[bvw]

The fact that the shape of the earth is evident by the shape of the shadow during a lunar eclipse, if not the shape of moon and sun themselves indicates to me that the spherical nature of the earth was known to prehistory. Another common observation that would suggest non-flatness is the disappearance of boats over the horizon -- since the water level is flat.

[Junior]

Not necessarily. Pythagoras deduced it because he determined (empirically) that the only shape to cast a circular shadow from any angle was a sphere. First, however, he had to determine it was the Earth's shadow blotting out the Moon. From early historic records we know that many cultures considered lunar eclipses to be magical phenomenon, such as giant monsters devouring the Moon.