12/06/2025
In 240 BC, the Chief Librarian of Alexandria determined the size of the world using nothing but a wooden stick.
Eratosthenes was a scholar living in Egypt during the height of the Ptolemaic dynasty.
He oversaw the Library of Alexandria, the single greatest collection of knowledge in the ancient world.
But scrolls and manuscripts could not answer his biggest question.
He wanted to know the exact physical dimensions of the Earth.
While most educated Greeks already understood that the world was spherical, no one had successfully measured its girth.
Most assumed it was impossible to calculate without traveling around it.
But Eratosthenes turned to geometry instead of travel.
His breakthrough began with a report from the southern city of Syene, near modern-day Aswan.
Travelers reported that at noon on the summer solstice, the sun cast absolutely no shadow.
The light shone straight down to the very bottom of a deep well.
Eratosthenes tested this observation hundreds of miles north in Alexandria.
He waited for the exact same date and time to conduct his experiment.
He placed a vertical rod, known as a gnomon, into the ground.
Unlike in Syene, the rod in Alexandria cast a distinct shadow.
He measured the angle of that shadow at precisely 7.2 degrees.
This was a simple matter of applied mathematics.
He knew that a full circle contains 360 degrees.
Since 7.2 is exactly one-fiftieth of 360, he deduced that the distance between the two cities was one-fiftieth of the Earth's total circumference.
Now he simply needed to measure the physical distance across the sand.
He did not rely on guesses or travel times.
He reportedly hired professional surveyors, known as bematists, to walk the track.
These men were trained to walk with perfectly equal stride lengths to measure distances for the state.
They determined the distance was approximately 5,000 stadia.
Eratosthenes did the multiplication.
He multiplied 5,000 by 50 to arrive at a circumference of 250,000 stadia.
In modern terms, his calculation converts to roughly 40,000 kilometers.
He didn't have satellites.
He didn't have lasers.
He didn't have modern aviation.
He relied entirely on logic and the sun.
Today, modern satellites confirm the Earth is about 40,075 kilometers around the equator.
His calculation, made over two thousand years ago, was accurate within a margin of one percent.
It remains one of the most durable facts in the history of science.
Sources: Encyclopedia Britannica / History Channel
