on quiet days.
Another way of looking at it is to ask what is the "curvature" of the
-earth's surface Over a considerable length, how much does the surface
+earth's surface. Over a considerable length, how much does the surface
deviate (on the average) from perfect flatness. The flat-earth theory
would make it seem that the surface doesn't deviate from flatness at
all, that its curvature is 0 to the mile.