An Islamic Astrolabe

The front of an Islamic Astrolabe

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The front (above) and back (below) of an Islamic Astrolabe in the Whipple Museum.

The back of an Islamic Astrolabe

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The earliest surviving Arabic astrolabe treatises are from the seventh and eighth centuries and are often translations of earlier Greek or Syriac texts. Eighth century literary references from Baghdad and Damascus indicate that by this time the use of the astrolabe was widespread throughout the Arab world. Land under Arab control stretched from North Africa and Spain to India, enabling a wide range of astronomical influences to be combined. The early ninth-century tables of al-Farghânî list the radii of the circles on the plate of the astrolabe for each degree of latitude. These simplified the process of astrolabe construction by removing the need for mathematical calculation of these values, indicating that astrolabes were being manufactured in substantial numbers since the effort involved in producing the tables would have been considerable. The earliest surviving Islamic astrolabes date from the ninth century, and these are of such quality and craftsmanship that they represent a continuing tradition rather than a new activity. By the eleventh and twelfth centuries there are many surviving texts and astrolabes, the instruments varying in style and artistry but retaining many fundamental similarities in functionality and design.

This astrolabe is signed "Husain b. Ali" and dated 1309/10 AD. It is probably North African in origin, and is made of brass. It has four plates (for the front of the astrolabe, representing the projection of the celestial sphere and marked with lines for calculation), each for a specific latitude, and 21 stars marked on the rete (the star map, with pointers, fitting over the plate). I have chosen this astrolabe since it is right in the middle of the time frame for Islamic astrolabe use (ca. 600 to ca. 1800) and because it demonstrates many of the features common to Islamic astrolabes.

On the back is a shadow square for measuring the heights of inaccessible things and other similar calculations (shadow squares are quite common, but not on all astrolabes), and scales for calendrical calculations and calculation of the qibla (the direction to face during prayers).

A typical text on the astrolabe describes more than forty uses of the astrolabe, indicating its versatility as an astronomical calculating device. Some of its principal uses to the Islamic astronomers were to provide answers to astrological, calendrical, and meteorological questions. Although less accurate than direct mathematical calculations (the astrolabe is only as accurate as the positioning of the rete and so on) it provided an easy and quick way to calculate values.

Recommended Reading

Sarah Schechener Genuth "Astrolabes: a Cross-Cultural and Social Perspective" introduction to Webster (ed) Western Astrolabes

David King Astronomy in the Service of Islam 1993

John North, The Fontana History of Astronomy and Cosmology, London 1994

Full Bibliography