How the ancient Greeks proved Earth was round over 2,000 years ago

  

• How the ancient Greeks proved Earth was round over 2,000 years ago


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      Dec. 27, 2017, 10:40 AM

• In the mid-20th century, we began launching satellites into space that would help us determine the exact circumference of the Earth: 40,030 km. But over 2000 years earlier, a man in Ancient Greece came up with nearly the exact same figure using just a stick and his brain. Following is a transcript of the video.

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  How an ancient Greek mathematician calculated the Earth's circumference. In the mid-20th century, we began launching satellites into space that would help us determine the exact circumference of the Earth, 40,030 km. 

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  But over 2,000 years earlier in ancient Greece, a man arrived at nearly that exact same figure by putting a stick in the ground. That man was Eratosthenes. A Greek mathematician and the head of the library at Alexandria. 

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  Eratosthenes had heard that in Syene, a city south of Alexandria, no vertical shadows were cast at noon on the summer solstice. The sun was directly overhead. He wondered if this were also true in Alexandria. 

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  So, on June 21 he planted a stick directly in the ground and waited to see if a shadow would be cast at noon. It turns out there was one. And it measured about 7 degrees.

  Now, if the sun's rays are coming in at the same angle at the same time of day, and a stick in Alexandria is casting a shadow while a stick in Syene is not, it must mean that the Earth's surface is curved. And Eratosthenes probably already knew that. 

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  The idea of a spherical Earth was floated around by Pythagoras around 500 BC and validated by Aristotle a couple centuries later. If the Earth really was a sphere, Eratosthenes could use his observations to estimate the circumference of the entire planet.

  Since the difference in shadow length is 7 degrees in Alexandria and Syene, that means the two cities are 7 degrees apart on Earth's 360-degrees surface. Eratosthenes hired a man to pace the distance between the two cities and learned they were 5,000 stadia apart, which is about 800 kilometers. 

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  He could then use simple proportions to find the Earth's circumference — 7.2 degrees is 1/50 of 360 degrees, so 800 times 50 equals 40,000 kilometers. And just like that, a man 2200 years ago found the circumference of our entire planet with just a stick and his brain. 

• This video was produced by Alex Kuzoian.

  This video was originally published on July 3, 2016.