That man's name was Eratosthenes. A Greek mathematician and the head of Alexandria's library. Eratosthenes had heard that at midday on the summer solstice, no vertical shadows were cast at Syene, a city south of Alexandria. The sun shone brightly overhead. He was curious if this was also true in Alexandria.
So, on June 21, he put a stick in the ground and waited for a shadow to be cast at midday. There was, in fact, one. And it was around 7 degrees.
If the sun's rays are coming in at the same angle at the same time of day, and a stick in Alexandria casts a shadow but a stick in Syene does not, the Earth's surface must be curved. And Eratosthenes was undoubtedly aware of it.
Pythagoras presented the notion of a spherical Earth in approximately 500 BC, and Aristotle supported it a few decades later. If the Earth truly was spherical, Eratosthenes' findings might be used to calculate the circumference of the entire world.
Because the difference in shadow length between Alexandria and Syene is 7 degrees, the two cities are 7 degrees apart on Earth's 360-degree surface. Eratosthenes recruited a man to measure the distance between the two towns and discovered they were 5,000 stadia (approximately 800 kilometers) apart.
He could then calculate the Earth's diameter using basic proportions - 7.2 degrees is 1/50 of 360 degrees, thus 800 times 50 = 40,000 kilometres. And thus, 2200 years ago, a guy using only a stick and his intellect discovered the circumference of our whole world. Below is another video, and Carl Sagan explains the same beautifully.
Still today, some people believe Earth is Flat and that's just sad.