Astrolabe: UC 4108
The instrument is too crudely made for any practical purpose other than teaching.
The astrolabe is an astronomical analogue computer: regular examples are capable of a variety of calculations. The pierced metal plate at the front is called 'rete'. It is a map of the fixed stars, with the north pole in the centre and the Tropic of Capricorn as the outer rim. Part of it is an inner circle, set off set, marked with the canonical twelve signs of the zodiac. The smaller pointers of the 'rete' indicate each the position of a bright star. Behind the 'rete' as part of the instrument itself there is a number of plates with a grid of co-ordinates of latitudes on Earth with a corresponding set of hour-lines.
The 'rete' could be move around freely. After using the sighting arm (here lost) one could find the altitude of the sun or of a star, which makes it possible to determine the time at day or night.
Astrolabes seem to appear already under the Greeks, though the written references to them are rather vague. The earliest secure evidence is found in the works of the writer Theon of Alexandria (about AD 335 - 400). His works were known in the Islamic world. Astrolabes became common in the Arabic world and were then taken over by the Spanish. The earliest preserved Islamic examples date to the tenth century.
Hill 1993: 48-52 (summary on Islamic astrolabes)
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