Ptolemy

:This article is about the geographer and astronomer Ptolemy. For Alexander the Great's general, see Ptolemy I of Egypt. For others named "Ptolemy" or "Ptolemaeus", see Ptolemy (disambiguation).

Geography

Ptolemy's other main work is his Geography. This too is a compilation of what was known about the world's geography in the Roman Empire during his time. He 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.

Related Topics:
Geography - Roman Empire - Marinos of Tyre - Gazetteer - Persian

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The first part of the Geography is a discussion of the data and of the methods he used. Like 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.

Related Topics:
Coordinate - Grid - Latitude - Equator - Degrees of arc - Midsummer - Polar circle - Meridian - Longitude - Canary Islands

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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 Geography 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.

Related Topics:
Topographic - Atlantic Ocean - China - East Indies - Africa

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The maps in surviving manuscripts of Ptolemy's Geography however, date only from about 1300, after the text was rediscovered by Maximus Planudes.

Related Topics:
1300 - Maximus Planudes

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Maps based on scientific principles had been made since the time of Eratosthenes (3rd century BC), but Ptolemy invented improved projections. It is known that a world map based on the Geography 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. The maps look distorted as 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 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.

Related Topics:
Maps - Eratosthenes - 3rd century BC - Projection - Autun - France - 15th century - Ulm - 1482 - Alps - Chronometer - 18th century

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In his Optics, a work which survives only in a poor Arabic translation, he writes about properties of light, including reflection, refraction and colour. His other works include Planetary Hypothesis, Planisphaerium and Analemma.

Related Topics:
Light - Reflection - Refraction - Colour

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