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How long have we known that the earth is a sphere?
(hp). Perhaps you have also heard that people allegedly still thought in the Middle Ages that the earth was flat. Christopher Columbus was one of the first people to believe in the spherical shape of the earth.
With his voyage in 1492 he wanted to prove that Asia and India can not only be reached by land with a journey to the east, but - since the earth is a sphere - one can also sail westwards by sea. This story is of course nonsense: the educated people of the Middle Ages knew that the earth was a sphere. However, they did not agree on how big this globe was. Christopher Columbus, for example, underestimated the size of the earth quite considerably - about a quarter. Columbus ‘attempt to reach India by sea to the west would have failed miserably because the seafarers would inevitably run out of drinking water and provisions on the way. Only the fact that Columbus "accidentally" discovered the continent of America on his way to India saved him and his team from dying of thirst and starvation.
In fact, knowledge of the spherical shape of the earth had been widespread since ancient times. This knowledge replaced the idea of the earth as a disk surrounded by an ocean, as it is often described in the creation myths of many peoples.
In Europe, people first suspected around 2,500 years ago that the earth could possibly be a sphere. The Greek philosopher Plato was one of the first people to deduce the spherical shape of the earth through logical conclusions.
He had noticed that of sailing ships leaving the harbor, first the hull disappears below the horizon line, then gradually the sails, until finally only the top of the mast can be seen and this too dips below the horizon a little later. Such a thing is of course only possible if the surface of the sea is curved like on a sphere.
It is similar with the constellations, noted Plato: For example, if you travel far enough from north to south, you can see that southern constellations are higher in the sky. Such an observation is only possible if the earth is a sphere.
About 200 years later, mathematician Eratosthenes observed that on a certain day of the year the sun was reflected in the water of a deep well in southern Egypt. From this he concluded that the sun was exactly at the zenith that day, i.e. vertically above the fountain.
On exactly the same day of the year, however, an obelisk casts a brief shadow in the city of Alexandria, about 800 kilometers to the north. From the length of the shadow, Eratosthenes was able to calculate that the sun is not in the zenith there on that day, but that its light falls at an angle of 7 degrees.
Such an observation can only be made if the fountain and obelisk are not on a flat, but rather curved surface.
Since Eratosthenes was an extremely brilliant mathematician of his time, he was able to calculate the circumference of the earth from the known distance between the fountain and obelisk and the angle of the (parallel) incident sunlight: his result was almost exactly 40,000 kilometers, which is ours today corresponds exactly to a few kilometers. However, this knowledge was lost again over the centuries and was only rediscovered in modern times - after Columbus.
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