Over 2,200 years ago, a man measured the Earth… using shadows and a stick.
In the 3rd century BCE, in the great Egyptian city of Alexandria, lived a man named Eratosthenes.
Some mocked him by calling him Beta — the second letter of the Greek alphabet — implying he was second-best at everything. But in truth, he was an Alpha in knowledge.
Astronomer, geographer, philosopher, poet, historian, mathematician, and director of the Great Library of Alexandria — he was a true Renaissance man… centuries before the Renaissance.
One day, he read in a scroll that in the city of Syene (modern-day Aswan), on June 21st, the summer solstice, a vertical stick cast no shadow at noon. The sun stood directly overhead. But in Alexandria, on the same day and same hour, a stick did cast a shadow.
Where most would overlook such a detail, Eratosthenes saw a cosmic clue.
He reasoned:
If the Earth were flat, the sun’s rays would hit both sticks equally — the shadows would match. But they didn’t.
So, what could explain the difference?
Only a curved Earth.
He measured the angle of the shadow in Alexandria. It was about 7 degrees — roughly 1/50th of a full circle.
He hired someone to measure the distance between Syene and Alexandria: about 800 kilometers.
Then he did the math:
800 km × 50 = 40,000 km — the circumference of the Earth.
That’s astonishingly accurate.
No satellites. No telescopes. No calculators.
Just observation, curiosity, and a mind that dared to ask why.
Eratosthenes became the first person to calculate the size of a planet — and he did it by watching how a stick cast its shadow.
~ Carl Sagan
#astronomer
#geographers
In the 3rd century BCE, in the great Egyptian city of Alexandria, lived a man named Eratosthenes.
Some mocked him by calling him Beta — the second letter of the Greek alphabet — implying he was second-best at everything. But in truth, he was an Alpha in knowledge.
Astronomer, geographer, philosopher, poet, historian, mathematician, and director of the Great Library of Alexandria — he was a true Renaissance man… centuries before the Renaissance.
One day, he read in a scroll that in the city of Syene (modern-day Aswan), on June 21st, the summer solstice, a vertical stick cast no shadow at noon. The sun stood directly overhead. But in Alexandria, on the same day and same hour, a stick did cast a shadow.
Where most would overlook such a detail, Eratosthenes saw a cosmic clue.
He reasoned:
If the Earth were flat, the sun’s rays would hit both sticks equally — the shadows would match. But they didn’t.
So, what could explain the difference?
Only a curved Earth.
He measured the angle of the shadow in Alexandria. It was about 7 degrees — roughly 1/50th of a full circle.
He hired someone to measure the distance between Syene and Alexandria: about 800 kilometers.
Then he did the math:
800 km × 50 = 40,000 km — the circumference of the Earth.
That’s astonishingly accurate.
No satellites. No telescopes. No calculators.
Just observation, curiosity, and a mind that dared to ask why.
Eratosthenes became the first person to calculate the size of a planet — and he did it by watching how a stick cast its shadow.
~ Carl Sagan
#astronomer
#geographers
Over 2,200 years ago, a man measured the Earth… using shadows and a stick.
In the 3rd century BCE, in the great Egyptian city of Alexandria, lived a man named Eratosthenes.
Some mocked him by calling him Beta — the second letter of the Greek alphabet — implying he was second-best at everything. But in truth, he was an Alpha in knowledge.
Astronomer, geographer, philosopher, poet, historian, mathematician, and director of the Great Library of Alexandria — he was a true Renaissance man… centuries before the Renaissance.
One day, he read in a scroll that in the city of Syene (modern-day Aswan), on June 21st, the summer solstice, a vertical stick cast no shadow at noon. The sun stood directly overhead. But in Alexandria, on the same day and same hour, a stick did cast a shadow.
Where most would overlook such a detail, Eratosthenes saw a cosmic clue.
He reasoned:
If the Earth were flat, the sun’s rays would hit both sticks equally — the shadows would match. But they didn’t.
So, what could explain the difference?
Only a curved Earth.
He measured the angle of the shadow in Alexandria. It was about 7 degrees — roughly 1/50th of a full circle.
He hired someone to measure the distance between Syene and Alexandria: about 800 kilometers.
Then he did the math:
800 km × 50 = 40,000 km — the circumference of the Earth.
That’s astonishingly accurate.
No satellites. No telescopes. No calculators.
Just observation, curiosity, and a mind that dared to ask why.
Eratosthenes became the first person to calculate the size of a planet — and he did it by watching how a stick cast its shadow.
~ Carl Sagan
#astronomer
#geographers
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