Although it provides a simpler and more literal representation of the heavens than the commonly encountered planispheric astrolabe, the spherical astrolabe is a more complicated and less robust instrument to use, which probably accounts for the fact that so few survive today. Only two others are known, both in brass. One, dated 1480-81 AD, is at the Museum of the History of Science at the University of Oxford (inv.no.49687; Francis Maddison, ‘A 15th Century Islamic Spherical Astrolabe’, Physis, Vol.IV, 1962, pp.101-09). The other is a fragment first recorded in the collection of the architect Federico Frigerio (1873-1959) in Como (Ernesto Canobbio, ‘An Important Fragment of a West Islamic Spherical Astrolabe’, Annali dell’Istituto e Museo di storia della scienza di Firenze, Vol.I, Florence, 1976, p.37). As with a planispheric astrolabe, the spherical astrolabe would have been used to perform astronomical calculations as well as to cast horoscopes, determine the time of prayers and indicate the direction of the qibla. They are unique to Islamic science and have no surviving tradition in the history of western science.
Until the Oxford astrolabe surfaced at auction at Sotheby’s on 26 February 1962, the spherical astrolabe was only known from contemporaneous descriptions by Islamic scholars. The first mention of the instrument occurs in a work by the mathematician, astronomer and geographer Muhammad bin Musa al-Khwarizmi (d. ca. 850 AD). The first treatise devoted solely to the subject is attributed to the Melkite scientist Qusta bin Luqa (d. circa 912). Other scholars such as al-Nairizi (d. ca. 922) – who described the instrument as superior to the planispheric astrolabe – al-Biruni (973-1048) and Hasan al-Marrakushi (d. ca. 1262) have also written on it. A detailed and admiring description of the construction and use of the spherical astrolabe, by Isaac bin Sid, was included in the ‘Libros del Saber’ of Alfonso X of Castile (1276-77) but there the interest from the medieval Christian European world seems to have ended.
Little research has been done on Islamic science in the period between the 15th and 19th century and although the sources go silent on the subject for a few hundred years, a later account of a spherical astrolabe is given by Muhammad bin Sulayman al-Fasi bin Tahir al-Rudani al-Susi al-Maliki al-Maghribi (AH 1037-94/1627-83 AD), an accomplished logician, grammarian, jurist and astronomer. Born in Taroudant in Morocco, he spent most of his life in the Ottoman territories eventually moving to the Hijaz where he became one of the most respected scholars in the area and was eventually appointed governor (Salim Ayduz, ‘Rudani: Abu ?Abdallah Mu?ammad ibn Sulayman (Mu?ammad) al-Fasi ibn ?ahir al-Rudani al-Susi al-Maliki [al-Maghribi]’ in Thomas Hockey et al, The Biographical Encyclopaedia of Astronomers, New York, 2007, p.990).
Al-Rudani wrote works on instruments, timekeeping and the qibla, seeking practical solutions and ways to simplify calculations. With this in mind he invented a spherical device into which another sphere with a different axis was placed. The second sphere was divided into two parts in which the zodiacal signs with their sections and regions were drawn. The purpose of this device was to facilitate timekeeping with the use of one instrument. The device, easily constructed, was a universal instrument in that it could be used for different longitudes and latitudes (Ayduz, op.cit., p.990).
Al-Rudani is known to have made a number of these instruments, and sold them to interested buyers (Khaled El-Rouayheb, ‘Opening the Gate of Verification: The Forgotten Arab-Islamic Florescence of the 17th Century’, International Journal of Middle East Studies, Vol.38, No.2, Cambridge, 2006, p.271). The Moroccan scholar ‘Abdullah al-‘Ayyashi who met al-Rudani in Medina wrote that ‘the like has not previously been made, rather he invented it with his acute mind and sophisticated skills’ (quoted in Rouayheb, op.cit., p.271). The Damascene biographer al-Muhibbi, who visited al-Rudani when the latter settled in Damascus towards the end of his life, noted, ‘he invented a sphere (kura) that was superior to previous spheres and astrolabes, and which spread to India and the Yemen and the Hijaz’ (Rouayheb, op.cit., p.271). Until now, none of Al-Rudani’s astrolabes have been known to survive. That offered here is the first published example. He signs simply Muhammad bin Sulayman al-Maghribi, probably in the interests of conserving space.
Al-Rudani wrote a description of and user’s guide to the instrument, called Al-Nafia fi amal al-Jamia, copied in Medina in 1662, the year to which our spherical astrolabe is dated (the text is described in some length in Charles Pellat, ‘L’Astrolabe Sphérique d’ar-Rudani’, Bulletin d’études orientales, Paris, 1975, pp.83-165). The signature on our astrolabe mentions that it was created in the courthouse (bi dar al-qudat) in Medina, perhaps no surprise for someone who was both governor and a known jurist. Not only is our astrolabe the only extant example of one of Al-Rudani’s acclaimed instruments, it might well also be the first, dated to the same year that he wrote his seminal work on the subject. Our astrolabe represents an extremely rare and important survival from a little studied period of Islamic science.