Rounded Earth is known since ancient Greeks. They used to compare the shadows of sticks in different locations. When the sun was directly overhead in one place, the stick there cast no shadow. At the same time in a city around 500 miles north, the stick there did cast a shadow.

If the Earth were flat then both sticks should show the same shadow (or lack of) because they would be positioned at the same angle towards the sun. The ancient Greeks found the shadows were different because the Earth was curved and so the sticks were at different angles. They then used the difference in these angles to calculate the circumference of the Earth.