As dusk is called the flowing transition between day and night before day start or after end of the day, caused by the scattering of light in the atmosphere. One distinguishes between
Dawn (before sunrise) - a morning transition from the darkness of the night to the brightness
Dusk (after sunset) - the evening transition from the brightness of the day for darkness.
In twilight course of three consecutive phases can be distinguished and separated according to how deep the sun is below the horizon (Sun indicated below the horizon as a low angle of the sun disk center), so evening:
Civil Twilight - reading outdoors possible (low angle to 6 degrees)
nautical twilight - horizon (chine line) still recognizable and quite a few stars visible (low-angle to 12 degrees)
astronomical twilight - deeper until they reach maximum darkness night (low angle to 18 degrees)
Uploaded: Aug 10, 2016