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Ptolemy (of Alexandria),

ptolemy’s described astronomy system

Clausius Ptolemaeus ( Lat ) [tol uhmee] c .90–170) Egyptian–Greek astronomer: wrote classic summary of Greek astronomy, geography and optics.

Little is known of Ptolemy’s life or original work. He was probably born in Egypt, and became Hellenized; he should not be confused with the kings of Egypt of the same name. His fame lies in his four books, which summarize 500 years of Greek astronomical ideas and which dominated Western thought on astronomy until the time of , 14 centuries later. His Almagest (Arabic for ‘the Greatest’; he called it ‘the mathematical collection’) described the motions of the heavens on a geocentric basis, making use of various devices such as epicycles (80 in all) to obtain a plausible match with observations (the Ptolemaic system; it owed much to work by , now lost). He gave distances and sizes for the Sun and the Moon, a catalogue of 1028 stars, descriptions of astronomical instruments, and computed p to be 377/120 (3.1417). In the Geography he described a system of determining latitude and longitude, and a map of the world (based largely on the travels of merchants and Roman officials). Ptolemy’s world did not include the Americas, or extend below the Equator (which he placed too far north); but his view that the Earth was spherical, and his exaggeration of Asia eastward, encouraged Columbus in his famous attempt to reach Asia by sailing to the west. Ptolemy’s Optics dealt with the basics of reflection and refraction, and the Tetrabiblios is the origin of much modern astrology. Many of his accounts are due to Hipparchus, including the trigonometry, whose basis remains today.

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