How did they prove the Earth is round?
Today it will be funny to say that the Earth is not round, but this was not the case forever. We humans at some point believed the Earth to be a flat disk, many ancient civilizations, including the Greeks, Egyptians, and Mesopotamians, used to believe this because of their everyday experiences of the Earth appearing flat when looking around. But some thought differently about the structure of the Earth and used their brains in those times to prove it to be round.
Many had the idea of a round earth in their minds but in the 15th century BC, Pythagoras, can say, was the person to bring his ideas out, he argued that when we see a ship moving in the sea, we should observe it to be becoming smaller and smaller if we are on a flat surface, but we observe it to be disappearing from bottom gradually. And this could only happen on a curved surface. Although he thought of it, he was not the one to prove it.
Then came Aristotle, he was a well-known Greek philosopher and scientist. In 350 BC, this man tried to explain the earth’s curvature by observing the shadow of the earth on the moon during the lunar eclipse. Every time the shadow formed was round and only a spherical object can form such. If the earth were to be a flat disk then the shadow on the moon must appear to be a straight line. He also gave a second proof by explaining that the constellations that were seen in Egypt were not seen in Cyprus (about 217 miles from Egypt) but on a flat surface one should see the same things despite the position of the person, this means the person must have moved the curved surface. These ideas were good and opened new ways of thinking. This is how we humans have come so far by proving and disproving things.
It was Eratosthenes, a Greek scholar who did calculations to prove the Earth to be round. We science people do not believe in the theories until math is involved. He first wanted a place where no shadow was formed in the daytime, which means the sun was exactly overhead, and this was the Egyptian city of Syene (now Aswan). He noted the time and at the exact time and date in Alexandria (Egypt), he used a pillar that was casting a shadow which means the sun was not directly overhead but was slightly south. So the rays made an angle with the vertical equal to about 1/50 of a circle (7°). The sun is so far from us that the rays that reach us are parallel, because the sun’s rays striking the two cities are parallel to one another, why would the two rays not make the same angle with Earth’s surface?
This is a great question, he reasoned the curvature of the earth. He hired a man to calculate the distance between Syene and Alexandria which was “5000 stadia” (The “stadium” was a Greek unit of length, derived from the length of the racetrack in a stadium). Alexandria, he saw, must be 1/50 of Earth’s circumference north of Syene. So he calculated the circumference of the earth to be 50 × 5000, or 250,000 stadia. Because we don’t know which stadium he used for the unit, we have two assumptions, if it was the Olympic stadium the common one, then he is 20% too much, but, according to another interpretation, he used a stadium equal to about 1/6 kilometer, in this case his figure is within 1% of the correct value of 40,000 kilometers. This is amazing to know because at the time of no proper tools, he did it. The circumference of the earth what we know today with our advanced tools is approximately 40,075 kilometers (24,901 miles) at the equator.
Later, Eratosthenes also made a map of the Earth while sitting in the Library of Alexandria, a great man of his time!
This is how we started believing in the roundness of the Earth!
