Eratosthenes was a smart man from ancient Greece who lived around 240 BCE in Alexandria, Egypt. He figured out how big the Earth is using a simple and clever idea. His method is easy to understand and still amazes people today. Let’s learn how he did it!

Step 1: The Sun in Syene

Eratosthenes heard that in a town called Syene (now Aswan, Egypt), on a special day (summer solstice, around June 21), the Sun was right above at noon. If you stuck a pole in the ground, it made no shadow because the Sun was straight up.

Step 2: A Shadow in Alexandria

On the same day, Eratosthenes checked the Sun in Alexandria, where he lived. He put a stick in the ground and saw it made a small shadow. The shadow showed the Sun wasn’t straight up but tilted a bit. He measured the angle of this tilt, and it was about 7 degrees.

Step 3: Using Math

Eratosthenes knew the Earth is like a big circle (360 degrees). The 7-degree angle was a small part of that circle. He calculated:

• 7 degrees out of 360 degrees is like 1/50th of the whole Earth.

So, the distance between Syene and Alexandria was 1/50th of the Earth’s total size.

Step 4: Measuring the Distance

Eratosthenes found out that the distance from Alexandria to Syene was about 800 kilometers. People who traveled between the towns told him this.

Step 5: Finding the Earth’s Size

If 800 kilometers was 1/50th of the Earth’s size, he multiplied it by 50:

• 800 km × 50 = 40,000 kilometers.

That’s how he guessed the Earth’s circumference (the distance around it) was about 40,000 kilometers! Today, we know it’s 40,075 kilometers, so he was super close.

Why Is This Cool?

• He used just a stick, a shadow, and his brain—no fancy tools!
• He believed the Earth was round, which was a big idea back then.
• His answer was almost perfect, even 2,000 years ago.

Why It Matters

Eratosthenes’ work shows how smart people can solve big problems with simple ideas. Kids and adults still study his method to learn about math, science, and the Earth. He also helped make maps and studied stars, making him a hero in history.