Around 500BC Pythagoras suggested the spherical Earth. He didn't have evidence to prove it but for him, the sphere was a perfect shape.
Many years later Aristotle (350BC) presented arguments, like when a ship sails away over the horizon, the hull vanishes first and the Earth casts a round shadow during an eclipse etc.
Around 240BC it was the great scholar Eratosthenes who first measured the circumference of the earth.
Many of you would have heard the "Sieve of Eratosthenes", a method of listing prime numbers and he is also famed for his solution for "doubling a cube".
He had heard from travellers about a well in Syene (Aswan, Egypt) with an interesting property: at noon on the summer solstice, about June 21 every year, the sun illuminated the entire bottom of this well, without casting any shadows, because it was directly overhead.
Eratosthenes then measured the angle of a shadow cast by a stick at noon on the summer solstice in Alexandria, and found it made an angle of about 7.2 degrees, or about 1/50 of a complete circle.
Then he realised that if he knew the distance from Alexandria to Syene, he could easily calculate the circumference of Earth.
He assumed that the Sun is very far away and it's rays are parallel and geometrically calculated the circumference of the Earth to a remarkable degree of accuracy.
Not only this, but Eratosthenes estimated the distance to both the sun and the moon, and measured the tilt of the Earth’s axis all with amazing accuracy.
