Eratosthenes measured the Earth with a stick and a shadow

Eratosthenes (c. 276–194 BCE), director of the great Library of Alexandria, learned that at noon on the summer solstice the Sun shone straight down a well in Syene (modern Aswan), casting no shadow. At the same moment in Alexandria, to the north, a vertical post cast a shadow at an angle of about 7.2°.

That angle is one-fiftieth of a full circle. Reasoning that the Sun’s rays arrive parallel, he concluded the distance between the two cities must also be one-fiftieth of Earth’s circumference. Multiplying that distance by 50 gave roughly 250,000 stadia.

Because the exact length of his “stadium” is uncertain, scholars can’t pin his error precisely — but his figure lands close to the true circumference of about 40,000 km. With nothing but geometry, shadows and careful measurement, he had sized the whole planet, and proved the Earth’s curvature could be calculated rather than merely assumed.