View source for Ptolemy

{{Infobox person
| name = Ptolemy
| image = Ptolemaeus.jpg
| image_size = 
| caption = Early [[Baroque]] artist's rendition<ref name="portraits">Since no contemporary depictions or descriptions of Ptolemy are known to have existed,
later artist's impressions are unlikely to have reproduced his appearance accurately</ref>
| birth_date = {{circa|AD 100}}<ref name="britannica.com">{{Britannica|482098|Ptolemy}}</ref>
| birth_place = [[Egypt (Roman province)|Egypt]], Roman Empire
| death_date = {{Circa|AD 170}} (aged {{age in years|100|170}})<ref name="britannica.com" />
| death_place = [[Alexandria]], Egypt, Roman Empire
| education = 
| occupation = {{hlist |[[Mathematician]] |[[Geographer]] |[[Astronomer]]|&nbsp;[[Astrologer]]}}
|}}
[[File:Claudius Ptolemy- The World.jpg|thumb|Ptolemy's AD 150 world map (redrawn in the [[15th century]]).]]
[[File:Cellarius ptolemaic system.jpg|thumb|Image from [[Andreas Cellarius]] ''Harmonia Macrocosmica'', 1660/61. Chart showing signs of the [[zodiac]] and the [[solar system]] with world at center.]]

'''Claudius Ptolemy''' ({{lang-grc-koi|Κλαύδιος Πτολεμαῖος|Klaúdios Ptolemaîos}} {{IPA-el|kláwdios ptolɛmɛ́os|}}; {{lang-la|Claudius Ptolemaeus}}; {{circa|AD 100}} – {{circa|AD 170}}) was a [[Ancient Greece|Greek]] who probably lived and worked in [[Alexandria]], [[Egypt]]. He is famous for his work on [[astronomy]] and [[geography]]. Very little is known about his personal life.

He was an [[astronomer]], [[mathematician]], and [[geographer]]. He described in his writing the Greek [[geocentric view]] of the [[universe]]. Earlier Greek philosophers changed the old assumption of a [[flat Earth]] below a "vault of the heavens". They substituted a spherical Earth surrounded by a [[Celestial sphere]]. Ptolemy built these two spheres into a system of nested spheres. He also thought out and described the apparent motions of the planets as they were known in his time.

Ptolemy explained and extended [[Hipparchus]]'s system of [[epicycle]]s and eccentric circles to explain the Earth-centered theory of the world. Ptolemy's system involved at least 80 epicycles to explain the motions of the [[Sun]], the [[Moon]], and the five [[planet]]s known in his time. He believed the planets and sun moved around the Earth in this order: [[Mercury (planet)|Mercury]], [[Venus (planet)|Venus]], [[Sun]], [[Mars (planet)|Mars]], [[Jupiter (planet)|Jupiter]], [[Saturn (planet)|Saturn]].<ref>Chapter 3 The scientific legacy of antiquity, in Dijksterhuis E.J. 1961. ''The mechanization of the world picture''. Oxford: OUP. p50{{ndash}}68.</ref>

This system became known as the ''Ptolemaic system''. It predicts the positions of the planets well  enough for naked-eye observations, so it seemed accurate at the time. This is described in his book ''Mathematical Syntaxis'' (widely called the ''[[Almagest]]''), a thirteen-book mathematical treatment of the [[phenomenon|phenomena]] of astronomy. It contains a wide variety of information ranging from earth conceptions to sun, moon, and star movement as well as [[eclipse]]s and an explanation of the length of [[month]]s. The ''Almagest'' also included a star catalog containing 48 [[constellations]], using the names we still use today.

In addition to his well-known works in astronomy, Claudius Ptolemy was important in the history of geography and [[cartography]] (making [[maps]]). He was influential up to the 16th century. Then his ideas were disproved by [[Nicolaus Copernicus]]. Ptolemy knew that the Earth is a [[sphere]]. Ptolemy's is the first known [[projection]] of the sphere onto a [[plane (geometry)|plane]]. His ''Geography'' remained the main work on the subject until the time of [[Christopher Columbus]].<ref>Watson, Peter 2009. ''Ideas: a history. From fire to Freud'', volume 1, p194/5. London: Folio Society.</ref> But he had [[Asia]] extending much too far east, which may have been a factor in Columbus's decision to try to reach India by sailing west from [[Europe]].

The Ptolemaic explanation of the motions of the planets was the accepted wisdom until the [[Polish people|Polish]] scholar Copernicus proposed a Sun-centered (heliocentric) view in 1543. Though the heliocentric idea is correct, its predictions were not better than Ptolemy's until [[Kepler's Laws]] were added.

Ptolemy may not actually have believed in the reality of his system. He may have thought of it only as a method of calculating positions.

== Related pages ==
*[[Geocentrism]]

== Notes ==
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{{Authority control}}
[[Category:100 births]]
[[Category:160s deaths]]
[[Category:Ancient Greek astronomers]]
[[Category:Greek geographers]]