Eratosthenes Measured Earth With Shadows

Around 276 BC, Greek mathematician and astronomer Eratosthenes estimated Earth’s circumference by comparing the Sun’s shadow angles in two different locations. He used the difference between those shadows to calculate the planet’s size, arriving at about 39,375 km. The real polar circumference is about 40,076 km, meaning his ancient estimate was only around 1.4% off.

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