Jantar Mantar Observatory
Palace Complex, Jaipur
A brief Introduction to the observatory is followed by notes on Jai Singh, the meaning of the name "Jantar Mantar," and Additional Resources for further exploration. A selection of instruments is described in the following pages: Laghu Samrat Yantra, Chakra Yantra, Kapali Yantra, Ram Yantra, Narivalaya Yantra, Jai Prakash Yantra, and Rashivalaya Yantra.
Jai Singh's astronomical observatory may look today like a giant playground for grownups, but in 1734, the year of its completion, it was the last outpost of medieval science. The observatory's eighteen fixed observational instruments are sighting devices that measure the position of the sun, stars and planets. Some are built entirely of masonry, others are engraved metal rings and plates set into masonry foundations.
During the day, masonry sundials cast the sun's shadow on a suitably engraved scale. A sundial has two functional parts: a gnomon, which is the part that casts the shadow, and a scale, from which the measurement is read. For example, in the instruments in the photo, the gnomon is an inclined ramp, and the scale is engraved on the circular part below the ramp. The sun, to the right, casts the gnomon's shadow on the curved scale to the left.
Metal instruments are used for nighttime observations. They consist of a small sighting tube attached to a circular ring or plate which can pivot in various directions. They are operated by aiming the sighting tube directly at a planet or star, and then reading off its position from scales on the body of the instrument.
Some instruments could be used for both daytime and nighttime observations. More information about this and other aspects of the observatory can be found in the Additional Resources below.
For all this to work, the position and orientation of the instruments and the calibration of their scales had to be minutely exact. The devices were built large, because the larger the scale, the more accurate the measurement. Once built and calibrated, they were fixed in place, could not be moved, and contained no moving parts (except of course for the pivots of the sighting instruments) or lenses. This restricted the kinds of observations that could be carried out, to those involving the positions and motions of the heavenly bodies which are visible to the naked eye.
Such observations are no different in principle from those carried out in ancient Babylon, although they are considerably more accurate, and some of Jai Singh's instruments are original in design. Basically, however, this is how astronomy was done in early Mesopotamia, Egypt, Greece, China, and everywhere in the world, from the dawn of civilization down to the end of the Middle Ages.
The projects carried out here included calculating the lunar calendar, predicting the start of the monsoon season, and creating astronomical tables. However, the observatory's main purpose seems to have been casting horoscopes, which requires a precise knowledge of the positions of the sun, moon, planets, and stars at the moment of birth.
Because of the size and careful construction of these instruments, their accuracy was impressive by any standard. However, devices of this sort are expensive to construct. Once built, they can not be corrected or improved, and the kinds of observations they can make are limited, in the ways previously mentioned. Because of this, the instruments preserved here were conceptually obsolete even before their construction. They were soon overtaken in both usefulness and accuracy by the smaller machined brass instruments and telescopes of the modern era. Their lasting value is the tangible record they carry, a summing-up in mortar and stone of 2,500 years of premodern astronomy.
Sawai Jai Singh, the first Maharaja of Jaipur, succeeded to the throne of Amber in 1700 at the age of thirteen. Abandoning that capital, he founded the city of Jaipur in 1727. A soldier, ruler, and scholar with a lifelong interest in mathematics and astronomy, Jai Singh built observatories in Delhi, Jaipur, Ujjain, Mathura and Benares. Jai Singh was conversant with contemporary European astronomy through his contacts with the Portugese Viceroy in Goa. He supplied corrections to the astronomical tables of de la Hire, and published his own tables in 1723. The good state of preservation of the Jaipur observatory is due first of all to Chandra Dhar Sharma Guleri, who restored it in 1901. It has been well maintained from then to the present day.
Jantar means "instrument." Mantar (the same word as "mantra") is usually translated "formula," but here it means "calculation." So, "Jantar Mantar" means something like "instrument for calculation."
Basic Celestial Phenomena, by Kerry Magruder and Mike Keas. A good introduction to basic observational astronomy including the ecliptic, the celestial equator, and the zodiac.
Jantar Mantar (1996), by Dr. Bonnie G. MacDougall at Cornell U. The Web version of an academic paper that places the observatory in its cultural context.
Astronomical Instruments, from the Jiva Institute, discusses ten of the instruments and their mode of operation.
Astronomical Observatory of Jaipur, by Daulat Singh Rajawat. Delta Publications, Jaipur, India. This book is sold near the observatory and elsewhere in Jaipur. It provides a useful and engaging description of the theory and practice of the observatory from a Vedic point of view.