Around 240 BC, Eratosthenes, the chief librarian of the Library of Alexandria, used simple geometry and shadows to pull off one of the most impressive scientific feats of the ancient world. He had heard that in Syene (modern-day Aswan, Egypt), the sun was directly overhead at noon during the summer solstice, meaning it cast no shadow. But in Alexandria, located north of Syene, a vertical stick did cast a shadow at the same time. By measuring the angle of that shadow to be about 7.2 degrees and knowing the distance between the two cities was roughly 5,000 stadia (around 800 kilometers), he used basic trigonometry to estimate the Earth’s circumference.
Eratosthenes calculated the Earth’s total circumference to be about 39,375 kilometers, remarkably close to the modern measurement of approximately 40,075 kilometers. His method relied on the assumption that the Earth is spherical and that the sun’s rays strike the Earth in parallel lines, assumptions that were surprisingly accurate for his time. His work not only demonstrated that the Earth was round but also showed that human curiosity and observation could yield answers to massive questions with surprisingly simple tools. His calculation was so respected that it influenced later scholars for centuries.
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