device to use.
The Narivalaya Yantra is comprised of two circles placed at an angle of 27 degrees toward the ecliptic. It is used to calculate time and follow the solar cycle.