Geographia (Ptolemy)
The
Geographia is
Ptolemy's main work besides the
Almagest. It is a compilation of what was known about the world's
geography in the
Roman Empire of the
2nd century. Ptolemy 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.
The
Geographia comprises two parts: Book 1, a discussion of the data and of the methods used; and Books 2‑5, an atlas. The original work included maps, but due to the difficulties involve in copying them by hand, they have fallen out. The work has been discovered and used through the ages by several noted people around the world. Arabic writer
al-Mas'udi, while writing around
956, mentioned a colored map of the Geography which had 4530 cities and over 200 mountains. Byzantine monk
Maximus Planudes found a copy of the Geography in
1295, and since there were no maps in his copy, he drew his own based on the coordinates found in the text. In
1397 a copy was given to
Palla Strozzi in
Florence by
Emanuel Chrysoloras. The first Latin translation of the
Geographia was made in
1406 by Florentine
Jacobus Angelus, and since this, various translations in other languages have been made available to people all over the world.
As 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 as 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.
Ptolemy also devised and provided instructions on how to create maps both of the whole inhabited world (
oikoumenè) and of the Roman provinces. In the second part of the
Geographia he provided the necessary
topographic lists, and captions for the maps. His
oikoumenè spanned 180 degrees of longitude from the Canary islands in the
Atlantic Ocean to
China, and about 80 degrees of latitude from the Arctic to the
East Indies and deep into
Africa; Ptolemy was well aware that he knew about only a quarter of the globe.
Maps based on scientific principles had been made since the time of
Eratosthenes (
3rd century BC), but Ptolemy improved
projections. It is known that a world map based on the
Geographia was on display in
Autun,
France in late Roman times. In the
15th century Ptolemy's
Geographia began to be printed with engraved maps; an edition printed at
Ulm in
1482 was the first one printed north of the
Alps. Also in 1482,
Francesco Berlinghieri printed the first edition in vernacular
Italian. The maps look distorted as compared to modern maps, because Ptolemy's data was inaccurate. One reason is that Ptolemy estimated the size of the Earth as too small: while
Eratosthenes found 700
stadia for a degree on the globe, in the
Geographia Ptolemy uses 500
stadia. It is not certain if these geographers used the same
stadion, but if we assume that they both stuck to the traditional Attic
stadion of about 185 meters, then the older estimate is 1/6 too large, and Ptolemy's value is 1/6 too small. Because Ptolemy derived most of his topographic coordinates by converting measured distances to angles, his maps get distorted. So his values for the latitude were in error by up to 2 degrees. For longitude this was even worse, because there was no reliable method to determine geographic longitude; Ptolemy was well aware of this. It remained a problem in geography until the invention of
chronometers at the end of the
18th century. It must be added that his original topographic list cannot be reconstructed: the long tables with numbers were transmitted to posterity through copies containing many scribal errors, and people have always been adding or improving the topographic data: this is a testimony to the persistent popularity of this influential work in the
history of cartography.
*Berggren, J. Lennart and Jones, Alexander. 2000.
Ptolemy's Geography: An Annotated Translation of the Theoretical Chapters. Princeton University Press. Princeton and Oxford. ISBN 0-691-01042-0.
*Cosgrove, Dennis. 2003.
Apollo's Eye: A Cartographic Genealogy of the Earth in the Western Imagination. John Hopkins University Press. Baltimore and London.
*Stevenson, Edward Luther. Trans. and ed. 1932.
Claudius Ptolemy: The Geography. New York Public Library. Reprint: Dover, 1991. (This is the only complete English translation of Ptolemy's most famous work. Unfortunately, it is marred by numerous mistakes and the place names are given in Latinised forms, rather than in the original Greek).
Primary sources
*
Ptolemy's Geography at LacusCurtius (English translation, incomplete)
*
Extracts of Ptolemy on the country of the Seres (China) (English translation)
Secondary material
*
Ptolemy the Geographer*
Alexander Jones, "Ptolemy and his Geography"*
Ptolemy's Geography of Asia - Selected problems of Ptolemy's Geography of Asia (in German)
*
Ptolemy's Geography of Northwestern Europe (in German)
*
History of Cartography including a discussion of the Geographia
