Eli5: How did people in olden days (such as Galileo) come to the conclusion that the earth was spherical without very advanced technology?
In: Physics
The first estimates were made by Eratosthenes, in about 240 bce.
Astronomical dates like midsummer and the solstice had been known for thousands of years. What Eratosthenes did was confirmed a rumour of a place where the midsummer sun perfectly illuminated the base of a well (the equator), then on the same date measured the length of shadow cast by a vertical pole at a location that was several hundred km away. By knowing the distance apart, and the fact that one place had vertical sun Ray paths, and the other Ray paths at an angle, he was able to use basic trigonometry to estimate the earth’s diameter. His value is within measurement error of the true value.
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