## The astrophysicist explains how the Greek polymath Eratosthenes was able to calculate the circumference of the Earth.

Incredible as it may seem, in the 21st century there is a current that assures that the Earth is flat. Those who believe in this, the Earth flatteners, have access to information gathered during centuries of scientific research: from the calculations of Eratosthenes to the NASA missions through which we now know what our planet looks like from the International Space Station. Even so, those who choose to lean toward the flat-Earth theory scorn this information and insist that the Earth is not spherical, but flat. As if it were a compact disc floating in space.

The flat-Earth theory has been around for many years: it is not a trend that emerged on the Internet, although it did find countless spaces online to spread. The ancient Greeks, however, knew that the Earth is round and Eratosthenes, who lived between 276 BC and 195 BC, calculated the circumference of the planet with great precision.

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At the time, the astronomer, astrophysicist, writer, and science popularizer Carl Sagan addressed the subject to demonstrate, with a drawing representing a map of ancient Egypt and a pair of obelisks placed in Siena and Alexandria, that the Earth’s surface is curved.

If the Earth were flat, the shadows cast on it would all be the same size. “The bigger the curve, the bigger the difference in shadow size,” explains Sagan.

In a video retrieved by Astro Adventure, Carl Sagan explains that, if the two obelisks were extended to intersect at the center of the Earth, they would do so at an angle of seven degrees. That represents one-fiftieth of the total circumference of our planet, which is 360 degrees.

Eratosthenes knew that the distance between the two was 800 kilometers because he hired a man to travel the distance between Alexandria and Siena; and 800 multiplied by 50 is 40 thousand kilometers: the circumference of the Earth.

“Eratosthenes’ only tools were rods, eyes, feet, brains, and a fervent yearning for experimentation,” says Sagan.

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Story originally published in Cultura Colectiva in Spanish