Today we can easily find out the circumference of our planet by simply 'Googling' it online.
By doing so, we will see that the exact circumference of the Earth is 40 075 km. There are some, however—the Flat Earth Society to be precise—who argue that the Earth is in fact not a sphere, but a flat plane.
However, more than 2000 years ago, ancient scholars managed to find out the Earth is a sphere—though an irregularly shaped one.
So, how on Earth did the ancient know the Earth was round, thousands of years ago, and without modern equipment?
The first man to suggest the Earth was a sphere—to put the term to paper—was no other than the Great Aristotle, one of the greatest Greek scholars, a polymath with extensive knowledge in politics, poetry, theater, music, natural sciences, and philosophy.
Aristotle, however, was also a great astronomer. Aristotle wrote in 350BCE in the treatise "On the Heavens," Book II, Chapter 14:
"Again, our observations of the stars make it evident, not only that the Earth is circular, but also that it is a circle of no great size. For quite a small change of position to south or north causes a manifest alteration of the horizon."
"Indeed, there are some stars seen in Egypt and in the neighborhood of Cyprus which are not seen in the northerly regions; and stars, which in the north are never beyond the range of observation, in those regions rise and set. All of which goes to show not only that the earth is circular in shape, but also that it is a sphere of no great size."
However, it was Eratosthenes who ‘nailed down’ the concept of a spherical Earth, and who managed to prove it more than 2,000 years ago.
When Eratosthenes was visiting Syene (now Aswan), he observed a phenomenon that caught his attention: at midday, on a summer solstice, there were no vertical shadows being cast down. This happened because the Sun was directly overhead.
Upon returning to his city, Alexandria, the mathematician tried to prove if the same thing happened there. He stuck a stick in the ground to see if, at noon, it generated shadows or not.
And of course, there was a shadow, of 7 degrees to be precise.
The conclusion that Eratosthenes drew from this phenomenon was the right one: The Earth must be spherical.
If it was not curved or round, he should not have seen any shadows in Alexandria, as it happened during his visit to Aswan.
However, Eratosthenes wanted to know more about the Earth, and after having discovered the importance of the shadows, he began to calculate how much the circumference of the Earth measured using the shadows.
He asked a man to calculate the distance between Syene and Alexandria, which was equivalent to about 5000 stadiums (800 kilometers).
Then, Eratosthenes drew a simple conclusion: if the 7 degrees—or as he put it, one-fiftieth of a circle—of difference between Alexandria and Syene are 800 kilometers, the 360 degrees of the Earth are about 40,000 kilometers. Turns out, he was not far from the real circumference.
In this way, the Greek mathematician obtained a number very similar to the one we know now, using nothing more than a stick, the shadows, a bit of common sense and math.
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