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A Report by Andrew Collins
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Author Topic: AL IDRISI MAP  (Read 3610 times)
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« on: December 05, 2007, 09:15:57 pm »

During this research, al-Idrisi and Roger compared data, keeping the facts on which travelers agreed and throwing out all conflicting information. This process of collecting and assessing material took 15 years, during which, according to al-Idrisi, hardly a day passed when the king did not confer personally with the geographers, studying accounts that disagreed, examining astronomical coordinates, tables and itineraries, poring over books and weighing divergent opinions.

Finally, however, the long preliminary study was finished and the task of map making began. First, under al-Idrisi's direction, a working copy was produced on a drawing board, with places sited on the map with compasses, following the tables that had already been prepared. Then a great disk almost 80 inches in diameter and weighing over 300 pounds was fabricated out of silver, chosen for its malleability and permanence.

Al-Idrisi explained that the disk merely symbolized the shape of the world: "The earth is round like a sphere, and the waters adhere to it and are maintained on it through natural equilibrium which suffers no variation." It remained "stable in space like the yolk in an egg. Air surrounds it on all sides.... All creatures are stable on the surface of the earth, the air attracting what is light, the earth what is heavy, as the magnet attracts iron." As his comment suggests, al-Idrisi thought that the world was round. Nor was he alone. Contrary to a still popular misconception that up to the time of Columbus everyone believed the world was flat, many scholars and astronomers since at least the fifth century B.C. had believed that the earth was a globe. In the third century B.C. the Alexandrian astronomer Eratosthenes measured a degree of the earth's circumference with amazing accuracy, arriving at a figure with an error of either 1.7 or 3.1 percent. (The variation in the amount of his error is due to modern uncertainty as to the exact length of the measurement he used.) Ptolemy, four centuries later, estimated the circumference with much less success—at almost 30 percent less than its true extent. And in the ninth century, 70 Muslim scholars, working under the patronage of Caliph al-Ma'mun, gathered in the Syrian Desert to determine the length of a degree of latitude. Rather than rely on travelers' guesses of distance, as previous astronomers had done, they used wooden rods to measure the road they traveled until they saw a change of one degree in the elevation of the polestar. Their calculation resulted in a figure for the earth's circumference equivalent to 22,422 miles, an error of 3.6 percent, almost as accurate as Eratosthenes' estimate and a considerable improvement over Ptolemy's.

By al-Idrisi's time, Muslim astronomers had made great strides in methods of reckoning latitude. (Longitude remained a problem until the 17th century.) Arab geographers had corrected some of the errors of Ptolemy and other Greek scientists. The mathematician al-Khwarizmi reduced Ptolemy's estimate of the length of the Mediterranean Sea from 62 to 52 degrees; the Spanish Muslim astronomer al-Zarqali further adjusted the figure to the correct 42. Other Muslim scholars, like the Iraqi astronomer al-Battani and the Persian al-Biruni, composed tables giving the latitudes of leading cities.
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