Originally Posted by pythos (Post 1332685)

er. Has anyone here been up in an airplane, or at sea.

My experience is there is a very definite line at the horizon. I know of this cause when flying VFR the horizon is my primary reference point, there is a definite line at the horizon. It is always lighter than the area below and above, what causes it, I don't quite know, but it is there.

It is visible in photos of the horizon as seen from space. Very visible in shots from the X-15.

It is almost like a corona around the Earth.