How was the size of Earth first measured?
More than 2000 years ago, earth’s circumference was first scientifically measured by a Greek astronomer Eratosthenes living in Alexandria, Egypt. He read somewhere that in the nearby town of Syene midday sunlight shines straight down to the bottom of deep wells on the same day each year. In Alexandria, however, sunlight on that date did not reach the bottom of a well, but instead fell on its side. Eratosthenes thought that if the earth was flat as was commonly believed then why the observation in Syene and Alexandria are different on the same day. He reasoned that this is possible only if the earth’s surface is curved. So, by measuring the angle of incoming sunlight in Alexandria on that day he could estimate the size of the round earth. It was measured to be seven degrees away from the zenith, meaning Syene must be seven degrees away from Alexandria as measured on the circle that is Earth’s circumference. Eratosthenes hired a man to pace the distance between Syene and Alexandria. Sinc
