Civilian night time, when the sun is >=6 degrees below the horizon
Without using a book, Brooklands, you can work this out.
The Earth turns 360 degrees in 24 hours (we call that one day
). This equates to 0.25 degrees per minute, or 4 minutes per degree depending what you're looking for.
So, if civilian night is when the sun is 6 degrees below the horizon, that means 24 minutes after the sun sunk below the horizon. Half an hour is near as dammit to that, perhaps this is the source of the half-hour rule.
GT's argument that it depends on latitude seems upside down. Surely in Florida, they'd have only a couple of minutes, if it gets darker quicker. However, back to the notes from Brooklands, perhaps they get their time from Nautical night (which'd make more sense, considering all the nautical terminology used in aeronautics).
"STOP WAFFLING RICHARD" I hear myself saying.