More than 2,000 years ago, long before GPS or satellites, an ancient Greek scholar named Eratosthenes used shadows and geometry to calculate the size of the Earth. His method was surprisingly simple—and astonishingly accurate.

Brilliance in Ancient Alexandria

In 240 BCE, Eratosthenes was the chief librarian of the legendary Library of Alexandria, one of the most important centers of learning in the ancient world. One summer solstice, he made an observation that would change how we understand our planet.

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Eratosthenes: Biography of a Great Thinke

In Syene (modern-day Aswan), located directly south of Alexandria, he learned that at noon on the solstice, vertical objects cast no shadows—meaning the Sun was directly overhead. But in Alexandria, about 800 kilometers away, objects did cast shadows at the same time.

The Power of Simple Geometry

Eratosthenes realized that this difference in shadow meant the Earth must be curved. He measured the shadow angle in Alexandria—it was 7.2 degrees, or 1/50 of a full circle.

If 7.2 degrees was 1/50 of the Earth’s total 360 degrees, then the full circumference of the planet would be:

800 km × 50 = 40,000 kilometers

Modern calculations place Earth’s circumference at 40,075 kilometers—meaning Eratosthenes was only 75 kilometers off, a stunning achievement for someone in the 3rd century BCE.

A Discovery Almost Lost to History

Sadly, much of Eratosthenes’ original work was lost when the Library of Alexandria was destroyed in ancient times. What we know of his method comes from later scholars who preserved and referenced his findings.

Ironically, Ptolemy, a later astronomer, offered a far less accurate estimation of Earth’s size. His model influenced generations of navigators—including Christopher Columbus, who underestimated Earth’s circumference and believed he could reach Asia by sailing west.

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How Eratosthenes calculated the Earth’s circumference

Why His Method Still Inspires Today

Eratosthenes didn’t have a telescope, calculator, or computer. He had only his mind, a measuring stick, and the sun. And with those tools, he proved that Earth is a sphere—and calculated its size with jaw-dropping precision.

His experiment is still taught in schools today to show how science can be both accessible and powerful.

Final Thoughts

Eratosthenes’ work was not just a scientific achievement—it was a triumph of human curiosity. He showed that with careful observation and logical thinking, we can uncover the truths of our world.

Even centuries later, his story reminds us that sometimes, a shadow is all it takes to see the light.