Sorry PigDog: Beat you by a few minutes... Glad to see we both came up with roughly the same numbers.
Would like to see someone try this out for real at 40000. Find 2 points along the route with a distance of around 200 nm, fly over one, and try to see the other.
I suspect the air might never be clear enough to see all the way, let alone having 40000/212 eyesight...
Yes, good idea, off to bed
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PieterPan