How Eratosthenes Proved the Earth Is Round 2200 Years Ago

Eratosthenes was an ethnic ancient Greek mathematician who managed the great library of Alexandria, Egypt circa 240 BC. He was born in Cyrene, Libya in 276 BC and died in Alexandria, Egypt in 194 BC.

One day he was reading a text that told of how at midday on the summer solstice in the city of Swenett,1 Egypt, the sun’s rays would shine straight into a well to illuminate the water at the bottom. Likewise, none of the buildings cast shadows because the sun was straight overhead.

This got Eratosthenes to thinking. So on the next summer solstice, at noon, in Alexandria, he went outside expecting to see the sun straight overhead and none of the buildings casting shadows. For that would be the case if the Earth were flat.

But that was not the case. Eratosthenes witnessed shadows being cast all over the place because the sun was not directly overhead. So he embarked on an epic experiment. Between then and the next summer solstice Eratosthenes hired a man to measure the distance from Alexandria to Swenett which turned out to be 800 kilometers.

On the day of the next summer solstice, Using a plumb line to insure verticality, Eratosthenes drove a pole into to the ground and then at noon measured the length of the shadow and the angle formed by the shadow and the pole:

Now is the time for everyone’s eyes to glaze over:

A is Alexandria. S is Swenett. E, on the lower left is the center of the Earth. AT is the length of the shadow. At Alexandria, Eratosthenes measured the angle formed by the top of his pole and the end of the shadow to be, 7 degrees. By geometric theorem, the angle formed by Alexandria, the center of the earth and Swenett must also be 7 degrees.

Consequently, 800 kilometers from Alexandria to Swenett corresponds with 7 degrees. Since the circumference of a circle is 360 degrees, 360 divided by 7 is approximately 50.

Therefore, 800 kilometers multiplied by 50 equals the circumference of the earth.

800 x 50 = 40,000 kilometers. Which turned out to be the case.

Here is a video of Carl Sagan doing a great job of explaining all this mumbo jumbo:

1 Swenett was called Syene by the ancient Greeks and today is called Aswan.

23 responses to “How Eratosthenes Proved the Earth Is Round 2200 Years Ago”

  1. Great Lion, Each molecule of water is acted upon by the earth’s gravity as are all atoms and molecules which make up the earth. This gravity pulls all matter toward the center of the earth. This is why sea level is possible on the spherical earth. Since your explanation does not account for gravity your flat earth explanation using water is in error. And you still have not explained how there can be no shadows at Swenett, Egypt on the summer solstice and shadows 800 kilometer north at Alexandria. This phenomenon can only happen if the earth is round.


  2. See friend, it’s easy to be civil while kinks are worked out. People are watching how carry our points.

    Gravity. Ah yes. Butterflies and ripe apples are oblivious. But the ant thing S- you and I were not designed to walk upside down on ceilings, a true magical act for they down under.

    The shadows? Eh, no big deal if the sun were close eh.


  3. Great Lion, Asked and answered. Obviously you are not reading my comments. Newton’s law of motion, F = ma explains why ants can walk on ceilings and man cannot. In the space station in orbit around the earth, man can walk on the ceiling.

    Even the light rays emitted by a flash light are parallel. So too, are the light rays emitted by the sun, even if it were close.

    Additionally, a simple reflecting telescope you can buy on Amazon demonstrates that the rays of light emitted by celestial objects are parallel.

    Great Lion, I have successfully refuted each of your arguments. The fatal flaw in all of them is simple misunderstanding of how the world works (science):

    1. You do not understand gravity.
    2. You do not understand properties of light.


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