Planispheric astrolabe

The numerous translations of Persian, Indian and Greek scientific texts bear witness to the fact that Arabs were interested in astronomy from very early on. Over time they developed tools enabling them to observe the sky, the most well-known being the planispheric astrolabe[2] . This instrument of Greek origin was adopted in the Muslim world from the eighth century. Its name comes from the Greek astron and labê, meaning “which takes the stars”.

Between the eleventh and fifteenth century, the Muslim West underwent tremendous development in the science of astronomy, particularly in Morocco and in Andalusia. Preserved astronomical treaties and astrolabes provide rich documentation on their development, and on transmissions that took place between Muslim and Christian lands.

This astrolabe, a round, portable brass instrument, is a mobile map of the world. It enables simulation of the celestial sphere in relation to the terrestrial sphere. It was used for measuring altitudes, latitudes and longitudes, as well as for finding out the time of day and night, and for establishing the direction of Mecca. Astrologists used the instruments for checking the position of planets to draw up horoscopes. [3]

Despite successive improvements, the astrolabe kept its traditional form throughout the centuries. It consists of a body, called the mater, made to hold several disks on the surface of which the latitudes are engraved. The background of this mater bears a series of inscriptions of the names of the lunar months, and numbering in abjad[4]. Its calibrated outer circle is crowned by the throne, which allows for the instrument to be fixed to a hanging ring. On the reverse, a fragmented inscription mentions the name of the astrolabe’s maker and the date it was made. The rete (the stereographic projection of the celestial sphere), which has deteriorated badly on this astrolabe, is a moving part made from an openwork metallic frame, onto which hooks are fixed to indicate the position of the major stars. On the back of the astrolabe, the very worn surface is engraved with a more recent and less careful inscription, in naskhî. The alidade, a pointer on the back of the instrument, is engraved on its ends and bears four small pins to enable rotation.

We know of several signed astrolabes. This one is the work of a Sevillian maker, of whose work we have at least five other instruments. We also know about this maker from the writings of astronomers. However, we do not have any of his texts (if, indeed, he wrote any). In any case, this astrolabe is evidence of the research that was carried out in the Muslim world as well as the numerous passages that took place via Spain, between the two shores of the Mediterranean. Al-Andalus experienced a major cultural radiation, enriched in particular by the presence of Jewish scholars, and its geographical proximity to the Christian world. We know of many astrolabes made in Spain, such as the one produced in Toledo by al-Sahlî in the eleventh century[5].

[3] An astrologist using the instrument can be seen in a painting from Hariri’s Maqamat, dated 1237 and kept at the Bibliothèque nationale de France (Arabe 5847).

[4] abjad is one of the eight mnemotechnical terms that corresponds to the order in which the consonants of the alphabet are arranged, each letter corresponding to a numeric value. Their use has always been limited: they are frequent on astrolabes, in versed historical texts, and divinatory procedures. A similar use can be found by Jews, in the Kabbalah.

[5]  Madrid, Museo Arqueologico Nacional, inv. 50762.