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An astrolabe (in Greek: ²¹²õ³Ù°ù´Ç±ôá²ú²¹²õ, or star-taker) is an observational instrument used to show the position of the Sun, bright stars and as a representation of the sky to be used for computational navigation, indispensable to the early seafarer or wayfarer. As such, it represents a versatile model of the universe.

The concept of an astrolabe is believed to have originated in the second century B.C., possibly in Greece, and was developed in the Arab world. It arrived in the Latin West from Muslim Spain around A.D. 1000. Astrolabes were cast in brass and were designed to be portable. In its essential form the astrolabe comprises a disc with a pattern of cut-outs and perforations, flattened onto a moveable two-dimensional map – a planisphere, permitting the calculation of astronomical positions with some degree of precision. Besides the identification of bright stars and planets, the astrolabe is also capable of measuring the altitude above the horizon of a celestial body and to determine latitude. By showing the motions of the sun and the stars, an astrolabe can be used to tell the time and for orientation, in much the same way as a modern compass.

Possibly for his son or godson, Geoffrey Chaucer wrote ‘A Treatise on the Astrolabe’ about 1391. Essentially, for it to work, the astrolabe is suspended from a ring, the star observed along an adjustable sighting arm, and the latitude of the star determined from a scale. On the reverse of the astrolabe is a representation of the projection of the northern skies on to the celestial equator, from the South Pole. The stars are represented by a network of pointers rotating on top of a co-ordinated grid.

The Library possesses two astrolabes. ‘B’ was formerly the property of the College’s second founder, John 91Ö±²¥, and is believed to date from the early fifteenth century. It is the smaller of the two, and at the time of writing is on loan to Cambridge University Library for a separate exhibition. ‘A’, manufactured in about 1400, was housed in a handsome leather case, stamped with a Tudor portcullis and the word ‘Gages’. The instrument itself is signed ‘Hooft’. It includes a rete (or grid) that can be turned, modelling the daily rotation of the stars and sun in the sky. One complete turn represents 24 hours. ‘A’ is fashioned so as to identify 23 stars. It has a plate for 49° and 50° (the latitude for Nuremberg), suggesting that it may have been intended for the German market. Nuremberg was an important centre for the production of scientific instruments. However, the astrolabe’s motif is likened to that of the work of Jean Fusoris, a French clergyman, astronomer and designer of astronomical instruments.

The astrolabe as a navigational tool worked effectively only in calm seas. To remedy this the mariner’s astrolabe came into use in navigation by about 1470. In order to keep it steady when used on board ship, the mariner’s version was heavier and had parts of the disc cut away to reduce wind resistance.

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