most of that is simple geometry (heh). The interesting parts creep in with the 'non-ideal' conditions.<p>You're not just referring to the percentage of the surface area of a sphere (which earth isn't), atmospheric distribution isn't uniform even if we go by volume (or should we be going by density?), then there's the curvature of light in the atmosphere to take into consideration, and the arbitrary descisions as to what height above sea level our observer is standing at, the variable nature of the tropopause ...<p>an interesting problem, but likely more interesting as a mental exercise than in actual execution.