As Clinton would attest [it] depends upon which definition of reasonable is reasonable. The Earth being flat was quite a reasonable assumption -until proved round...
I'm not sure it's true that "the Earth being flat was quite a reasonable assumption". Consider the implications of that position. If the Earth were flat, it would either stretch to infinity in all directions (which might have been true, but would not have been reasonable to suppose since it would have lacked the support of additional evidence—nothing else in nature appears to be infinite), or else it would arbitrarily stop at some distance away from the observer (which is also not reasonable, since one would then have to postulate an irrational arbitrariness in nature—why does it stop here rather than there?).
For further fun on such questions, read the Antinomies in Kant's Critique of Pure Reason.