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Ptolemy....
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Claudius Ptolemaeus, known
in English as Ptolemy, was a Greek geographer, astronomer, and astrologer
who probably lived and worked in Alexandria in
Egypt. Ptolemy was the author of two important
scientific treatises. One is the astronomical treatise that is now known
as the Almagest "The Great Treatise". It was preserved, like most of Classical
Greek science, in Arabic manuscripts (hence its familiar name) and only made
available in Latin translation (by Gerard of Cremona) in the 12th century.
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In this work, one of the most influential books of
Antiquity, Ptolemy compiled the astronomical knowledge of the ancient
Greek
and Babylonian world; he relied mainly on the work of Hipparchus of three
centuries earlier. Ptolemy formulated a geocentric model of the solar system which remained the generally accepted model in
the Western and Arab worlds until it was superseded by the heliocentric
solar system of Copernicus. Likewise his computational methods (supplemented
in the 12th Century with the Arabic computational 'Tables of Toledo') were
of sufficient accuracy to satisfy the needs of astronomers, astrologers and
navigators, until the time of the great explorations. They were also adopted
in the Arab world and in India. |
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The Almagest also contains a star catalogue, which is
probably an updated version of a catalogue created by Hipparchus. Its list
of 48 constellations is ancestral to the modern system of constellations,
but unlike the modern system they did not cover the whole sky. Ptolemy's
other main work is his Geography. This too is a compilation, of what was
known about the world's geography in the Roman empire at his time. He relied
mainly on the work of an earlier geographer, Marinos of Tyre, and on
gazetteers of the Roman and ancient
Persian empire, but most of his sources
beyond the perimeter of the Empire were unreliable. |
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The first part of the Geography is a discussion of the
data and of the methods he used. Like with the model of the solar system
in the Almagest, Ptolemy put all this information into a grand scheme. He
assigned coordinates to all the places and geographic features he knew, in
a grid that spanned the globe. Latitude was measured from the equator, as
it is today, but Ptolemy preferred to express it in the length of the
longest day rather than degrees of arc (the length of the midsummer day
increases from 12h to 24h as you go from the equator to the polar circle).
He put the meridian of 0 longitude at the most western land he knew, the
Canary Islands. |
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