Investigations of Astrolabe Metallurgy Using Synchrotron Radiation
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Investigations of
Astrolabe Metallurgy Using Synchrotron Radiation
G. Brian Stephenson, Bruce Stephenson, and Dean R. Haeffner Introduction The planispheric astrolabe occupies a special place among early scientific instruments because of its mathematical sophistication, its elegant appearance, and its usefulness to astronomers in pre-telescopic times. The operation of the astrolabe is based on the mathematical properties of stereographic projection, concepts that were understood by the time of Ptolemy in the second century A.D.,1 although surviving instruments are from much later periods. The astrolabe combined a simple instrument for measuring angles (stellar altitudes in particular) with a rotating map of the heavens—in effect, an analog computer that used stereographic projection to preserve angular relationships during its rotation. The key to its success was its compact design, for it combined an observing tool and a calculating device in a portable instrument that an astronomer/ astrologer could carry and use to solve many problems. The oldest extant astrolabes are from 9th-century Syria. Astrolabes were made and used in Western Europe until the 17th century. In some Islamic countries, they were used into the 20th century to determine the times of prayer. Because of their beauty and intricacy, astrolabes are valued as historic artifacts by museums and private collectors. Museum curators want to know the date and place of an astrolabe’s making and as much as possible about the maker, in order to understand the manner in which it was made and used, and to appreciate the cultural context in which it existed.2 These are the same questions asked of other artistic relics from the past. One way in which some of these questions can be answered
MRS BULLETIN/JANUARY 2001
is by metallurgical analysis of the material (usually brass) from which an astrolabe was fashioned.3
Initial Study: Comparison of Two Astrolabes The Adler Planetarium and Astronomy Museum in Chicago has one of the world’s great collections of astrolabes, with more than 70 instruments from the 11th through the 19th centuries, including examples from most of the major regions and of the predominant styles.4 We wished to explore whether modern synchrotron x-ray techniques offer particular advantages for investigations of the metallurgy of these historical artifacts. To that end, we present here synchrotron x-ray analyses of two astrolabes: M-33a (Adler Planetarium and Astronomy Museum, from the Mensing Collection), and DW0595 (Harvard University, from the David P. Wheatland Collection) (Figure 1). As shown in Figures 1a and 1b, the two astrolabes are made in a strikingly similar style, and are both signed and dated “Ioannes Bos,” and “24 March 1597” (Figures 1c and 1d). Astrolabes were not mass-produced in the 16th century. The identical signatures and dates suggest strongly that there is a problem with at least one of the instruments. These are both, in fact, part of a larger group described in a previous study of potentially fake antique scientific
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