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.

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. 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. 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. 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 actually
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. 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. 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.

Via Businessinsider

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