For most people, in most situations, the Earth is flat. We look at flat maps, we make small-angle assumptions on the direction of gravitational force. Even civil engineers building huge structures make this assumption in their calculations. Of course we know the Earth is not flat... In some cases we need to assume the Earth is round - e.g. using the stars for navigation works better with a round Earth assumption. The seasons and yearly weather cycles also fall into this category. Of course we know the Earth is not round... The Earth is actually a squashed ball, fattest at the equator. Well, actually there's also a little bulge at the North Pole. Plus a bunch of mountains and oceans... And on top of that we need to account for general relativity and the non-Euclidean nature of space with respect to mass and velocity etc... Little fuzzy on that. And that's not even including whatever quantum effects there are. But if you think about it, the Earth only curves about 0.67 feet over 1 mile. That's the flattest thing most people will ever come across. I was thinking about this after reading Asimov's "The Relativity of Wrong". He was more arguing that science is never right but always getting closer to the truth. But I think he left out something - the first claim, "the Earth is flat", while being the most "wrong", is also the most useful basic assumption to make in most cases.