Originally Posted by steve64
It's the curvature of the earth.
Think "Line of Sight" literally...
A rough Rule of Thumb:
1.23 * (altitude (ft.) ^ 1/2) = horizon (sm).
@ 40000 ft:
1.23 * 200 = 246 st. miles
I read somewhere once that a circle w/ a radius of this is something like 8% of the surface area of the US.
added: That dumb thumb is supposed to be an exponent operator.