Updated June 01, 2013

**Ptolemy** (Claudius Ptolemaeus, Greek: Κλαύδιος Πτολεμαῖος), was born around 90. It is a Greek astronomer and astrologer who lived in Alexandria (Egypt).

Ptolemy was the author of two major scientific treatises. One deals with astronomy, the Almagest, the other, Geography, based on the knowledge of Graeco-Roman world. Ptolemy's work is a synthesis of ancient science based on verification by observation. The Almagest includes the astronomical models of antiquity, from the Babylonians to the Greeks, the set covers a period of almost nine centuries.

This is the first day of the Egyptian calendar, that is to say 26 February 747 BC. J. C. the first century AD. Ptolemy uses the data tables Egyptian perfect models and outlines in a comprehensive book on astronomy of the time. Ptolemy's geocentric model of the valid Hipparchus, which will be widely accepted for over 1300 years. Its model is slightly modified as adopted by the Arab and India, simply because it serves the needs of browsers at the time.

Ptolemy would have had an intuition of emptiness "the stars are swimming in a perfect fluid which opposes no resistance to their movements." His world view remained geocentric reference scientific minds and paralyzed during the thirteen centuries that followed. Astronomy stagnated until the late Middle Ages until the advent of observation instruments. It is with great sadness that Copernicus (1543), Galileo (1630) and Kepler will end this geocentric view of the world, proposing a heliocentric system.

Its geography is also a synthesis of all knowledge of world geography at the time of the Roman Empire during the reign of Hadrian (125).

Ptolemy defines the latitude from the equator depending on the length of the longest day, the duration of the summer solstice from 12 am to 24 pm at As one approaches the poles.

It sets the meridian of longitude 0 at the furthest point west that he knows, the islands 'Fortunata' which may correspond to the islands of Cape Verde. His maps were the Atlantic Ocean to China, from 80 degrees latitude to the Arctic to Africa.

Ptolemy was aware that his cards only covered a quarter of the planet. The first copy of Ptolemy's Geography published with maps, dates from 1477. Ptolemy died about 168 to Canopus.

A convex quadrilateral is inscribed in a circle, if and only if the product of the diagonals equals the sum of products of opposite sides. Ptolemy's theorem.

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