In fact, the Greek philosopher Pythagoras was the first to come up with the concept of a spherical Earth back in the 6th century BCE. Pythagoras’s idea was based on his observations of the moon during a lunar eclipse. He noticed that the shadow cast by the Earth on the moon was always round, no matter what part of the planet was facing the moon.
But it wasn’t until about 200 years later that another Greek scholar, Eratosthenes, came up with a way to actually measure the circumference of the Earth.
Eratosthenes was the chief librarian at the Library of Alexandria in Egypt.
He knew that at noon on the summer solstice, the sun was directly overhead at the city of Syene (now called Aswan). But he also knew that in Alexandria, about 800 kilometers to the north, the sun was not directly overhead at that same time.
By measuring the angle of the sun at Alexandria, and knowing the distance between the two cities, Eratosthenes was able to calculate that the circumference of the Earth must be about 250,000 stadia. (Astadia was a unit of measurement used by the Greeks.
There is some debate about how long one stadia actually was, but estimates range from 160 to 210 meters. This would put the circumference of the Earth at anywhere from 40,000 to 50,000 kilometers.)
This was an amazingly accurate estimate, considering that Eratosthenes came up with it more than 2,000 years ago! And he didn’t even have access to modern instruments or technology.
The ancient Greeks figured out the Earth was round and even calculated the diameter pretty accurately.
https://magnifymind.com/who-discovered-the-earth-is-round/