Whats the closest distance I can see from the track of an airliner while in the flight levels? From my estimates it’s about 2.5 to 5 nm. Next time I’m flying I’ll bring a sectional with me and find out what it actuall is.

Lines “a” and “b” are parallel,
lines “g” and “e” are parallel,
lines “e” and “g” are perpendicular to lines “a” and “b”,
angle DAE is equal to angle FAG,
the distance of AF is 13 in (+/- 2 in),
the distance of FG is 8 in (+/- 2 in),
point “A” is where your eye is.

Excellent! Also need to add A and A’ to allow for a person to move their eyes from the bottom to the top of the window, thus changing the angles of view.

You can get a good approximate solution with a simple ratio.

AF:FG::altitude:distance on ground that you can’t see

So the distance on the ground you can’t see, in nm, is approximately your altitude (in feet) divided by 10000 (6080*13/8). 1.5 nm at 15000 feet, 4 nm at FL400, etc.

Well that calculation has many factors like wing dimensions, where you’re siting, if the sun is reflecting on the window (see pic below), and type of aircraft (this REALLY matters if it’s a CRJ or one of those nasty jets).

View of the South satellite from a NWA 757-300 seat 32A. Note the UA 777-200, NW 757-300 and A330, Horizon Dash 8B (Q200), and Hawaiian 767-300.