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Author Topic: AL IDRISI MAP  (Read 3519 times)
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« on: December 05, 2007, 09:17:10 pm »

Al-Idrisi himself gave three figures for the earth's circumference, without deciding among them: Eratosthenes' approximately correct estimate, a slightly smaller figure arrived at by Indian asronomers, and a still smaller number—though larger than Ptolemy's—which was apparently agreed on by Sicilian scholars.

Cartography, nevertheless, remained in a primitive state. Although Ptolemy had discussed several kinds of projection, the problem of flattening out the surface of a sphere so that it could be represented on a map was not solved until the 16th and 17th centuries—the Age of Exploration—and none too satisfactorily even then. The great geographer Gerardus Mercator commented, "If you wish to sail from one port to another, here is a chart . . . and if you follow it carefully you will certainly arrive at your port of destination . . . You may get there sooner or you may not get there as soon as you expected, but you will certainly get there." Al-Idrisi's silver disk, or "planisphere," was a form of projection considerably in advance of others of its time.

On the disk, according to al-Idrisi's own account, were incised "by skillful workers'' lines marking the limits of the seven climates of the habitable world, arbitrary divisions established by Ptolemy running east and west and bounded by parallels of latitude, from the Arctic to the Equator. Below the Equator, an unexplored southern temperate zone was thought to be separated from the familiar northern one by an impassable area of deadly heat. Following the rough sketch prepared by al-Idrisi, the silversmiths transferred the outlines of countries, oceans, rivers, gulfs, peninsulas and islands to the planisphere.

To accompany the silver map, al-Idrisi prepared for Roger a book containing the information gathered by the geographers: Nuzhat al-Mushtaq fi Ikhtiraq al-Afaq (The Delight of One Who Wishes to Traverse the Regions of the World), or more simply, al-Kitab al-Rujari (Roger's Book). The text contained 71 part maps, a world map and 70 sectional itinerary maps, representing the seven climates each divided longitudinally into 10 sections.

Modern geographers have attempted to reconstruct the features of the silver planisphere by using a combination of the maps of Roger's Book, which has survived in several texts, and its tables of longitudes and latitudes. From this reconstruction it is evident that, like Ptolemy, al-Idrisi pictured the habitable world as occupying 180 of the 360 degrees of the world's longitude, from the Atlantic in the West to China in the East, and 64 degrees of its latitude, from the Arctic Ocean to the Equator. The planisphere showed the sources of the Nile—not explored by Europeans until the 19th century, but evidently known to 12th-century Muslim travelers—and the cities of central Sudan. The Baltic area and Poland were represented much more precisely than on Ptolemy's maps, showing the fruit of the geographers' investigations. The British Isles also were treated with a surprising knowledgeability, probably due to contacts between Norman England and Norman Sicily. An element of subjectivity entered into the fact that southern Italy was represented as larger than the north, and that Sicily occupied a substantial part of the Western Mediterranean, in contrast to Sardinia and Corsica, which shrank in scale. Not surprisingly, the best part of both map and text, accurate and detailed, dealt with Sicily itself. 
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