2,000 Years in the Making

03/06/2021

'Give me a stick, and I'll tell you the size of the world'

Long ago, in ancient Alexandria, there lived a librarian named Eratosthenes. And he had heard a remarkable story, about a town with no shadows.

Where on one day* each year, at noon, the Sun stood directly overhead in the sky. So the sun casting down, left no shadows at all. Making everything perfectly bright.

And that gave Eratosthenes an equally brilliant idea. In his day, people already knew that the world was round. But no one knew how big the world was. Eratosthenes realized that by using sunshine and shadows, he could find out.

In his diagram, the Sun casts shadows where he lived, but not at that far-away town. So if he could measure the angle of the shadows, and then the distance to that town, he could measure the Earth.

Measuring the shadows was easy, using a device like a sundial. But finding the distance to the other town was not. It was about 500 miles away, but Eratosthenes needed to know exactly. According to legend, he counted how many paces a camel needed to walk the journey!

But his effort paid off. By measuring carefully, Eratosthenes calculated that the Earth was 252,000 'stadia' around. Which in our units, is about 25,000 miles. Very close to our best modern measurements, and better than other attempts made centuries later.

Eratosthenes' experiment went down in history. It also lives on, in a word we use today. Geometry, the math we use to study shapes, comes from two greek words, 'geo' & 'metry'. Which mean 'measure the Earth'!

(*The special day he used was the Summer Solstice; when the Sun stands highest in the sky)