Astronomical Efforts of Sawai Jai Singh
- A Review

Astronomical Instruments

Jai Singh had been interested in instrumentation long before he decided to erect observatories, and as such, he collected literature on the subject. In 1716, he purchased two books on Turīya Yantra (File No. 424/1, Jaipur records, Rajastan State Archives). He initiated his ambitious program of observing the heavens with instruments of brass constructed according to the Persian-Arabic school of astronomy (see Reference Zīj Jadīd Muhammad Shāhī, f. 1). However, the metal instruments did not measure up to his expectations, and he noted with disappointment that their axes soon wore down, displacing the center and shifting the planes of reference. A reason for Jai Singh's difficulty could have been the fact that the technology of fabricating large metal instruments had not yet been developed in India. If the surviving specimens at the Jaipur Jantar Mantar are indicative of his attempts to construct metal instruments, it is obvious that his early instruments were too heavy, with no attention paid to eliminate the excessive weight. They were simply magnified versions of the small medieval instruments of the East that may still be seen in museums around the globe.

Jai Singh discarded his metal instruments in favor of ones of stone and masonry that he himself designed. DeBois, an eyewitness of his efforts, writes that the Raja prepared the models with his own hands (DeBois). Altogether, Jai Singh constructed 13 different types of instruments, ranging from a few cm to 24 meters in height, for his observations at Delhi, Jaipur, Varanasi, Ujjain and Mathura (Sharma, V.N. 1987; Kaye 1918). In 1981- 82, the author spent several months studying Jai Singh's instruments at these locations, and noted that the instruments may be classifies into three main categories, namely:

The instruments such as Dhruvadarshk and Nadīvalaya have been clearly designed with a layman in mind, and are of little value to a researcher. The instruments such as Jai Prakāśa and Rāma Yantra, on the other hand, belong to the medium-precision category.

Jai Prakāśa is a multipurpose device and has a varying degree of accuracy. The Jaipur instrument, for instance, measures time with an uncertainty of ± 0.5 to 1 min and the zenith direction and declination both with uncertainty of ± 3' of arc. The uncertainty in the measurement of azimuth and right ascension could be anywhere from ± 3' to ± 1°.

Despite the fact that Jagannath lauded the Jai Prakāśa as the finest of instruments, its utility is rather limited for the kind of precision Jai Singh had in mind (Sharma, R.S. 1967). The instrument is a good teaching tool nonetheless, displaying the relationship between the local and equatorial systems of coordinates. Its ability to indicate the approach of a sign on the meridian is quite valuable to a Hindu astrologer.

The Rāma Yantra, another of his low-precision instruments, is most sensitive near the 45° marks around the base line of its vertical walls. The theoretical accuracy of the Jaipur yantra is ± 1' of arc for the angles of 40 - 50°. However, due to the finite width of the penumbra associated with the shadow of the pillar, the accuracies of this order are not possible in practice. And as such, a precision of ± 6' of arc is the most that one can expect with this instrument¹. For the zenith angle readings, the accuracies deteriorate at a rapid rate to ± 1° or worse, as the angle approaches zero.

The Samrāta Yantra and the Sasthāmsa both belong to the third category, that of high-precision instruments, and are the two most sensitive instruments indeed. The Samrāta of Jaipur is apparently designed to measure time with precision of ± 2 seconds, right ascension ± 15" of arc, and declination ± 1' of arc. With this instrument, an accomplished observer can measure time with at least ± 3 seconds, and the two angles with ± 1' of arc accuracies, provided the instrument is constructed properly. However, the instruments of Jaipur and Delhi both suffer from a number of constructional defects. As a result, the time measurement accuracy for the Jaipur instrument ranges between ± 10 and ± 90 seconds, depending on the section of the instrument used. Similarly, the uncertainty in declination measurements could be anywhere from ± 1' to ± 15' of arc. With his Samrātas, Jai Singh seems to have measured time with a greater accuracy than the instrument of Jaipur is capable of today.

The Sasthāmsa of Jaipur is the only sensitive instrument of Jai Singh's that still maintains its intended accuracy of ± 1' of arc. In 1981-82, the author took numerous readings with this instrument, and after becoming familiar with the device, his readings began to fall within the intended uncertainty. On December 23, 1981, for example, he measured the apparent declination of the meridian Sun as 23°:25'± 1'. This result compares well with the calculated value of 23°:26' for the day.

Jai Singh did not stop after having designed these two high-precision instruments, but continued on with his search for better ones. Accordingly, he sent a delegation to Europe in 1728 with one of its goals being to learn about the old and new instruments for astronomical observations (Gazeta 1). He apparently intended to incorporate these instruments into his Jaipur observatory which, according to De Bois, was still being expanded at the time (De Bois).

The questions that the Raja addresses to Boudier also reflect his continued interest in instrumentation. "How is the longitude of the moon observed in its off meridian position, and with what instrument?" he wrote to Boudier in 1730-31 (Lettres, pp. 610-11).

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