NavList:

Josh,

You wondered about the invisible true horizon, the uncertainty of refraction, and the transition to the vacuum for most of the light path up to the orbit of the ISS. There's a way around all of these issues at once. Use the oxygen level (air glow level) as a pseudo-surface for the Earth. It's high, about 95 km up, where the atmosphere has zero refractivity at visual wavelengths. And we can see it clearly from orbit. It's in all of those photos that started this. Just change one thing.

To use the air glow line as the Earth's surface, measure observer height of eye from the remainder of the altitude above 95 km. Just "pretend" that's the surface of the Earth geometrically. Also, use an unrefracted dip equation 1.06*sqrt(ht_in_feet) will yield good results. This is not perfect. The air glow line is still "fuzzy" but it's better than trying to guess the ground level. Also, the orbit of the ISS varies up and down, both due its elliptical form and also due to the oblateness of the Earth beneath it.

I did not use the air glow pseudo-surface in my earlier analysis. What do you get if you try that?

L.A.M.