Geography (Ptolemy)
Early Life and Education
Claudius Ptolemy was a Greek-Roman citizen of Egypt who wrote in Greek. He was a mathematician, astronomer, geographer, astrologer, and poet of a single epigram in the Greek Anthology. He lived in Alexandria, in the Roman province of Egypt, and wrote in Koine Greek, and held Roman citizenship. Beyond that, few reliable details of his life are known. His birthplace has been given as Ptolemais Hermiou in the Thebaid in an uncorroborated statement by the 14th-century astronomer Theodore Meliteniotes. This is quite late, however, and there is no other 14th-century evidence to support it. He died in Alexandria around AD 168.
Ptolemy was the author of several scientific treatises, three of which were of importance to later Byzantine, Islamic and Western European science. The first is the astronomical treatise now known as the Almagest, although it was originally entitled the Mathematical Treatise (Μαθηματικὴ Σύνταξις, Mathēmatikē Syntaxis) and later known as The Greatest Treatise (Ἡ Μεγίστη Σύνταξις, Hē Megistē Syntaxis). The second is the Geography, which is a thorough discussion of the geographic knowledge of the Greco-Roman world. The third is the astrological treatise in which he attempted to adapt horoscopic astrology to the Aristotelian natural philosophy of his day. This is sometimes known as the Apotelesmatika but more commonly known as the Tetrabiblos from the Greek (Τετράβιβλος) meaning "Four Books" or by the Latin Quadripartitum.
Ptolemy's Geography
Ptolemy's other main work is his Geography (also called the Geographia), a compilation of geographical coordinates of the part of the world known to the Roman Empire during his time. He relied somewhat on the work of an earlier geographer, Marinos of Tyre, and on gazetteers of the Roman and ancient Persian Empire. He also acknowledged ancient astronomer Hipparchus for having provided the elevation of the north celestial pole for a few cities.
The first part of the Geography is a discussion of the data and of the methods he used. As with the model of the solar system in the Almagest, Ptolemy put all this information into a grand scheme. Following Marinos, 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 climata, the length of the longest day rather than degrees of arc: the length of the midsummer day increases from 12h to 24h as one goes from the equator to the polar circle. In books 2 through 7, he used degrees and put the meridian of 0 longitude at the most western land he knew, the "Blessed Islands", often identified as the Canary Islands, as suggested by the location of the six dots labelled the "FORTUNATA" islands near the left extreme of the blue sea of Ptolemy's map here reproduced.
Ptolemy also devised and provided instructions on how to create maps both of the whole inhabited world (oikoumenē) and of the Roman provinces. In the second part of the Geography, he provided the necessary topographic lists, and captions for the maps. His oikoumenē spanned 180 degrees of longitude from the Blessed Islands in the Atlantic Ocean to the middle of China, and about 80 degrees of latitude from Shetland to anti-Meroe (east coast of Africa); Ptolemy was well aware that he knew about only a quarter of the globe, and an erroneous extension of China southward suggests his sources did not reach all the way to the Pacific Ocean.
The maps in surviving manuscripts of Ptolemy's Geography, however, only date from about 1300, after the text was rediscovered by Maximus Planudes. It seems likely that the topographical tables in books 2–7 are cumulative texts – texts which were altered and added to as new knowledge became available in the centuries after Ptolemy. This means that information contained in different parts of the Geography is likely to be of different dates.
Maps based on scientific principles had been made since the time of Eratosthenes, in the 3rd century BC, but Ptolemy improved map projections. It is known from a speech by Eumenius that a world map, an orbis pictus, doubtless based on the Geography, was on display in a school in Augustodunum, Gaul in the third century AD. In the 15th century, Ptolemy's Geography began to be printed with engraved maps; the earliest printed edition with engraved maps was produced in Bologna in 1477, followed quickly by a Roman edition in 1478 (Campbell, 1987). An edition printed at Ulm in 1482, including woodcut maps, was the first one printed north of the Alps. The maps look distorted when compared to modern maps, because Ptolemy's data were inaccurate. One reason is that Ptolemy estimated the size of the Earth as too small: while Eratosthenes found 700 stadia for a great circle degree on the globe, Ptolemy uses 500 stadia in the Geography. It is highly probable that these were the same stadion since Ptolemy switched from the former scale to the latter between the Syntaxis and the Geography, and severely readjusted longitude degrees accordingly. See also Ancient Greek units of measurement and History of geodesy.
Legacy
Ptolemy's geographical work was revived in the West and modified during the Renaissance, a period during which more accurate maps were drawn and better latitude and longitude estimates were made. The rediscovery of the Geography, along with the innovations by figures like Amerigo Vespucci, Martin Waldseemüller and Gerardus Mercator, helped pave the way for the Age of Exploration. Despite his generally accurate conception of the world for his time, Ptolemy erred in his estimation of the size of the Earth, a point which Christopher Columbus would exploit in his pitch for a westward voyage to the Indies.
Ptolemy's work in geography, as in astronomy, influenced a wide array of scholars from different epochs and cultures. Islamic scholars, for instance, were well-acquainted with his geographical work, and a modified version of his Almagest was used by the Andalusian astronomer al-Zarqali in the 11th century. In China, the Jesuit missionary Matteo Ricci would make a map of the world that incorporated Ptolemy's geography with Chinese traditional cartography.
Today, historians of cartography regard Ptolemy's work as a significant development in the history of geography, one that would influence not just the Middle Ages and the Renaissance, but the modern era as well.