> In other words, cases where it's not reasonable to assume
> that the earth is flat (as when you're building a house), but
> need to take into account the earth's curvature (as when
> you're sending a rocket to the moon and then need to bring
> it back).
My brain must be fried after teaching so many classes back to back since early this morning. The curvature of the earth doesn't react to your moves or anticipates them. :/ What I mean is that you cannot skip that obvious fact in economics that, when things get interesting, some usual givens get in a state of flux that results inter alia from what you do.