Observatories of Jai Singh

18th Century Observatories of Maharaja Sawai Jai Singh II

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 Benares), 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 Ulugh 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 large Samyat Yantra, equatorial sundial, at Jaipur.

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.

Drawing of the small equatorial sundial showing the shadow, quadrants, gnomon, and Sun rays.

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 it will fall on the marked hemisphere segments in the other instruments.

One of the hemispherical depressions of the Jai Prakash Yantra. The dark areas are passages.

There are a number of sundials; one is a beautiful vertical sundial. There is also a smaller equatorial sundial. The large equatorial sundial has a gate across the steps leading up the gnomon but the smaller equatorial sundial is access able.

The small equatorial sundial is in the center with the two hemispheres of the Jai Prakash Yantra below it and a vertical sundial to the right.

For the astronomically more advanced reader: There are also twelve smaller instruments similar in design to the equatorial 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 masonry instruments built to help read the altitude and azimuth of celestial bodies. Each is a circular arrangement of tall rectangular pillars with markings up their sides to read altitudes. The circle has degree markings around its circumference to help read azimuths.

See For Yourself

The Jantar Mantar in Jaipur, Delhi and Varanasi are open to tourists. If you make the trip to Jaipur, be sure to buy the book, Astronomical Observatory of Jaipur by Daulat Singh Rajawat, from the sellers near the entrance to the site (35 Rupees). It contains the history of the observatory, diagrams, pictures and descriptions of all the instruments, and even includes the horoscope of Sawai Jai Singh II. It also gives an explanation of the Indian system of measuring angles, 6 ghatis of 60 palas each equals one circle of 360 pales. The book has a table to help you to convert from mean solar time to India standard time, which you will need if you want to set your watch with the equatorial sundial readings you make. The guides at the Jaipur observatory will eagerly show you (for a small negotiable fee) how the small equatorial sundial can be used to read the time of day, but they will probably not explain how you can read the latitude of the Sun. Spend some time, as I did, studying the markings, and then with the help of a friend make your own measurements. I looked at the gnomon shadow, but I suspect that better measurements of the position of the Sun can be made by sighting from the quadrant edge up past the gnomon edge toward the Sun; bring along a appropriate solar filter for this measurement. Do not look directly at the Sun with the naked eye. The large equatorial sundial, Samrat Yantra, at the Delhi observatory has been partly filled in so that the lower portions of the quadrants are unusable. If you have a choice visit the Jaipur observatory.

The palace of Jai Singh overlooking Man Mandir Ghat on the Ganges River. The observatory is on the skyline at the right of the photograph behind the wall.

The observatory at Varanasi is spectacularly located overlooking the bathing ghats on the Ganges; it is next to the Dasaswamedh Ghat, one of the more popular and important ghats in Varanasi. The observatory sits high above the water next to the palace that Jai Singh built there. The instruments are small, compared to those at Jaipur and Delhi, and few in number, but there is a very nice equatorial sundial in working order. It is small enough so that one person can easily make measurements. Even though you are surrounded by tens of thousands of people in and around the ghats below the observatory, you will most likely be alone in the observatory compound. When I was there, in 1995, there was an attendant. There was no charge for admission but the attendant expected a tip for his efforts. The attendant will let you into the large receiving hall built by Jai Singh below the observatory. Strangely not many tourists visit this site. The setting alone makes the trip worth while. (I have noticed that a few guidebooks talk about the Varanasi observatory being in disrepair and imply it is not worth visiting. Wrong!)

This page prepared by C.Hartley, Director of the Ernest B. Wright Observatory at the Department of Physics at Hartwick College in the City of Oneonta, NY. More things from C. Hartley at his home.