Twilight means either dawn (in the morning) or dusk (in the evening).
In the morning, dawn is when it is not dark any more, and the light of the sun is just starting to appear. In the evening, it is the other way around and that phenomenon is called dusk. The sun has disappeared, and darkness is setting 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 the sea which is flat.