It's a valid question, and it also raises some other questions. Why are there time zones, or at least so many time zones, with a flat planet? Newton, besides articulating the laws of gravity and motion, also articulated the laws of optics and light - and the peculiar excuses offered by FEers about why stuff disappears over the horizon but doesn't actually fall off, or why we can't see London or even Boston from the top of the Empire State Building, etc., on a flat planet seem to violate those laws.
Here's another question: At or near the north pole, watching the stars move around the pole they move counter-clockwise; at or near the south pole, watching the stars move around the pole they move clockwise --- that makes sense if the earth is a globe but it seems impossible to explain if the earth is flat.