HOW ERATOSTHENES CALCULATED THE EARTH'S CIRCUMFERENCE

 

Ancient Greeks made some of the most impressive astronomical discoveries in history, including when Eratosthenes calculated the circumference of the earth.

 

It wasn’t until the mid-20^th century that we managed to launch satellites into space and determine that the exact kilometres of the circumference of the earth 40.030.2 kilometres.

 

But how could then the ancient Greek mathematician, Eratosthenes, manage almost precisely the same number, without having any image of the earth from space or even proper measuring tools?

 

Amazingly, Eratosthenes didn’t have much more than a stick and his brilliant brain when he made this amazing discovery.

 

Born in Cyrene, the ancient Greek colony in modern-day Libya, in 276 BC, Eratosthenes was a polymath meaning that he had enormous knowledge of many different subjects, including mathematics, astronomy, music theory and poetry.

 

Over two thousand years ago Eratosthenes heard that Syene, a town south of Alexandria, in Egypt, no vertical shadows were cast at noon, during the summer solstice as the sun was directly overhead.

 

He decided to conduct an experiment.  On the 21^st of June he went to Alexandria and put a stick directly in the ground and waited to see if a shadow would be cast at noon. It turned out that there was one and when measured it was seven degrees.

 

After concluding the experiment, Eratosthenes came to  the conclusion: If the sun’s rays are coming from the same angle,  at the same time of day, and a stick in Alexandria casts  a shadow of seven degrees, while the stick in Syene does  not cast a shadow at all, it must mean that the earth’s surface is curved.

 

The idea of a spherical earth was already known by Pythagoras, around 500 BC and was validated by Aristotle a few centuries later.

 

So if the ancient Greeks before him were right, and the earth was a sphere, Eratosthenes could use his discoveries to calculate the circumference of the planet.

He found out that the two cities were 5000 stadia apart, which is about 800 kilometres. He could then use simple proportions to find the earth’s circumference.  7.2 degrees is 1/50 of 350 degrees, so 800 x 50 = 40.000 km.

 

Eratosthenes calculated precisely the circumference of the world over 2.000 years ago!         

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