Varieties of twilight

Twilight occurs at dawn and dusk, the times when the sun itself is entirely below the horizon but sunlight glows over the horizon. People who live inside the Arctic Circle like in northern Alaska or Norway get their only daily light from twilight for several months out of the year, during the months of polar night (24 hours with no sun over the horizon).

Twilight comes in several strengths, each one named for the type of function that can be done by it. These strengths listed below are important to a variety of people like
-photographers
-pilots of ships and planes
-people in Arctic regions who get long periods of twilight, sometimes their only light of the day if they live inside the Arctic Circle where during polar night months the sun does not rise over the horizon

In order of dark, darker, darkest:

Civil twilight - brightest twilight, can distinguish terrestrial objects clearly (i.e. perform "civil" functions like driving, etc.) Only the brightest stars appear.
Sun angle is up to 6 degrees below horizon
Civil twilight in London

Nautical twilight - sea horizon line is visible which allowed sailors to measure star heights from the horizon. Can distinguish most stars and objects' general shapes but not their details. Need artificial light to see objects clearly.
Sun angle is 6 to 12 degrees below horizon
Nautical twilight overlooking St. Peres de Ribes, Spain and the Mediterranean Sea

Astronomical twilight - can distinguish stars easily, twilight is barely perceptible, cannot distinguish sea horizon.
Sun angle is 18 degrees below horizon
Astronomical twilight, Simi California

When each period of twilight ends, there is a dusk as shown in the fishing diagram below.