Moroccan astrolabe

The astrolabe is a plane image of the celestial sphere, usually observed from the South Pole. From this single observation point, the stars of the northern hemisphere were visible; those located below the Tropic of Capricorn (the horizon represented by the edge of the disc) are too close to the observer to be visible.

Thanks to its engraved lines, the astrolabe enables the user to establish and predict the course of the stars, to determine the time of day or night, the latitude of the location, to determine the height of something and to calculate a direction. Thought to have been invented during the era of Ptolemy (second century), great improvements were made to the instrument by the Arabs; first in Syria, under the Umayyads (particularly in Harran), then in Iraq, thanks to the scientific activity supported by Caliph al-Mamūn. Up to the nineteenth century, and even later in Morocco, Muslim countries produced astrolabes.

To use an astrolabe, it must be held vertically, at eye level. On the back of the instrument is a revolving rule, the alidade, with two holes. The ray of the star being observed was passed through the alignment of these two holes: the sighting. The reading in degrees was taken on the edge of the astrolabe equipped with divisions and numeral letters. According to the position of the star observed, the rete found on the other side of the instrument was rotated to correspond. The rete is the cut-out disk with pointers indicating the fixed stars and a zodiac scale (ecliptic). Due to its apparent incline, this scale is decentred. Thus, when a star was observed, it was seen in the context of the entire celestial vault of the northern hemisphere with its brightest stars. Below the rete, a detachable plate, known as the tympan, is marked with circles and coordinates. Its outer edge represents the Tropic of Capricorn (the horizon); the second circle, the Equator; the third, the Tropic of Cancer. These three circles are crossed by a north-south vertical line, or “meridian of the location”, and an east-west horizontal line, or six o’clock line, which cross the horizon line at the equinox points. Arcs indicating the celestial meridians, circles parallel to the horizon (almucantars), representing the altitude circles of the stars, allow the position of the sun and the principal stars to be determined throughout the day and night.

The tympan, a representation of the local celestial sphere, was made for a given latitude. It must be changed when the latitude changes. An astrolabe usually has several tympans. This astrolabe has two: one made especially for the cities of Essaouira (31°) and Marrakech (32°), the other for the cities of Fez (33°) and Meknès (34°). On the back of the instrument are the names of the client, Muhammad bin Abd Allah, and the maker, Charaf Allah.

It was via Muslim Spain, then Catalonia with its many Benedictine abbeys, and via the exchanges between monasteries and cathedral schools that the science of the astrolabe, which was so advanced in Muslim countries, spread in the Latin West beginning in the eleventh century.