Eratosthenes, a brilliant scholar from ancient Greece, lived in Alexandria, Egypt, around 240 BCE. He made history by figuring out the Earth’s circumference—the distance around the planet—using just a stick, a shadow, and some smart math. His method was so simple yet so accurate that it still inspires students, scientists, and history lovers today. Let’s break down how he did it in an easy-to-understand way!
The Sun’s Clue in Syene
Eratosthenes heard an interesting fact about a town called Syene (now Aswan, Egypt). Every year, on the summer solstice (around June 21), the Sun shone straight down at noon. If you placed a pole or stick in the ground, it cast no shadow at all. This happened because the Sun was directly overhead, making a perfect 90-degree angle with the ground.
A Shadow Tells a Story in Alexandria
Curious, Eratosthenes decided to check what happened in Alexandria, where he worked as a librarian. On the same day (summer solstice) at noon, he stuck a pole in the ground and noticed something different. The pole did cast a shadow, meaning the Sun wasn’t directly overhead. He carefully measured the angle between the pole and the shadow. This angle was about 7.2 degrees, showing the Sun was slightly tilted compared to Syene.
Connecting the Dots with Math
Eratosthenes knew the Earth was round, like a giant ball, and a full circle has 360 degrees. He realized the 7.2-degree angle between Alexandria and Syene was a tiny slice of the Earth’s full circle. To find out how tiny, he divided:
- 7.2 degrees ÷ 360 degrees = about 1/50th of the Earth’s circumference.
This meant the distance between Alexandria and Syene was 1/50th of the total distance around the Earth.
Measuring the Distance Between Cities
Next, Eratosthenes needed to know how far it was from Alexandria to Syene. Based on reports from travelers and traders who moved between the two cities, he estimated the distance to be around 800 kilometers. In those days, they used a unit called a “stadium,” and he calculated it as about 5,000 stadia. (One stadium is roughly 157.5 meters, so 5,000 stadia equals about 800 km.)
Calculating the Earth’s Circumference
Now came the exciting part. If the 800-kilometer distance between Alexandria and Syene was 1/50th of the Earth’s circumference, Eratosthenes could find the whole distance by multiplying:
- 800 km × 50 = 40,000 kilometers.
So, he concluded that the Earth’s circumference was approximately 40,000 kilometers. Amazingly, modern measurements show the Earth’s circumference is 40,075 kilometers—Eratosthenes was incredibly close!
Why Was This So Special?
Eratosthenes’ achievement was a big deal for several reasons:
- Simple Tools: He used only a stick, a shadow, and basic math—no fancy machines or technology.
- Bold Idea: He assumed the Earth was round, which wasn’t a common belief in his time.
- Super Accurate: His answer was almost the same as what we know today, even with the limited tools of 2,000 years ago.
- Smart Thinking: He combined observation, logic, and math to solve a huge question about the planet.
Eratosthenes’ Lasting Impact
This discovery wasn’t just about measuring the Earth. Eratosthenes’ work helped shape science and geography. He also created maps, studied stars, and invented a way to find prime numbers called the “Sieve of Eratosthenes.” His method for measuring the Earth is still taught in schools because it shows how curiosity and clever thinking can solve big problems.
Today, people search for terms like “Eratosthenes Earth circumference,” “how to measure Earth’s size,” and “ancient Greek scientists” to learn about his incredible story. His work reminds us that even with simple tools, humans can uncover amazing truths about the world.
Fun Fact
You can try Eratosthenes’ method yourself! On a sunny day, measure a stick’s shadow and use math to estimate distances. It’s a fun way to feel like an ancient scientist!
