The Astronomical Observatories of Jai Singh c15.s2.t1: The Marāgha instruments

The Astronomical Observatories of Jai Singh c15.s2.t1

Language: English

The Marāgha instruments

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A brief description of the instruments used by Naṣīr al-Dīn-Ṭūsī at the observatory at Marāgha is available.² The theory of these instruments was probably known to Jai Singh (see p. 4).

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² For an account of earlier instruments see al-Battānī, Opus astronomicum, LVI-LVII. Ed. C. A. Nallino.

(1) A quadrant, or mural circle, constructed of wood, the radius of which was about 11 feet. The arc was of copper, 3 inches wide, and was graduated. At the centre was a copper pin, round which the alhidade (furnished with two sights) turned. The alhidade terminated in a point, and was moved by a cord passing over a pulley attached to the wall.

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(2) An armillary sphere of five circles, viz., the ecliptic, colure, great circle of latitude, the meridian, and the small circle of latitude. The ecliptic, meridian and small circle of latitude were graduated down to minutes, and the last mentioned was furnished with an alhidade, or sighter. A sighting tube appears to have been used on the alhidade (see figure 68).

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(3) A meridian circle of about 11 feet in diameter, furnished with an alhidade.

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(4) An equatorial circle fixed in the meridian.

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(5) An instrument for measuring the diameter of the sun or moon. This consisted of two sights fixed on a bar. The objective was pierced with a comparatively large hole, and was moveable along the bar, over appropriate graduations. Special discs (like camera stops) were used with the objective.

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(6) The instrument of two pillars. At the centre of a bar supported by two pillars a pin was fixed, round which revolved an alhidade, or sighter, 12 feet long. Vertically beneath this pin was another, to which was fastened a graduated bar, 17½ feet long, and along this bar the end of the sighter was free to run. This appears to have been a modification of Ptolemy's parallactic rulers.¹

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¹ The graduated bar, of course, measures the chords of the arcs, and "as it was much easier to graduate a straight line than an arc, the triquetrum continued to be the favourite instrument down to the end of the sixteenth century." (DREYER, p. 332.) This is the Zāt al Shu' batayn mentioned by Jai Singh (see page 12).

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(7) A large azimuth circle fixed on a pillar, traversed by two diameters directed to the four points of the compass. At the centre were fixed two vertical quadrants, furnished with alhidades, so that the altitudes and azimuths of two stars could be taken simultaneously.²

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² Nos. 7 and 8 are interesting to us, as being the same in principle as the Rām Yantra (see page 37).

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(8) Sine and azimuth instrument. An azimuth circle, similar to the preceding, but in place of the vertical quadrants were two bars, moving in a groove, and supported by two other bars perpendicular to them.

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(9) Similar to No. 6, but with the bars horizontal, for measuring azimuths.

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The notion of increasing the size of the instrument as far as possible has already been referred to (page 35). It is perhaps to Abu'l Wafā that we owe some of the immense instruments which the Arab works mention. With a quadrant of over 20 feet radius the obliquity of the ecliptic was observed in A.D. 995. The sextant of Abu M. al-Khojendi (C. A.D. 992) was of nearly sixty feet radius. In the tenth century the aperture dial was used, and Naṣīr-al-Dīn, by utilising a hole in the dome of a large building, obtained excellent results. According to Greaves the quadrant used by Ulugh Beg was 180 feet high.⁴

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⁴ See L. P. E. A. Sédillot, Prol. Tab. Astr. d'Ouloug Beg, pp. lxxx, xcviii, cxxix.

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The astrolabe,⁵ the theory of which was due to Ptolemy, was improved by the Muslims, almost to perfection, and many of these instruments were so beautifully made, as to become valuable works of art, as well as efficient calculating machines, and useful instruments of observation. As a portable instrument of observation it was only superseded about A.D. 1731 by Hadley's quadrant; for purposes of astrological calculations the astrolabe is still in use in the East. The flat astrolabe has already been described in some detail, as also has al-Zarqālī's modification of it, and al-Bīrūnī's invention has also been referred to (p. 37). There is also the linear astrolabe, or *‘aṣā ’l Ṭūsī* (the rod of al-Ṭūsī),¹ called *yaṣhṭi* by the Hindus. Great ingenuity was exercised in devising improvements and variations of the astrolabe, and there are numerous Arabic and Persian works describing the theory and construction of the instrument. The term al-Aṣṭurlābī, as a name suffix, was not at all uncommon.²