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Ptolemy's Geography

Claudius Ptolemy was born around the year 85 AD. Like Eratosthenes, he lived in Alexandria, Egypt. Ptolemy wrote books about astronomy and geography. His books were very famous and were used for more than a thousand years. Ptolemy's ideas were not all original, but he made them popular.

Ptolemy wrote Guide to Geography around 140 AD. Like Hipparchus, Ptolemy used a system like the modern latitude and longitude. He measured distances in degrees north or south of the equator and east or west of a reference line. Ptolemy's reference line went through the Fortune Islands, the farthest west place that he knew about. Today we think these were probably the Canary Islands, off the coast of Africa in the Atlantic Ocean.

Ptolemy's map of the world might have looked something like this:

But Ptolemy's map had some problems. To measure distances in degrees, you need to know the circumference of the earth. And Ptolemy's estimate of the circumference of the earth was not as good as the one Eratosthenes had made. In modern units, Ptolemy's estimate comes out to about 28 800 000 meters. This is much less than the correct value of 40 000 000 meters, so Ptolemy's maps were not very accurate.

Ptolemy's mistake had big consequences. Much later, Christopher Columbus read his Geography. Since Ptolemy said the earth was fairly small, Columbus thought it would be easy to sail around the world. He didn't know there was enough room for another continent!


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