In addition to very many curiosities offered by this country, Rajasthan includes several old observatories of which the most renowned is Jaipur. It is undoubtedly wise, here, to point out why these observatories were built.

November 3, 1688, was born the Maharajah Sawai Jai Singh II. He ascended the Throne of Amber in 1699. At 14 years, he shone already in his military career. Later, his life was devoted entirely to his love of the sciences and the arts. Thus, he gave up the Amber Throne after having set up his palatial city: Jaipur (this new city became independent and the capital of the state of Rajasthan).

In 1719, in Delhi, in the public courtroom of the Red Fort, an animated meeting took place. The mogol emperor Mohammad Shah and Maradjah Sawai Jai Singh II were witness to a court debate, and the discussion was extremely animated so as to establish certain astronomical calculations.

The situation was that the emperor was to leave for a long expedition, and the best day for departure had already been decided on. The controversy came about because of the positions of certain planets, which were supposed to influence the life on ground. No astronomical observatory existed to check calculations. The debate finished in an inconclusive way, but planted an idea in the head of the Amber Maharajah who had studied and acquired good knowledge in astronomy and mathematics. Sawai Jai Sîngh II decided to build astronomical observatories.

Starting in 1724, the first observatory was completed in Delhi, the place where the controversy began regarding planetary positions. Thus, India began to appreciate for the first time instruments of masonry and massive stone. The Maharajah astronomer continued to carry out experiments and observations for nearly seven years, during the time that the observatories were constructed. Thereafter, he reformed the imperial calendar and established the value of ecliptic obliqueness as 23° 28 '.

Sawai Jai Singh II became immortal thanks to the work completed after these stone observatories were set up: Delhi, Mathura, Varanassi, Ujjain and Jaipur.

With these creations, Sawai Jai Singh II made studying astronomy and astrology easier. His observatories were used as laboratories, where one could test oneself on various calculations and check them by practical observations. Formerly, these observatories were used to accommodate academic seminars, conferences, discussions, and, especially, to prepare ephemeris tables, calendars and almanacs.

Jaipur was the only observatory that our group visited. Our full schedule did not allow us time enough to visit the Delhi Observatory.

The Jaipur Observatory was presented to us by an Indian guide who, knowing we were interested in astronomy, did not hesitate to give us all his time: it was a great joy for us.

The site, located at an altitude of 430 meters, occupies a longitude of 75° 49 ' 9 " east and a latitude of 26° 55 ' 27 " north. This observatory is largest, and was built after the Delhi Observatory was successfully tested by the Maharajah in 1724. Construction was completed in 1728, except for the final touches, which still required a few years. The observatory is very close to the Royal Palace, allowing the Maharajah astronomer to carry out his observations when he wanted to.

Although located at the heart of a turbulent city, the observatory appears bathed in a calmness and tranquility. The monument is very well preserved, and was also restored in 1901. Since the independence, it has become a national monument. Known under the name of Jantar Mantar (Jantar = instrument; Mantar = calculation) it is regarded as the largest stone observatory in the world.

Instruments of Jaipur. (Click on the images to enlarge them)

The small equatorial sundial,made of red sandstone and white marble, appears as a triangle placed on the local meridian line, with two quadrants on each side which have a slope in the plane of the celestial equator. The quadrants are graduated in size to represent the hours.

The pole star instrument is located on the eastern side, close to the equatorial sundial. Built out of red sandstone, this apparatus indicates celestial north and helps to locate the pole star " Dhurva Tara ".

The hemispherical sundials are made on the plane of the celestial equator. The northern face functions like a sundial between the vernal and autumnal equinoxes. The southern face functions between the autumnal and the vernal equinoxes.
Picture, far left: the sundial, southern face.
Opposite picture: the sundial, northern face.

The horizontal sundial is visible only if one goes up higher than the preceding instrument. The hemispherical sundials can check the hour read here. Let us specify that this dial has the advantage of functioning during the days of equinox, contrary to the hemispherical sundials.

The astrolabe or Yantra Raj (which means " the king of all the instruments ") results in a carefully engraved celestial map.

The gigantic equatorial sundial is, in principle, identical to the first dial referred to above, but it is five times larger. The base of its right triangle measures 44 meters, the hypotenuse, tilted at 27°, rises up 27 meters. The triangle is flanked by two enormous quadrants of 15m long rays, graduated in hours, minutes and seconds. The shade of the gnomon moves approximately 4 meters per hour, that is to say 6 cm per minute, which is itself divided into thirty fractions with an incredible accuracy of two seconds! A detail that the designer had perhaps not thought of, is the result of shade diffraction. Indeed, the shade is not limited by a clear line but by a broadband of ten centimeters of half-light. Therefore, any precision in the reading of the hour would be lost, were it not for the imagination of the Indian astronomers. They could nevertheless read with precision, by using a fine needle, wisp of straw, or other object. From these objects, the umbra, mixed with the penumbra, makes it possible to determine the hour exactly. (To know more about this method, contact the author). This large sundial is certainly the largest and most precise in the world.
The weather forecasting viewpoint is located at the very top of the giant dial. The astrologers and the pundit's astronomers went there until sundown, to learn the direction of the wind and, consequently, time, using a flag of muslin.

The armilliar sphere instrument is located between the southern sundial and the zodiac instruments. The concave hemispheres are approximately 5,50 m in diameter. The white marble plates each constitute one hour of observation. To compensate for the space between two plates, there is a plate on the other sphere, thus, the two spheres function alternately.

The instruments of altitude/azimuth are built in masonry and stone. They are similar but are made to function in alternation. Made of twelve columns and an identical number of horizontal plates, these instruments represent the celestial sphere. Reading the shade on a column or a horizontal plate makes it possible to know the height and the azimuth of the Sun.

Other instruments, no less interesting, appear at this astronomical site. This is the case of the ecliptic instrument, the circular instruments, the meridian wall instrument, etc.

If you go to India, don't miss Jaipur!

M. Audejean.

Thanks to Geospace d'Aniane. Thanks to Liz Berzin, who helped out with the English translation.