ECT & MCT are well defined, being when the geometric centre of the sun is 6* below the horizon.
Note that the "horizon" is the astronomical or theoretical horizon viewed from sea level in this definition. The visible horizon (what you actually see) is a little lower due to atmospheric refraction and the elevation of your eyeball.
I think CASA has done a reasonable job warning pilots that terrain and cloud may make the world a very dark place even during the period of civil twilight. Let the user beware.
Latitude also changes the duration of twilight a lot. There is a very nice graph of twilight duration vs. latitude, on wikipedia - twilight.
Last edited by Oktas8; 20th Aug 2014 at 07:33.