The Astronomical Observatories of Jai Singh c16: Chapter XIII.—The Evolution of Jai Singh's Instruments
The Astronomical Observatories of Jai Singh c16
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Chapter XIII.—The Evolution of Jai Singh's Instruments
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79. The history of the evolution of Jai Singh's astronomical instruments would, if it could be recorded, step by step, be of great interest; but detailed descriptions of the larger Arabic instruments are not generally available, and we must, for the present, be content with general indications of the lines of development. Generally speaking, Jai Singh's instruments are copies of, or direct developments from, those used by Ulugh Beg and his predecessors and successors. The flat astrolabe played an important part. Jai Singh's first attempt at improvement was apparently the construction of huge astrolabes, such as those shown in figures 28 and 29, and the construction of large graduated circles. He had some excellent Arabic and Persian models (figures 5—20) but the metal instruments he had constructed do not appear to be of that fine workmanship that adds so much to the value of many of the mediæval astrolabes. As far as can be gathered Jai Singh did not use the ordinary sextant and quadrant instruments, such as were used by Naṣīr al-Dīn Ṭūsī, Tycho Brahe, Flamsteed, and others.
c16.2
It has been related how he discarded brass instruments and pinned his faith on large immoveable masonry instruments; and some of these he claims to have devised himself. As has already been pointed out (pages 12, 13), the basic idea was not peculiar to Jai Singh. The Arab, Persian and Tartar astronomers had constructed huge instruments; and they had formulated the notion, that the only bar to accuracy of observation was the limit imposed by circumstances on the size of the instruments. Jai Singh was prepared to carry out the idea, on which this proposition is based, to, what he thought, a reasonable extent.
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80. The bases of the designs of the particular instruments are always obvious, but Jai Singh showed very considerable ingenuity in the actual constructions. The Jai Prakāś is practically the hemisphere of Berosus, somewhat elaborated, and the Samrāṭ Yantra may also be considered as evolved from that instrument. This only means, however, that the dial of Berosus was of a very general nature. It consisted of a hemispherical bowl, placed with its rim horizontal, and in the centre and in the same plane as the horizontal edge, was fixed a bead, whose shadow on the concave surface of the hemisphere marked the trace of the sun's diurnal path. The resemblance to the Jai Prakāś is striking enough, but it is doubtful whether Jai Singh had any knowledge of the earlier instrument: he could only have learnt of it from the Muslim astronomers (e.g., al-Battānī, who refers to the principle of the instrument). The Jai Prakāś, however, is something more than the bowl of Berosus, for it is fully graduated, and appears to have been based upon the Muslim instrument known as al-Masāṭirah, descriptions of which are found in the works of the Muslim astronomers.¹
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¹ See L. A. Sédillot's *Memoire sur les instruments astronomiques des Arabes*, p. 151f.; also a description by al-Barjendi; also Blagrave's *Art of Dyalling*, 1600. One section of Blagrave's book is "How to make a dyall on a concave hemispheare of a globe two severall wales," and the second way is that of the Jai Prakāś.
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The Samrāṭ Yantra might be considered as a section in the plane of the equator of the hemisphere of Berosus, and with the bead extended into a line parallel to the axis; but so could any dial be referred to the same origin. In the British Museum are many dials of the seventeenth and early eighteenth centuries constructed exactly on the same principle as the Samrāṭ.¹ The Samrāṭ instrument is an equal hour instrument, or equinoctial, as such instruments are often called. The evolution of the equal hour instrument is of considerable interest. In early instruments the time from sunrise to sunset is divided into 12 equal portions. These portions of time vary in length from one-twelfth of the longest day to one-twelfth of the shortest day. They were therefore called unequal hours, also temporal hours and planetary hours (see page 22). Of these varying hours, naturally, the equinoctial hours were the mean, and, on the introduction of mechanical clocks, the equinoctial hour became the standard for sun-time measurements. According to Delambre, Abdul Ḥasan (al-Ḥasan b. ‘Alī b. ‘Omar al-Marrakōshī, Abū ‘Alī) was the first to introduce the constant hour notion among the Muslims; but he seems to have employed, in the usual manner, the horizontal plane for the shadow traces, while Jai Singh’s instrument receives the shadow of the inclined gnomon on a circular arc lying in the plane of the equator, and, thereby, secures, in the simplest manner, equal hours throughout the year. The direct origin of the tangential scales on the gnomon, for measuring the declination of the sun, is not known; although Ibn Yūnus, and other Muslim writers on astronomy, had worked out the theory.² Hindu astronomers did not employ tangent scales, and refer to no other dial than the vertical gnomon, and to no other dial measurements than the length of the shadow. They made no direct angular measurements, and an angular dial would have been almost contrary to the spirit of their teaching.³ The only other instruments that can be attributed to Jai Singh’s genius are the Digamśa Yantra and Rām Yantra, but these are simply enlargements, in masonry work, of the azimuth and combined azimuth and altitude instruments of the Muslims.
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¹ e.g., a pocket dial made by Elias Allen about A.D. 1620; a more elaborate dial invented by John Paul Kraus and engraved by T. G. Gutwein; one by Rugendas of Augsburg, 17th century; one by Laurenz Grassl of Augsburg; etc., etc.
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² Indeed they worked out the complete theory of the horizontal, vertical, inclined, cylindrical and conical dials, etc.
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³ This is a curious point in the history of science. The Hindus seemed to avoid direct angular measurements. Their mathematical works contain no theorems or rules relating to angles (see my Indian Mathematics, page 20). Whitney wrote (page 259): "Lest it seem strange that the Hindus should have derived from abroad the name (*koṇa* from *γωνία*) for so familiar and elementary a quantity as an angle, we would direct attention to the striking fact, that in that stage of their mathematical science, at least, which is represented by the *Sūrya Siddhānta*, they appear to have made no use whatever, in their calculations, of the angle."
c16.9
An indicator of the course of evolution of Jai Singh’s instruments is still to be seen at Benares, on the instrument known as the Chakra Yantra. The wedge (*faras* or ‘horse’) which fastens the parts of this instrument together is of the traditional Arabic design.
Linked texts (1)
- Sūryasiddhānta — Maya, Sanskrit