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To estimate the Earth’s circumference, Eratosthenes calculated the length of a meridian arc by measuring the shadow cast by a vertical axis at noon on the summer solstice. In Syene (northern tropic), no shadow is cast whereas 800 kilometres further north, in Alexandria, the shadow is cast at an angle of 7.2° from the vertical, that is to say 1/50 of 360°. The circumference is therefore equal to 50 times the distance between the two cities.

**Click** and **drag** the obelisk along its terrestrial meridian.

- To illustrate the Eratosthenes's experiment: The measurement of Earth's circumference.

The Greek scholar **Eratosthenes **(approx. 276 – 194 BC) postulated that the Earth was spherical in shape; that the two cities -- the distance between which had been accurately measured (5000 stadia,…