Al-Sahli astrolabe

Astrolabes are multifunctional instruments: they are used to calculate the position of the sun and stars, to measure heights and distances, and as calendars and clocks. They were also used as navigation instruments up until they were replaced by the quadrant. They proved to be very useful for measuring time, during the day as well as at night, and  to determine the prayer times. They also have an application in astrology, agronomy, etc.

This planisphere-shaped astrolabe is made up of several master components: a grid (rete in Latin) or spider (‘ankabût in Arabic) bearing the indications relative to twenty-four stars, an alidade, a base plate or "mater", and five plates corresponding to the various latitudes[1]. The back of the astrolabe has a calendar of the Zodiac where the spring equinox corresponds to the middle of the fourteenth day of the month of March, and an altimetric scale called a "shadow square". The throne (kursî in Arabic) at the top of the astrolabe is linked to a large ring used for suspending the instrument when it is used.

This is the oldest astrolabe made by Ibrahim b. Sa‘id Al-Sahli preserved to date. One of the things that makes it special is that it indicates, for several cities of the Islamic world, the duration of the longest and shortest diurnal arcs (presence of the sun above the horizon) in the year. This type of information is not common on ancient astrolabes, nor is it in the context of the measurement instruments used in Al-Andalus during that period.

This is representative of the Andalusian astrolabes, famous as early as the period of the Umayyad Caliphate of Cordoba, then under the taifas. It is during this period that astronomy practiced in Al-Andalus developed the specificities that characterized it until the Nasrid period.

The astrolabe was transmitted to Europe via Al-Andalus. It then became the most famous instrument of the Middle Ages and Renaissance.

Ibrahim b. Sa‘id Al-Sahli is associated in the written sources to the site of (Castellón de) La Plana. We know, thanks to information reported by Qadi Sa‘id in his History of Science, that he was a young scholar building astrolabes in Toledo, and that after the death of Azarchel he continued to work, first in the capital of ??Tage, then in Valencia, and that he made many instruments until the end of the XI^th century. In 1080, it seems that he was already established in the Oriental part of Al-Andalus, since he produced one of the first celestial globes still preserved, and dedicated it to Qadi Isa b. Labbun, the governor of Murviedo near Sagunto. The latter is mentioned in Dhakhira fî mahasin ahl Al-Gazira by Ibn Bassam. Analysis of the instruments that he created shows that Al-Sahli built, at least in one of the cases documented, more plates corresponding to latitudes than the mater of the instrument could hold.

Ibrahim b. Sa‘id Al-Sahli made another astrolabe signed with his name and preserved at the Museum of the History of Science in Oxford. He also made, in collaboration with his son, a celestial globe dated Hegira 478, currently kept at the Museo di Storia della Scienza in Florence. He is also attributed another piece that belongs to the Bibliothèque nationale de France[2], comparable to the previous piece.

[2]Paris, Bibliothèque nationale de France, Ge A 325.