*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.*

*From BusinessInsider by Alex Kuzoian*

How an
ancient Greek mathematician calculated the Earth's circumference.

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 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.

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