Nautical twilight

Nautical twilight starts in the morning when the geometric center of the Sun is 12 degrees below the horizon and ends when the when the geometric center of the Sun is 6 degrees below the horizon. It starts in the evening when the he geometric center of the Sun is 6 degrees below the horizon and ends when the geometric center of the Sun is 12 degrees below the horizon.

Nautical twilight refers to sailors being able to navigate during this time of day, because both stars and the horizon are visible. Terrestrial objects may be difficult to discern, so artificial lighting may be needed.