Around 240 BC the chief librarian of Alexandria measured the circumference of the Earth using two shadows and a piece of arithmetic. The two shadows were on the same day — the summer solstice — at?
Eratosthenes knew that on the summer solstice, the sun was directly overhead at Syene (modern Aswan), because it lit the bottom of a deep well. At Alexandria on the same day, a vertical pole cast a shadow at an angle of about 7.2 degrees. Since 7.2° is one-fiftieth of a circle, the distance between Alexandria and Syene had to be one-fiftieth of the Earth's circumference. He knew the distance (about 5,000 stadia, by professional surveyors who measured it by counting camel paces). The arithmetic gave a circumference of approximately 250,000 stadia — about 40,000 km, within two percent of the modern value. He had no instruments more complex than a pole.
Read the full story →Around 240 BC the chief librarian at Alexandria measured the planet's circumference using two shadows and a piece of arithmetic. He was off by about two percent.
Related questions
- How accurate was Eratosthenes's measurement of the Earth's circumference around 240 BC?
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