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HISTORY OF SCIENCE AND TECHNOLOGY
CLAUDIUS PTOLEMY (c. 85 (87) – (150) 165 AD)
Claudius Ptolemy, also referred to as Claudius Ptolemaeus, lived in Alexandria. His name suggests that he was of Greek parentage, although his first name Claudius indicates that he was a Roman citizen. Very little is known about his early life; even the dates of his birth and death are uncertain, but it is assumed that he would have studied at the Alexandria academy. Ptolemy was an astronomer, mathematician and geographer. In the realm of astronomy he is best know for his publication “Almagest”, also called “Mathematical Syntaxis”. This publication was an enormous 13 volume edition that included a star catalogue detailing 48 constellations with names that we still use to the present day.
Ptolemy used his own observations and the works of other astronomers, such as Hipparchus, to build a geocentric model to explain the movement of the sun and the planets. A geocentric model is based on the idea that the sun and all the planets orbit the Earth. He extracted all of the best features of other geocentric models and formulated a complicated explanation to explain the observed movements of the sun and planets, while keeping the Earth at the centre. It is to his credit that, despite being completely wrong about his model, he did manage to predict the movements of the planets with amazing accuracy. His model became known as the Ptolemaic system. Even though other astronomers, such as Aristarchus (310 BC - 230 BC), had rejected the geocentric model and correctly advocated the heliocentric theory, it was Ptolemy’s model that was to dominate for another 1500 years. The explanation for this is most likely to be the fact that Ptolemy published his work while most of the earlier writings, such as those of Aristarchus, did not survive.
His other publication of importance was “Geography” which was the first book to provide maps of the world, or at least the world as it was known at that time. Ptolemy used the work of Marinus of Tyre (c.70-130 AD) as a basis for this eight volume edition. His maps were inaccurate because he had miscalculated the circumference of the Earth; he had made it too small. This meant that Asia appeared much farther east than it actually is. It is believed that Christopher Columbus, who was using Ptolemy’s maps in 1492, made the decision to sail westward because the eastern route appeared too far!
Claudius Ptolemy has been honoured by having a crater on the Moon named after him and a crater on Mars also bears his name.