Tribute Tuesday: Eratosthenes

A few nights ago I sat on my bed staring at the small screen of my iPad as the NASA Atlas V rocket launched into space. As I watched the giant missile slowly fade into a glowing light marking the dark sky, it struck me what an amazing time we live in. We ignited a gigantic explosion under thousands of tons of metal to forcefully push that payload out of the Earth’s gravitational field; this occurred in Florida and I sat many thousands of miles away watching it live; watching it live on my portable computer that was extracting the video from thin air with no wires or connections to the outside world of any kind. Amazing!

Atlas 5

Too often we experience these miracles of science without even a thought to the thousands of dedicated scientists, engineers and mathematicians who paved the way to our current technological state. Let’s change that!

Let’s cherish the memory of the great minds whose work makes our lives so much better every single day. I propose starting Tribute Tuesday, one day each week to recall the life and achievements of one great scientist. Join me as we explore the genius minds that gave us controlled electricity, vaccinations against disease, the telephone, or the atomic bomb. Let’s recall how the greats discovered relativity, gravity, atomic theory, evolution, or the speed of light. There are so many amazing stories!

To start us off I bring you the story of one of my favorite ancient scientists, Eratosthenes. You’ve probably never heard of him, but he used simple observations to figure out something incredible literally thousands of years before anyone else could even check if he was correct (spoiler alert, he was).


The year was roughly 250 BC. Rome was still a Republic, and was just one of many military powers in the Mediterranean. Alexander the Great’s conquest of the ancient world was still fresh in everyone’s mind, and our Eratosthenes lived in one of the greatest cities on Earth which Alexander had founded. He was the head librarian in the famous library of Alexandria.

The simple observation that Eratosthenes made was this: in the ancient city of Swenet, at noon on the summer solstice when you looked into a well there was no shadow whatsoever because the sun was directly above you. But, if you looked down a well at the same time in his home city of Alexandria, which was about 600 miles north, the sun was not directly above. Instead it cast a shadow down the well, which meant it had to be at an angle from the ground. If I had noticed this I think I would have just shrugged my shoulders and thought something like “weird” and moved on with my life. But Eratosthenes was clever.

He figured that if the sun was directly above in Swenet but at an angle in Alexandria, this meant that even though the land seemed flat it had to be curved. That’s right, he confirmed in 250 BC that the Earth was round!

But this isn’t the end of the story. Based on where the shadow was hitting the well in Alexandria, he calculated what angle the sun was from the ground which told him that the distance between Swenet and Alexandria must be about 1/50 of the total circle of the globe. So he took that distance and multiplied it by 50 and calculated the size of the Earth! What’s really amazing about this story was he got it right. Modern scholars have gone back and looked at his numbers, and his calculations were accurate to within about 2%! Here is an image showing his calculations. Note that in this image the city of Swenet is called Siena.

Eratosthenes calculation


Eratosthenes calculated the circumference of the Earth long before ships that could sail across the oceans and without leaving Egypt. And that is truly amazing.

Politicus Cerebri


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