Eratosthenes of Cyrene
The painting below is called Eratosthenes Teaching in Alexandria, (1635) by Bernardo Strozzi. (A bit of an anachronism here--in Eratosthenes time The Library of Alexandria would have had scrolls rather than books!)
Eratosthenes of Cyrene (276BC-194BC) was first person to show that the earth was a sphere and then calculate its circumference. How did he do it?
Well, he had heard that in Syene, a city south of Alexandria, no vertical shadows were cast at noon on the summer solstice. He wondered if this were also true in Alexandria. Then he did a bit of an experiment. On June 21 he planted a stick directly in the ground and waited to see if a shadow would be cast at noon. It turns out there was one. And it measured about 7 degrees.
He then deduced that if the sun's rays are coming in at the same angle at the same time of day, and a stick in Alexandria is casting a shadow while a stick in Syene is not, it must mean that the Earth's surface is curved. Beautiful deduction!
Then he hired a person to pace the distance between the two cities and learned they were 5,000 stadia apart,(800 kilometers). He then did a little calculating: 7.2 degrees is 1/50 of 360 degrees, so 800 times 50 equals 40,000 kilometers which had to be the earths circumference.
He was not very far off: the actual circumference of the earth is 40,075 Kms! Pretty nifty for a polymath that lived 2200 years ago. He was also the chief librarian at the Library of Alexandria.
Of course Eratosthenes remains, to this day, 'persona non grata' at the flat-earth society!