Twilight

Twilight means either dawn or dusk when the Sun is below the horizon but still lights the sky.

In the morning, dawn is when it is not dark any more, and the light of the sun is just starting to appear, before sunrise. In the evening, it is the other way around and that phenomenon is called dusk. At sunset, the sun disappears, and darkness is settling in. Light fades away.

The following have been defined:
Civil twilight is when the sun is less than 6° below the horizon. It ends at sunrise in the morning or follows sunset in the evening.
Nautical twilight is when the sun is between 6° and 12° below the horizon.
Astronomical twilight is when the sun is between 12° and 18° below the horizon.
For all these definitions, the horizon is at sea level.

 Related pages 
Aurora (mythology)
Eos

 Other websites 

Definition of Twilight , US Naval Observatory
Twilight time calculator 
Formulae to calculate twilight duration , by Herbert Glarner

Category:Parts of a day