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Eratosthenes
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The man who measured the Earth with a shadow
Long before satellites circled above or GPS told us where we stand, a curious Greek scholar tilted his head towards the Sun and asked a question: How big is the Earth? That man was Eratosthenes of Cyrene, a multi-talented mind — astronomer, mathematician, poet, philosopher and geographer — who etched his name into the annals of science with a brilliant experiment and a simple stick.
A mind that measured the world
Born around 276 BCE in Cyrene (modern-day Libya), Eratosthenes eventually found his intellectual home in Alexandria, Egypt — where he directed the famed Library of Alexandria, the greatest storehouse of knowledge in the ancient world. There, amid scrolls and starlight, he asked one of history’s most profound questions: How big is the Earth?
Using keen observation and logical thinking, he measured the Earth’s circumference with remarkable accuracy. Here’s how:
•At Syene (now Aswān), the Sun was directly overhead at noon on the summer solstice.
•Meanwhile, in Alexandria, about 800 km north, he noticed the Sun cast a shadow — 7.2 degrees from vertical.
•Eratosthenes reasoned that if the Sun’s rays are parallel (given the Sun’s great distance), then this angle was proportional to the arc between the two cities — 1/50 of a full circle.
•Multiplying the distance between the cities by 50, he estimated the Earth’s circumference to be 2,50,000 stadia. While the exact length of a stadium is debated, his estimate was within a few percent of today’s accepted value — a staggering feat for the 3rd century BCE.
The man behind the math
Eratosthenes didn’t stop with Earth’s size. He also:
•Calculated the Earth’s axial tilt (obliquity of the ecliptic)
•Wrote on the octaëteris, a complex eight-year lunar-solar calendar cycle
•Mapped the known world, laying the foundations for scientific geography
•Developed the “Sieve of Eratosthenes”, an ingenious method to find prime numbers, still taught in classrooms today
•Authored Catasterisms, a poetic and mythological guide to the constellations, though its full authorship remains debated
A philosopher of precision
Though only fragments of his writings survive, Eratosthenes’ legacy endures in both science and spirit. He believed knowledge was best pursued across boundaries — of disciplines, of geography and of imagination. He was a true polymath, unafraid to combine poetry with mathematics, myth with measurement.
One of his most quoted reflections remains:
“A man can’t step into the same river twice, nor can he stand under the same sky twice — everything changes, but the pursuit of knowledge endures.”
A quiet exit
In his later years, Eratosthenes suffered from blindness — a cruel fate for someone who had once measured the heavens. Unable to read, write or observe, he chose to end his life through voluntary starvation around 194 BCE.
Yet his legacy, like the Earth he once measured, endures through time.
Legacy beyond measure
Eratosthenes showed humanity that the world could be measured not just with tools, but with thought. With a stick, sunlight and geometry, he proved the Earth wasn’t flat, but fantastically round — and brilliantly understandable.
He didn’t just calculate a planet’s size.
He showed that even the vastness of Earth can be grasped — by a curious mind, standing in sunlight.
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