How Did the Ancient Greeks Calculate the Size of the Earth?

The ancient Greeks used clever tricks and simple tools to figure out how big Earth was, just like you use a ruler to measure your toys.

Eratosthenes, a smart Greek man, was the first to do this. He noticed that on a certain day in Syene, the sun shone straight down a well, no shadow at all! But in Alexandria, a few hundred miles away, he saw a shadow from a stick. That meant the sun wasn't directly overhead there.

He measured how long the shadow was and used it to figure out the angle between the two cities. It was like comparing the corners of a triangle, if you know one angle and how far apart the corners are, you can guess the whole size.

Then he used that angle to calculate Earth’s circumference, that's how around Earth is. He guessed it was about 25,000 miles, which is really close to what we know today!

Imagine you and your friend are standing on opposite ends of a playground. You both look at the same swing set. If you see the swing set straight ahead but your friend sees it tilted, you can use that tilt to guess how big the playground is, just like Eratosthenes used shadows!