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Inventory no. 49687 - Former Display Label

The Spherical Astrolabe

This Eastern Islamic spherical astrolabe of A.D. 1480/1 is the only complete spherical astrolabe known to have survived. Until the discovery of this instrument, and of the sphere only of a maghribî spherical astrolabe, the spherical astrolabe was known only from descriptions by Islamic authors and in the Libros del saber of Alfonso X, el Sabio, of Castile. The spherical astrolabe was used for the solution by analogue computation of the same problems in astronomy, astrology, and horology as the planispheric astrolabe. Its history may be at least as long as that of the planispheric astrolabe (as no knowledge of stereographic projection is required for its construction); it may, therefore, have been known to Hellenistic astronomers, but it is probably an Islamic invention. The first certain mention of the instrument occurs in a work by al-Khwârizmî (d. c.A.D. 850). The first treatise on the subject is perhaps that attributed to Qustà b. Lûqâ (d. c.912). Other Islamic writers on the subject were an-Nairîzî (d. c.922), al-Bîrûnî (973-1048), and al-Hasan al-Marrâkushî (d. c.1262). A detailed description of the construction and use of this type of astrolabe, by Isaac b. Sid, was included in the Libros del saber (1276-7). Though the spherical astrolabe is there called "uno de los buenos estrumentes que fueron fechos", there does not seem to have been much further interest in the subject in medieval Christian Europe. The spherical astrolabe was less convenient in use, and was less robust, than the planispheric astrolabe; this may account not only for the lack of European interest in the instrument but also for the fact that, despite the Islamic treatises on the subject and an-Nairîzî's opinion that the spherical astrolabe was superior to the planispheric, only this specimen has so far come to light.

This spherical astrolabe is signed, on the sphere, 'Work of Mûsà. Year 885 [H. = A.D. 1480/1]'. From the style of the Kufic lettering and the system of abjad numerals used, it is certain that this instrument is not of maghribî origin, but was made in Eastern Islam.

The sphere is of brass with inscriptions, hour-lines, meridians, and the almucantars at 5° intervals damascened in silver; the concentric rete is also of brass, laminated with silver on the ecliptic and equatorial circles, on the vertical quadrant, and, for ornament, near the suspension piece; the suspension piece is of silver.

The rete has pointers for 19 fixed stars, all named and all above the ecliptic. In addition to the star-pointers, the ecliptic circle, and the small circle for equatorial measurements, there is a vertical quadrant of degrees with scales numbered for celestial latitudes and polar and zenith distances. Between the two scales on this quadrant, there is a slot within which slides a gnomon for measuring solar altitudes. The rete is pierced at the equatorial pole and also at the pole of the ecliptic (the latter is marked by a small strip across the slot in the quadrant). The rete is retained in contact with the globe (around which it should be able to move freely) by three bands, below the ecliptic circle, which are pinned to the circle.

The sphere is engraved with almucantars for every 2° and damascened with almucantars for every 5°; it is also engraved with azimuths for every 10°. Along one quadrant of a meridian, the sphere is pierced at each of the almucantars at 2° intervals. By means of an axis (now missing) passed through the pole of the rete and the appropriate hole in the sphere, the instrument may be set for use in any latitude. Below the equatorial band are marked hour-lines for unequal hours.

See Hugo Seeman, Das kugelförmige Astrolab nach den Mitteilungen von Alfons X von Kastilien und der vorhandenen arabischen Quellen (Abhandlungen zur Geschichte der Naturwissenshaften und der Medizin, Heft VIII), Erlangen, 1925; Francis Maddison, 'A 15th Century Spherical Astrolabe', Physis. Rivista di storia della scienza, anno IV (1962) fasc. 2, pp. 101-9; and Francis Maddison & Anthony Turner, Catalogue of an Exhibition, 'Science and Technology in Islam' ... Science Museum, London, 1976 [Paris, 1976]; pp. 130-33 and nos. 68 & 69.

[62-25; B-M: MUSA 1]
Purchased with grants received from the Goldsmiths' Company, Lord Leigh's Fund, the Hulme Surplus Reserve Fund, the National Art-Collections Fund, and the Higher Studies Fund.

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