Marcel Destombes's astrolabe, bequeathed to the Institut du monde arabe in 1983, is considered to be the oldest surviving astrolabe in the Christian West. It is known that the astrolabe is a flattened form of the celestial sphere viewed from its southern pole. The lines engraved upon it enabled astronomers to establish and forecast the path of the stars, tell the time during the day or at night, calculate the latitude of a place, and also to determine the height or orientation of a building, natural object, and so on. It appears to have been invented during the Ptolemaic period, and was then developed in Umayyad Syria. Further development took place in Iraq thanks to the scientific activity patronized by the caliph al-Ma'mun (reigned 813–833). In Muslim Spain, during the Caliphate of Córdoba, the astronomer Maslama al-Majriti wrote a commentary on the instrument.
The astrolabe was used by holding it vertically at eye level. On the back of the instrument there was a rotatable pointer (alidade), which had two holes. When these two holes were aligned and a star was sighted through them (the sighting), the star's altitude in degrees could be read on the graduated edge of the astrolabe. Depending on the height of the star, the rete on the other side of the instrument could be rotated to a position corresponding to the angle measured by the alidade. The rete was an openwork metal plate, which served to indicate the positions of the principal fixed stars in the sky and was also marked with the Zodiac scale. So, when a star was sighted, it was seen in the context of the entire canopy of heaven with its principal stars and zodiac. Under the rete, there's a plate called the tympan, engraved with circles and curves. These represent the circular path of the stars according to their height and angle in relation to the local horizon.
This astrolabe has some unusual features. All the inscriptions are in Latin. On first inspection, the instrument appears to have been made in Christian Europe. The pointers on its rete indicate eighteen stars: ten boreal and eight austral stars (i.e. situated below the equator). Eleven of them correspond with the date 980. However, no names of stars have been engraved on the rete. The words ROMA and FRANCIA are engraved in Roman capitals on one of the tympans. These are accompanied by the numerals 41-30. The capitals are similar to those used at the end of the tenth century in the Latin manuscripts of Catalonia, which was at that time part of Carolingian France. This explains the presence of the word FRANCIA. The figures 41° 30’ refer to the latitude which, give or take a few minutes, corresponds with that of Barcelona. However, at the end of the first millennium, there wasn't enough knowledge of astronomy in the Latin West to make an astrolabe. This would therefore suggest that the instrument was made in Muslim Spain, left unfinished, and then completed in the Benedictine monasteries of Catalonia—Vic, Sant Gugat del Vallès, and Ripoll—, which maintained relations with the Mozarabs, Jews, and Muslims of Andalusia. Llobet (also known as Lupitus), one of the learned monks of the time and the archdeacon of Barcelona, wrote one of the first treatises on the astrolabe, in which there are clear influences from the Córdoban Maslama al-Majriti and the Iranian Al-Khwārizmī. There is evidence of these multi-cultural contacts on various parts of the instrument: some of the classical numerals were replaced by abjad (a corresponding alphanumerical system).
It is exactly at this time that the monk (and future pope Sylvester II) Gerbert d’Aurillac, went to Auvergne in Catalonia to study maths and astronomy. The science of the astrolabe, which was so advanced in Muslim countries, came to the Latin West through northern Spain and was relayed by monasteries and cathedral schools.
Beaujouan, G., ‘L’enseignement du “Quadrivium” ’, in Estratto da Settimane di Studio del Centro italiano di studi sull’alto medioevo, XIX, La Scuola nell’Occidente Latino dell’alto medioevo, Spolète, 1971–1972.
Destombes, M., ‘Un astrolabes carolingien et l’place of fabricatione de nos chiffres Arabs’, in Archives internationales d’histoire des sciences, XV, n. 58–59, 1962, pp. 3–45.
Destombes, M., Contributions sélectionnées à l’Histoire de la Cartographie et des Instruments Scientifiques, Paris, 1987, pp. 153–161.
Michel, H., Traité de l’astrolabe, Paris: Éditions Alain Brieux, 1976, pp. 3–13.
Millàs Vallicrosa, J. M., Assaig d’historia de les idees fisiques i matemàtiques a la Calunya medieval, Barcelone, 1931, vol. I, pp. 137–211.
Millàs Vallicrosa, J. M., Nuevos estudios sobre Historia de la Ciencia espanola, Barcelone, 1960, pp. 93–103.
Mouliérac, J., Contribution de la civilisation islamique à la culture européenne, Paris: Institut du monde arabe, 1989, pp. 28–29.
Samsó, J., ‘Andalusi Science and the Monastic Scriptoria’, in Islam and Catalonia, (exhibition catalogue, Institut Català de la Mediterrània, 1988), pp. 187–191.
Savoie, D., ‘Les astrolabes’, in L’Age d’or des Sciences arabes, (exhibition catalogue, Pairs, Institut du monde arabe, 2005), Paris, Institut du monde arabe/Actes Sud, 2005, pp. 93–94.
Wesley, S.; Baujouan G.; Turner, A., ‘The oldest Latin Astrolabe’, in Physis, No. 32, 1995, issues 2–3.
Les Andalusias de Damas à Córdoba, (exhibition catalogue, Paris, Institut du monde arabe, 2000), Paris, Institut du monde arabe / Hazan, 2000, p. 242, n. 245.