2

Proceedings 2

An Introduction to Islamic and Medieval European Astrolabes: a talk by Francis Maddison, Curator of the Museum for the History of Science, Oxford

March 5, 1987The Danson Room Trinity College, Oxford

Dr Crombie expressed his pleasure that M. Jean Gimpel, who had given the Society’s inaugural lecture, was in the audience for the meeting. Introducing to Mr Maddison, Dr Crombie alluded to his valuable work at the Museum for the History of Science and welcomed him as a friend and colleague of many present.

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Announcing his theme, Mr Maddison explained that he would be speaking on the planispheric astrolabe, just one of four types, the others being the linear, spherical and mariners’ astrolabes. He emphasised that the planispheric type was never used as a navigational aid. In Western Christendom, indeed, it was principally used as a demonstration instrument in the teaching of astronomy. One of the few scientific instruments to have survived from the medieval period and the symbol of the astronomer/astrologer it was, in modern terms, an analogue computer for solving certain problems and also capable of a few observations. It was useful to the modern scholar as a diagnostic tracer of the transmission Hellenistic science to Islam and of Islamic science to Moslem Spain, Christendom and India. Manufactured in Islam from the 11th to the 20th and from the 13th to the 18th centuries in Europe, about 2000 examples survived. They were generally made of ‘brass’; commonly a quaternary alloy of copper, lead, tin and zinc. The rim and suspension piece were usually of cast metal, the plates and back of beaten metal.

Models could range in diameter from 4 or” 5 to t8 inches or more. The larger, the more efficient. Using a slide of a model made in Louvain in 1555, Mr. Maddison pointed out the back disc or’ ‘mater’, the different plates made to suit different latitudes and the pointer or alidabe. Although the observational function was not by then very important, John Greaves, in the course of his scientific journey to the Levant, 1537-40, had used an astrolabe to check Ptolemy’s latitude for the Island of Rhodes and had found it badly out.

Antecedents to the astrolabe include 3rd century BC references to a celestial model of Archimedes’, and the Antikythera machine of c.80 BC, a geared device for computing planetary positions, recovered from an ancient sea wreck. The celestial globe had also been known in antiquity, though none survives. A 14th century Persian example was illustrated, so as to explain how it could be adjusted, for demonstration purposes, to the user’s latitude. The 10th-century Arab astronomer, ar-Sufi, in his famous ‘Book of the Fixed Stars’ had set a pattern for the constellation images which was widely followed for many centuries.

Using a 16th-century armillary sphere (from Latin, armilla, a ring), Mr Maddison explained the significance of the various rings defining celestial coordinates and the way in which the stars were shown. By adjusting the sphere in the horizon ring to the observer’s latitude, the apparent rotation of the stars about the celestial pole and their risings and settings in that latitude could be demonstrated. The planispheric astrolabe was, in fact, a flattened version of the armillary sphere achieved by a geometrical procedure, known as stereographic projection. By sighting a star’s altitude above the horizon and by then rotating appropriately the star map (rete) over the plate, it was, in theory, possible to tell the time. The origins of stereographlc projection were uncertain; knowledge of it has been claimed for Hipparchus of Nicaea, c. 150 BC, certainly it was well known by the 1st century BC because it was used on the anaphoric water clock described by Vitruvius Ptolemy, 2nd century AD, described stereographic projection in his Planisphaerium. In .A.D. 1020 a Byzantine astrolabe was made in Constantinople according to Islamic models for a Persian client.

A 15th-century Astrolabe, showing plate, rete and alidabe or pointer, mounted unconventionally on the front.

Islamic astronomy had three purposes: to enable determination of the astronomically defined hours of Moslem prayer; to determine the azimuth of the qibla, i.e.; the direction of Mecca; to assist the operations of astrology. When it was taken up by the West only astrology would have any interest, at first, perhaps, a subject in search of a purpose.

One of the earliest examples shown was a late 9th-century Syrio-Egyptian model, made possibly at Harran (the date is confirmed by the star positions). The Saabaeans of Harran are something of an enigma. A pagan community classified as one of the Peoples of the Book, they were devotees of astrology and there is some evidence that the first astrolabes in the Islamic world were made at Harran, which had a long tradition of metal work. Inscriptions make it possible to trace family and apprenticesh7p links between the early makers of astrolabes. In every case the makers were astronomers on the widest sense; there is no overlap between the lists of astrolabists and metal workers in general.

It is possible that Islamic makers were working for export to the Latin West as early as the 11th century- a Toledan model from A.D. 1068 (AH 460) had Latin month names. These and other Islamic models continued long in use in the West-in 1486 Martin Bylica was using an astrolabe made in Cordoba’ in 1054-55. Islamic astrolabes did not use Hindu-Arabic numerals but letters of the Arabic alphabet used numerically.

An Isfahani example from 1221/2 had a gear train mechanism to demonstrate the relative movements of sun and moon, correlated with the date. There is evidence of such a mechanism in a Byzantine sun dial of the 5th century and al-Biruni described one in c. A.D. 1000.

A sequence of slides showed Mughal and Safavid Persian astrolabes of the 17th and 18th centuries. The Mughal examples, mostly from Lahore, had much astrological detail; the Safavid instruments also, but with fine decorative engraving. Turning to the transmission between Islam and the West in more detail, Mr Maddison showed first, an astrolabe quadrant developed in late 13th-century Europe-a model adopted in Islam. A rare, perhaps unique, instance of transmission from West to East in the middle ages, it was a simplified version of an astrolabe proper, much easier to use for the basic purpose of determining the hours of prayer.

The main route of transmission was from Cordoba across the Pyrenees. In the late 960s Gerbert, Pope Sylvester II, was a populariser of Islamic science. Europe’s first treatises on the instrument from the first half of the 11th century; an astronomer had an astrolabe in England in c. 1092. The earliest dated western made astrolabes date from 1326 and 1342. Thirty years later comes a treatise in French (i.e. a vernacular language rather than Latin,). The first treatise in English was Chaucer’s for his son Lewis, c. 1390.

Astrolabes were physical models to show the astronomical movements of the celestial sphere. Some of the first mechanical ‘clocks’ were in fact mechanical astrolabes; in the 1270s, it seems probable, the first effective verge and foliot escapement was developed. The great town clock (1410) on the old town hall in Prague, may give some idea of the appearance of the St Alban’s clock made by Richard of Wallingford in the early 14th century-a section through the astrolabe dial from the unique manuscript published in Dr North’s book, may qualify as the first machine drawing. In conclusion, the lecturer referred to Dondi’s astrarium (7-dial clock: with elliptical gears) completed in 1364. He also showed a mariner’s astrolabe of the late 15th century. The one representing the most refined, the other the most primitive European derivatives of the Islamic astrolabe,.

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Dr Crombie thanked Mr Maddison for a comprehensive and thought-provoking lecture and survey.

Questions from the floor included the central matter of interest:

Why if the Islamic astrolabe had little or no practical application in Europe was it adopted there. The lecturer pointed out that it did have limited applications for surveying, time measurement and estimating heights, for casting horoscopes and in the teaching of astronomy but surmised that its principal appeal was as a trendy gadget.