Ptolemy on Astrolabes

An astrolabe

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Relevant to astrolabes is Ptolemy's Planisphaerium, so called in the Middle Ages because it dealt with the problem of mapping figures from the celestial sphere onto a plane, by a specific method, now known as 'stereographic projection', that preserves circles. The early interest in this technique, well before Ptolemy, may have been a desire to simplify certain problems in spherical geometry by reducing them to plane geometry. However, the stereographic projection later became the mathematical basis of the plane astrolabe, the most popular of medieval astronomical instruments.

Recommended Reading

M.Hoskin (ed.) The Cambridge Illustrated History of Astronomy, Cambridge 1997

J.North The Fontana History of Astronomy and Cosmology, London 1994

G.J.Toomer "Ptolemy", pp.186-206 in The Dictionary of Scientific Biography, New York 1970

Full Bibliography