How far away is that airliner...?
Okay, here's the scene:
It's early morning, the sky is crystal clear. You look up, and cruising along at 37,000' (or whatever), you see a dinky little airliner, blatting it's way across the skies, leaving 4 contrails for miles behind it. You keep watching the 747 or whatever it happens to be.
Ten minutes later, you can still see it in the distance, many many miles away.
What I want to know is : HOW FAR AWAY WOULD IT BE?
(I know there are in infinate number of variables which would affect the answer, like atmospheric quality, light, blah blah blah - I'm just after a rough answer)