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Dead medium: the Astrolabe; Ctesibius's Clepsydra Orrery
From: kadrey@well.com (Richard Kadrey)
Source(s): http://myhouse.com/mc/planet/astrodir/ASTROLAB.HTM (((kadrey remarks: This page was constructed and written by James E. Morrison, who teaches an Introduction to Astronomy course at Montgomery College at Tokoma Park in Connecticut. If you're really interested in astrolabes, he sells cool, working models through the site.)))

"Astrolabe History

"Origins of Astrolabe Theory

"The origins of the astrolabe were in classical Greece. Apollonius (ca. 225 BC), the great codifier of conic sections, probably studied the astrolabe projection.

"The most influential individual on the theory of the astrolabe projection was Hipparchus who was born in Nicaea in Asia Minor (now Iznik in Turkey) about 180 BC, but studied and worked on the island of Rhodes. (...) Hipparchus did not invent the astrolabe but he did refine the projection theory.

:The earliest evidence of use of the stereographic projection in a machine is in the writing of the Roman author and architect, Vitruvius (ca. 88 - ca. 26 BC), who in *De architectura* describes a clock (probably a clepsydra or water clock) made by Ctesibius in Alexandria. Apparently, Ctesibius' clock had a rotating field of stars behind a wire frame indicating the hours of the day.

"The wire framework (the spider) was possibly constructed using the stereographic projection with the eye point at the north celestial pole. Similar constructions dated from the first to third century and have been found in Salzburg and northeastern France, so such mechanisms were apparently fairly widespread among Romans.

"The first major writer on the projection was the famous Claudius Ptolemy (ca. AD 150) who wrote extensively on it in his work known as the *Planisphaerium.* There are tantalizing hints in Ptolemy's writing that he may have had an instrument that could justifiably be called an astrolabe. Ptolemy also refined the fundamental geometry of the Earth-Sun system that is used to design astrolabes.

"Early Astrolabes

"No one knows exactly when the stereographic projection was actually turned into the instrument we know today as the astrolabe. Theon of Alexandria (ca. 390) wrote a treatise on the astrolabe that was the basis for much that was written on the subject in the Middle Ages.

"Synesius of Cyrene (378-430) apparently had an instrument constructed that was arguably a form of astrolabe. This is plausible since Synesius was a student of Hypatia, Theon's daughter. The earliest descriptions of actual instruments were written by John Philoponos of Alexandria (a.k.a. Joannes Grammaticus) in the sixth century and a century later by Severus Sebokht, Bishop of Kenneserin, Syria, although it is likely that Sebokht's work was derivative of Theon. It is certain that true astrolabes existed by the seventh century."

Richard Kadrey (kadrey@well.com)