Engines of Our Ingenuity

No. 2594
PTOLEMY'S GEOGRAPHIA

by Andrew Boyd

Today, a man and a map. The University of Houstonís College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

Most of us know the name Ptolemy for the Ptolemaic or earth-centered model of our solar system. Ptolemyís model, laid out in the second century, described the motion of the stars and planets reasonably well for the time; and his work, the Almagest, is one of the most important surviving treatises we know about early Greek astronomy.

Ptolemy believed the earth was round, which, perhaps surprisingly, wasnít a new idea even in the second century. Aristotle had argued that the earth was round some five centuries earlier. Eratosthenes devised a clever geometric experiment to actually measure the earthís diameter, and did so four centuries before the Almagest was written. And a round earth played a central role in Ptolemyís other important work, the Geographia. In the Almagest, Ptolemy laid out a map of the heavens. In the Geographia, he laid out a map of the earth.

Key to Ptolemyís earth map was the notion of latitude and longitude — a handy coordinate system for locating points on a round surface. Ptolemy didnít come up with the idea — he freely borrowed it from a contemporary, Marinos of Tyre. But he used the idea to make his own lasting contributions. Ptolemy spent a lot of effort on projection, the mathematical process of taking the earthís round surface and drawing it on a flat sheet of paper. Itís not a trivial problem, and all projections distort maps in one way or another. Ptolemy didnít skirt the issue — he hit it head on.

Ptolemy drawing picture

He also compiled information from many disparate sources, providing long lists of latitudes and longitudes for important locations. And he used that information to put together an all-encompassing map of the known world. Ptolemy believed the known world covered about a quarter of the earthís surface, and he wasnít far off. His effort led to what weíd now call a world atlas — an atlas that was updated and referenced for well over a millennium.

Itís easy to find problems with Ptolemyís world map. The known land masses are there, but some are wildly misshapen. Today we can call upon an elaborate global positioning system and fix the latitude and longitude of any point on earth to within about a foot. Thatís mind-boggling. But early civilizations couldnít come close to that. In fact, longitude couldnít be properly measured until the eighteenth century, when sufficiently accurate clocks were devised — clocks that kept good time as they were shaken and rattled while moving from one point to another.

But what really stands out in Ptolemyís world map are the beautiful, gently curving lines of latitude — a clear, visual reminder that we live on the surface of a ball, and that weíve known it for thousands of years.

Ptolemy World Map picture

Iím Andy Boyd at the University of Houston, where weíre in-terested in the way inventive minds work.

(Theme music)


Notes and references:

For related episodes, see 230, THE ROUND EARTH, and 235, HARRISON'S TIMEPIECE.

Geography (Ptolemy). From the Wikipedia Web site: http://en.wikipedia.org/wiki/Geography_%28Ptolemy%29. Accessed March 23, 2010.

Ptolemy's Geographia. From the Oxford University Computing Services Web site: http://ota.ahds.ac.uk/headers/2422.xml. Accessed March 23, 2010.

D. Sobel. Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time. New York: Walker and Company, 1995.

The mathematical figure of Ptolemyís map projection is taken from the translation of Geographia found at the Oxford University Computing Services Web site.

Most unfortunately, while copies of the text of the Geographia were handed down through the ages (with changes and additions as new and better data became available), none of Ptolemyís original maps are known to have survived. However, Ptolemy provided an adequate description to allow later cartographers to recreate faithful reproductions — especially with respect to the projection, which Ptolemy carefully described. The map presented here, taken from Wikimedia Commons, is a fifteenth century rendition of Ptolemyís world map.


The Engines of Our Ingenuity is Copyright © 1988-2010 by John H. Lienhard.