RELAZIONI E FUNZIONI PRACTICE WITH CLIL Eratosthenes of Cyrene and the calculation of the Earth s radius Eratosthenes of Cyrene and the calculation of the Earth s radius Eratosthenes of Cyrene was a Greek scientist who lived between the third and second centuries BC in Athens and Alexandria where he was head of the Library. He devoted himself to many disciplines, being very versatile. Above all, however, he is remembered for having measured the length of the earth s meridian obtaining much more accurate results than those achieved by his predecessors and contemporaries. zenith in Alexandria Sun at its zenith 7°12 5000 stades Alexandria Syene 7° Eratosthenes observed that at noon of the summer solstice the sun s rays perfectly illuminated the bottom of a well in Syene (modern Aswan). At the same time in Alexandria, located on the same meridian as Syene (or so he believed) but 5,000 stades further north, he observed the shadow cast by a stick planted perpendicular to the ground and calculated that the direction of the sun s rays and the stick formed an angle of 7.2° = 7° 12 that is 1/50 of the round angle. Hence, he deduced that the length of the earth s meridian must have been 50 times the distance between Alexandria and Syene, that is 250,000 stades. Questions 1. Check if Aswan and Alexandria in Egypt are actually on the same meridian. 2. How many meters equalled a stade? Search in books or online. 3. Using the equivalence found in the previous point, what was Eratosthenes estimate of the length of the earth s meridian? 64

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