Eratosthenes' experiment (205 BC) gave the first estimation of the Earth’s radius simply by observing the shadow of a vertical stick at noon local solar time. 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 Cyrene (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.
You might also have a look at the "Eratosthenes Experiment" animation.
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