What is The Gnomon?
The gnomon is an astronomical instrument, consisting of a horizontal dial with a vertical rod, the upper end of which casts a shadow on the surface of the dial.
The discovery by Eratosthenes of the almost exact circumference of the earth.
Eratosthenes discovered that at Syene (Aswan) in Egypt, on the day of the summer solstice, the stem of a gnomon did not cast any shadow on the dial. He thought that Syene was on the line of the Tropic of Cancer, in fact he was 70 km away. At the same time a measurement taken in Alexandria, 5,000 furlongs away, showed that the vertical rod of the gnomon was casting a shadow, the length of which was length revealed an angle with the sun’s rays of 1/50 of the circle, ie 7 degrees 12 minutes. Eratosthenes deduced that the distance from Syene to Alexandria must have been the fiftieth part of the earth’s circumference, or about 250,000 stadia, or 40,075 km, a measurement close to one hundredth of today’s figure.
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