Ptolemy was born in about 85 AD in Egypt and lived until 165 in Alexandria, Egypt.  He studied astronomy, mathematics and geography.  He did his astronomy from Alexandria in Egypt starting from when he was about forty years old until 141 AD.  He wrote Almagest which dealt with mathematical theories of the motions of the planets, Sun and Moon.  It was partially a summary of
the work of Greek astronomers.  In this book he considered the Earth the center of the universe.
He had a variety of arguments to prove this. One was that the Earth must be immovable, and that
all matter falls towards the center of the universe. If something is thrown up it then returns to it's
original position falling towards the center of the Earth (at least for small projections).  He argued
that objects had always been observed to fall towards the center of the Earth, therefore it must be true. He also made geometric models to predict the positions of these bodies.  This system was accepted by western civilization until the fifteenth century.  Ptolemy also worked on many geometrical theorems and proofs. He also wrote a Guide to Geography which had lists of
places with their latitudes and longitudes and information on map construction.  Because he used a
bad estimate of the circumference of the Earth, the book was inaccurate.  He made a calender that had weather information as well as risings and settings of the stars in the morning and evening.

Author: Charles DeBoer

References:
http://www-groups.dcs.st-and.ac.uk/~history/HistTopics/Longitude1.html#5
Wednesday, September 23, 1998

http://www-groups.dcs.st-and.ac.uk/~history/Mathematicians/Ptolemy.html
Tuesday, August 25, 1998

http://www.mizar.com/GeoCoordinator/Biographies/Ptolemy.htm
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