The Earth’s circumference
- Eratosthenes (276BC to 195BC) relied on measuring the different lengths of shadows cast by poles stuck vertically into the ground, at midday on the summer solstice, at different latitudes. The difference in the shadows demonstrated how much the Earth's surface curved. Eratosthenes estimated the Earth's circumference as approximately 40,000km.
- Another scientist called Posidonius (135BC to 51BC) who lived on the island of Rhodes, arrived at almost exactly the same answer. He observed the bright star Canopus would lie very close to the horizon, but when in Alexandria, in Egypt, Canopus would ascend to some 7.5 degrees above the horizon. 7.5 degrees is 1/48th of a circle, and multiplying the distance from Rhodes to Alexandria by 48, arrived at approximately 40,000km.
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