Maharaja Sawai Jai Singh II ruled the state of Jaipur in Rajasthan, India from 1699 to 1743. In 1728 he founded the capital city of Jaipur about 200 km southwest of Delhi. As a scholar he read the works of Ptolemy, Euclid and Persian astronomers.
Wanting to improve the Indian calendar and the ability to precisely locate the Sun, for purposes of map making, he built five astronomical observatories in India, at Delhi, Jaipur, Varanasi (or Banares), Ujjain and Mathura. The instruments at these observatories, based on Moslem design, perhaps copies of the large 15th century instruments at Samarkand, Uzbekistan built by Ulgh Beg, were large masonry structures equipped with protractors and marked grids to aid in the precise measurements of the location of celestial objects.
Observatory at Jaipur
The Jantar Mantar, or “House of Instruments” at Jaipur is the largest of the observatories and contains eighteen instruments. They are in excellent repair after a 1901 reconstruction project supervised by Chandra Dhar Sharma Guleri.
The largest instrument is the Samrat Yantra or equatorial sundial. It consists of a straight 90 foot high ramp, or gnomon, which is aligned north south and elevated at an angle of 27 degrees above the northern horizon. The latitude of Jaipur is 27 degrees north, thus the ramp points toward the celestial pole. On each side of the ramp there are two quarter circles, or quadrants, fashioned in masonry. The quadrants are centered on the nearest edge of the ramp. The Sun casts a shadow of the ramp edge on one of the quadrants; before noon the shadow is cast on the quadrant to the west of the ramp, and afternoon it is cast on the eastern quadrant.
The local solar time can be read from markings on the quadrant at the edge of the shadow. With careful measurements one can measure the local solar time to a precision of a few seconds. There is a stairs up the ram and one can move a stick up and down the edge of the ramp until the shadow of the stick falls on the appropriate edge of the quadrant. Markings on the ramp can then be read, at the location of the stick, which give the celestial longitude of the Sun. Repeated determination of the longitude of the Sun allows one to determine the time that the Sun crosses the celestial equator and thus the date and time of the equinoxes.
One of the more interesting instruments is the Jai Prakash Yantra. It is claimed to have been invented by Sawai Jai Singh II. One part of the instrument is a hemisphere, fashioned of mortar, sunk in the ground, sliced with passages so that the instrument reader can be close to the remaining segments of the hemisphere. The second part of the instrument is another hemisphere, again fashioned of mortar, sunk in the ground, sliced with passages to compliment the first instrument.
The parts of the hemisphere in the first instrument which are missing because of the passageways are present in the second and visa versa. The hemisphere surfaces are of marble and are scribed with celestial latitude and longitude lines. A small marker is suspended at the center of each hemisphere by wires.
The location of the shadow of the sun, and thus the celestial coordinates of the sun, can be read from the markings on the hemisphere segments. If the shadow happens to fall in a passageway in one instrument is will fall on the marked hemisphere segments in the other instruments.
There are a number of sundials; one is a beautiful vertical sundial. There is also a smaller equitorial sundial. The large equatorial sundial has a gate across the steps leading up the gnomon but the smalleer equatorial sundial is accessable.
For the astrnomically more advanced reader: There are also twelve smaller instruments similar in design to the equitorial sundial. They are unique in that they are constructed to help read the ecliptic coordinates of celestial objects. Because the ecliptic pole is not a fixed point relative to an observer on the rotating Earth, one can not align a single gnomon to always point to the ecliptic pole. The angle of the gnomon would have change with time to point to the ecliptic pole. As an approximate solution to this problem Jai Singh built twelve “equatorial” sundials with differing gnomon slopes. Thus at any one time one could use the dial with the angle closest to the true angle of elevation of the ecliptic pole and use that dial to read the ecliptic coordinates of celestial objects. This would be particularly valuable for tracking the Sun and planets. There is one instrument for each sign of the zodiac.
There are two large masonary instruments built to help read the altitude and azimuth of celestial bodies. Each is a circular arrangment of tall rectangular pillars with markings up their sides to read altitudes. The circle has degree markings around its circumfrance to help read azimuths.