Introduction

From the Maqluqat of al-Hariri
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Few astronomical instruments have had an active lifetime as long as the astrolabe. Although it perhaps never made an impact in astronomy comparable to that of the telescope, its compactness and versatility easily made it the most treasured instrument of astronomers and astrologers from the flowering of Greek science until well into the modern age. It was regarded as the instrument par excellence of the stargazer and Urania’s children are rarely seen without it in the art and the literature of the East and the West.

Description of the Astrolabe

In essence an astrolabe is a portable, usually flat instrument made of brass which depicts the positions of the prominent stars in the sky relative to the observer’s horizon. With a well made astrolabe, an experienced user had a powerful observing instrument and computational device at his disposal. The correct time could be determined to the nearest few minutes at any time of the day or the night, and conversely, for a particular moment and place, the positions of the celestial bodies relative to the observer’s horizon could be obtained. Further diagrams served for solving practical surveying problems such as measuring the height of a tower or the width and depth of a moat. Islamic astrolabes often featured additional diagrams for determining the prayer times and tables which served as astrological compendia or as gazetteers from which the correct orientation to Mecca could be read.

Although many of these operations could also be done just as well, or even more accurately, with other instruments such as sundials or armillary spheres, the unique combination of all these functions in a single instrument light enough to carry around easily explains its popularity. In the West it held this special status until the late seventeenth century, when it was rapidly made obsolete with the development of accurate and relatively cheap mechanical time-keepers and the introduction of precise geodetic surveying instruments equipped with telescopic sights. In the East, however, they were to be used until well into the nineteenth century.

The astrolabe was developed in three distinct varieties. Mathematically speaking, they can be viewed as instruments which project the three-dimensional celestial sphere on to three-, two- or one-dimensional reference surfaces.

Major Astrolabe Surveys and IC Numbers

In the literature (and also on this website) astrolabes in museum and private collections are usually identified by an International Instrument Checklist (CCA, IC, ICA or IIC) number for medieval and Renaissance instruments. Currently, the IC numbers are assigned as follows:

For more details, see the following astrolabe surveys and related publications: