Erastostème was the first man who calculated the Earth ‘s circonférence and radius. He lived in Egypt between 273 and 192 before J C.
He observed that there were differences about drop shadows seen in the town of Alexandry in the north of Egypt and another town called Syène in the South of Egypt (near Assouan) : at the summer solstice (21 of june) at midday, in Syène there was no shadow (sun strictly at the vertical) while at the same hour in Alexandry there were shadows. The angle was mesured with a special sphere between the shadow and the vertical and was a whole angle divided by 50 (7,2°).
The Egyptians had yet evaluated the distance between Alexandry and Syène couting the number of steps done by a camel to go from Alexandry to Syène ; the distance was 5000 stadium (1 stadium’s value is 157,5 meters) or 787,5 km.
After that, Eratostème had two possibilities :
– if the earth was flat, the difference of the shadows should be explain by the difference of direction of the sun rays in Alexandry and in Syène; but it also meant that the sun should be very near from the earth.
– in the other explication, the sun should be very far from eath and the sun rays should have the same direction in the two towns (parallel). So the difference of shadows should be explained by the curve of the earth.
By many others observations (moon eclipses), he thank that the second possibility was right. So he deduced the circonférence of earth 50 x 787,5 = 39760 km. Today the Earth circonference value is 40075 km.
To know the Earth Radius value, Erastostème mesured the shadows done by a stick called « gnomon » and he calculated the radius value : 6550 km. Today the Earth Radius is known to be 6378 km.
Not so far and so elegant, only with a camel, a sphere and a stick !
ps: mais pourquoi un billet en anglais ? parce que ! (j’ai repris une préparation que j’ai faite pour mon cours d’anglais çà l’UPE Strasbourg).
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Bonjour et Bienvenue dans ma Bouteille à la Mer !!