Jose writes:

When you are at flight levels, due to the curvature of the earth, on first
apperance, the "higher" aircraft may appear to be lower,

The curvature of the earth is greater in the bugsmasher altitudes.

How far away are you seeing these airplanes?

FWIW, at FL360 aircraft will be already be in line of sight as much as
200 nm away (ground distance). Of course, human vision and
atmospheric conditions put more restrictive limits on visibility.

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