Eratosthenes

What if I told you someone measured the size of the whole Earth with a stick and the Sun—over 2,200 years ago?

Meet Eratosthenes. He was a Greek scholar who lived around 276–194 BC and ran the great Library of Alexandria. His biggest and most famous idea was figuring out how large the Earth is.

Here’s what he did. In Syene (now Aswan) the Sun shone straight down into a well at noon on the summer solstice—no shadow. In Alexandria a vertical stick did cast a shadow. Eratosthenes measured that shadow's angle—about 7.2 degrees, which is 1/50 of a full circle.

Because the angle was 1/50 of a circle, he reasoned the distance between the two cities must be 1/50 of Earth's full round. He multiplied and got a number very close to the modern value—about 40,000 kilometers. He did this with simple tools and smart thinking.

He also created the Sieve of Eratosthenes, a neat way to find prime numbers by crossing out multiples. As a librarian, mapmaker, and teacher, his work helped later explorers and scientists understand the world better.

Try this at home: stick a ruler in the ground at noon, measure the shadow, and notice how math helps you figure out angles and distances. Eratosthenes shows that big discoveries can start with curiosity and a simple experiment!