Around 240 BCE, Eratosthenes heard that at noon on midsummer, the Sun shone straight down a well in Syene, Egypt. In Alexandria, a stick cast a short shadow at the same moment. From the angle and the distance, he worked out Earth's size remarkably closely.
Truth label: Established. Strong evidence and broad agreement among experts.
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The maths
The shadow angle was about 7.2 degrees, one fiftieth of a circle. So the Earth's circumference was 50 times the distance between the cities.
How close?
Depending on how long his unit of distance was, he was within a few percent.
Learn
- Try it. Schools around the world repeat the experiment together each year, comparing shadows at the same moment.
Test yourself
What did Eratosthenes use to measure the Earth?
Shadows. He compared shadow angles in two cities.
Sources
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