[johnnyOnTheSpot]

Given people have always known the earth is round based on the shadow it casts on the Moon...

[saghm]

EDITED: Obviously not trying to argue that the Earth is flat, but flat things can still cast round shadows if they're, well, round

(ORIGINAL: Obviously not trying to argue that the Earth is flat, but there are round things that are also flat)

[Kenji]

A disk will also cast a round shadow in some cases.