Rethinking the Spherical Astrolabe
SIS Grant Holder Taha Yasin Arslan (Istanbul Medeniyet University, Turkey), explains how his funding helped him discover more about this unusual instrument from the Islamic world.
The spherical astrolabe is one of the most mysterious astronomical instruments in the history of instrumentation. Until a fully functional one surfaced in an auction in 1962 and was acquired by the History of Science Museum, University of Oxford (HSM), the reality of this type of instrument as a practical device was uncertain. Its use was only known through descriptions in historical texts. Although parts of another spherical astrolabe are now known to exist, the HSM version is complete and remains unique.
About 8 cm in diameter, this instrument was dated 885 Hijra (1480/81 CE) by an instrument maker simply signed as ‘Mūsā’. Spherical astrolabes are universal in nature and can be used at any latitude but the unequal hour markings below the horizon on the mater indicate that this instrument was made to be used around 41-degrees latitude, which usually corresponds with Istanbul. Coincidentally, there is an unsigned treatise on the use of spherical astrolabes that was copied in either Istanbul or Edirne around the 1450s or 1460s that is preserved in the Suleymaniye Library’s Hamidiye Collection (no. 1453). The goal of my research was to provide tangible evidence, if possible, between the Hamidiye text and the HSM instrument. I began my journey by examining the Hamidiye text and took notes about the detailed descriptions of the instrument. I then visited HSM for examination of the instrument. With the help of Dr. Stephen Johnston, head of research, teaching, and collections at HSM, I had the chance to thoroughly examine the spherical astrolabe and to compare my notes on both the written and physical instruments.
From this research, I can confidently say that the HSM astrolabe was made with only artistic intentions, most likely as a demonstration tool to show what a spherical astrolabe looks like and how it operates. The small size of the instrument, the choice of not-so-bright stars to form a symmetry on the rete, and lack of an alidade that can be used for stars makes the instrument visually appealing but impractical for observations. The instrument that is described in the Hamidiye text, however, is much more accurate and functional for both observations and calculations. It is hard to imagine that the author of this text constructed or commissioned a much less accurate instrument such as the HSM astrolabe. I will conclude my research by examining some small details in the text on the use of the instrument to further define if there are some similarities and/or connections between the text and instrument.