Eratosthenus was the first person to calculate the earth’s circumference with a fair accurate measurement. It took us another 2500 years to confirm his calculation!
His calculation is very simple and logical. He was intrigued that at Syene, a southern city of Egypt the sun was directly overhead at noon time (which means there were no shadows) on June 21 (summer solstice). At the same time, there was a considerable amount of shadow in Alexandria, a city 800 km from Syene. If earth were to be parallel this cannot be the case. At both places there won’t be a shadow from any pillar or building.
But in reality, in Syene there is not a shadow, but in Alexandria there is a shadow. He thought logically about this problem. He could conclude that only if the two places are not parallel that can happen, which means that earth is not flat but is curved or rounded!!!
He applied simple geometry and found the angle of shadow from the pillar to be 70. He extended the angle to the center of the earth which corresponded to 800 kms, the distance between Syene and Alexandria. He then easily calculated the circumference of the earth to be 41, 000 kms which is very true with less than 1% of error!
Imagine he did this feat 2500 years back and it took almost 2000 years for the mankind to re-discover his calculation!
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