Ptolemy and Geography
The Ancient Greek Ptolemaic Geographical System
Ptolemy's geography contained many errors, but it also pioneered many concepts that we still use today.
Ptolemy
Ptolemy (aka Claudius Ptolemaeus) lived and worked in Alexandria, Egypt during the second century. We know few details of his life, but he made important lasting contributions to astronomy, geography, optics, mathematics, and music. Although he was born and lived in Egypt, Ptolemy was of Greek ancestry, therefore he is considered an ancient Greek rather than Egyptian. He lived in Alexandria, where the famous ancient library was located, so Ptolemy had access to the best library of his time.
Ptolemaic Geography
In addition to his better known Ptolemaic system of astronomy, Ptolemy made very important contributions to geography. Ptolemy's geography builds on previous workers, particularly Eratosthenes and Hipparchus. Ptolemy also added original contributions.
We mark locations on Earth with the latitude and longitude grid system. Ptolemy did not invent the idea of a grid system for locations. He did however improve the system that was handed down to him. Ptolemy coined and popularized the words longitude and latitude. Ptolemy also originated the modern map making custom of placing east on the right and north at the top.
Because the Earth is spherical, map makers must use some type of projection to map the spherical Earth onto a flat map. Ptolemy knew that the Earth was spherical and was the first map maker to devise a projection system for projecting the spherical Earth onto a flat map.
Errors in Ptolemaic Geography
In Ptolemy's time, the Earth was not yet very well explored. This lack of knowledge led to errors in Ptolemy's geography. Ptolemy simply did not know what large regions of the Earth were like. One serious error resulting from this lack of knowledge was the size and extent of Asia. It is much smaller than Ptolemy thought.
In addition to thinking that Asia was much larger than it really is, Ptolemy underestimated the size of the Earth. Ptolemy knew about Eratosthenes' estimate for the size of the Earth, but used his own estimate. Unfortunately for Ptolemy, Eratosthenes was surprisingly accurate in his estimate. Ptolemy however estimated that the Earth's circumference was only about 18,000 miles.
The combination of these two errors led Christopher Columbus to believe that he could reach the East Indies and Asia by sailing west from Spain. The distance turned out to be much greater than Columbus thought from reading Ptolemy's geography. Of course there was also another continent in the way.
The errors in Ptolemy's geographical knowledge led to a monumental discovery.
Further Reading
Berry, A. A Short History of Astronomy. Dover, 1961.
Boorstin, D.J. The Discoverers. Vintage Books, 1983.
Dreyer, J.L.E. A History of Astronomy From Thales To Kepler. Dover, 1953.
