The Astronomical Observatories of Jai Singh c15.s2: Muslim Astronomy

The Astronomical Observatories of Jai Singh c15.s2

Language: English

c15.s2.t1 The Marāgha instruments — 21 parts

Muslim Astronomy

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74. The Muslim astronomers frankly acknowledged their indebtedness to Greek writers. Indeed they were to some extent the direct successors of the Greeks in intellectual matters, and the historical problems of their astronomy are much less complicated than is the case with the Hindus. In the middle ages they were the foremost astronomers of the world. They accepted the fundamental features of the Ptolemaic system of the universe. They were aware of the precession of the equinoxes, and discovered the slight movement of the apogee of the sun, and also they perceived the variation in the obliquity of the ecliptic. They discussed the possibility of the earth rotating on its own axis, but generally rejected the theory.

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They fully realised the necessity for methodical observation, and, in practical astronomy, they excelled the Hindus and Europeans of their time.¹ The first series of regular observations, with the aid of fairly accurate instruments, appears to have been made at Gondeshāpūr, in the south-west of Persia, in the first years of the ninth century of our era. During the Califiate of al-Ma'mūn (A.D. 813—833), at the observatory at Baghdād, all the fundamental elements of the Almagest were verified—the obliquity of the ecliptic, the precession of the equinoxes, the length of the solar year, etc. A measurement of an arc of the meridian in the region of Palmyra was also carried out during the same period, and similar observations continued to be made throughout the Muslim world until the middle of the fifteenth century. The observatory at Cairo was founded in the tenth century, and the observations there were recorded in the ‘Ḥākimid Tables’ (al-zīj al-Ḥākimī). In Persia an observatory was founded, in A.D. 1074, at Nishāpūr, and there, in A.D. 1118, al-Khāzinī complied his ‘Sanjaric Tables’ (al-zīj al-Sanjari). In 1259 a great observatory was founded at Marāgha in North-West Persia, and there Naṣīr al-Dīn-Ṭūsī (mentioned by Jai Singh), published his famous ‘Ilkhānic Tables.’

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¹ It is related that the works of Āryabhaṭa and Brahmagupta were introduced into Baghdād in the ninth century of our era, and that these works possibly had some influence in directing the scientific study of astronomy by the Arabs; but the 'spirit' of the Arab astronomy is entirely different from that of the Hindu.

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With Ulugh Beg, the grandson of Tamerlane, the study of scientific astronomy throughout the Islāmic world ceased. He founded a large observatory at Samarqand, to which he summoned such renowned astronomers as Jamshid al-Kāshī (mentioned by Jai Singh), Kāḍī Zāde, al-Rūmī, ‘Alī al-Qūsjī, and others. He undertook a complete revision of the catalogue of the stars—based upon direct observation—and himself wrote a preface to the tables, a few months before he perished by an assassin's hand. Jai Singh professedly followed Ulugh Beg in his astronomical work.

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The names of many Muslim astronomers of the middle ages, such as Ibn Sīnā or Avicenna, al-Bīrūnī, ‘Omar Khayyām and Averroës, are familiar to everybody.

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75. The practical view taken by the Muslim astronomers led to attempts to improve the instruments in use, and to the design of others.

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.

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The Marāgha instruments were—

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(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