How Ancient Greek Philosopher Eratosthenes Measure the Size of Earth ?
Eratosthenes is most famous for making the first good measurement of the size of Earth. He did that sometime around 240 B.C. He knew that there was no shadow at the bottom of a well in the town of Syene on the summer solstice. That meant that the Sun must be straight overhead in Syene on that day. He measured the length of the shadow of a tall tower in Alexandria on the same day. He also measured the distance between Syene and Alexandria. With this information, he was able to calculate the circumference of the Earth. We aren’t quite sure about his exact answer, because we don’t know the exact length the distance units he used (called “stadia”). He may have gotten it right to within 1%, or he may have been off by about 16%. Either way, his answer was pretty good… and he was the first person to make a good measurement of the size of Earth!
