Explanation of twilight.
Twilight is the period of approximately one hour’s duration before the sun rises in the morning and the period of approximately one hour’s duration after the sun sets in the evening, ie between the times when the sun is 12° below the horizon and sunrise in the morning and between the times of sunset and when the sun is again 12° below the horizon in the evening.

Civil twilight (duration approximately half an hour).
During this time, it is considered there is not sufficient natural light to perform the normal day to day tasks

• Morning civil twilight. This is the period between the time the sun is 6° below the horizon in the morning and the time of sunrise.
• Evening civil twilight. This is the period between the time of sunset and the time when the sun is 6° below the horizon in the evening.

Nautical twilight (duration approximately half an hour).
It is during this period under normal weather conditions that star sights are possible at sea. During this period the horizon is still reasonably defined and only the brightest stars and planets appear. This makes it easier to identify them and to measure their altitude above the horizon.

• Morning nautical twilight. This is the period between the time the sun is 12° below the horizon in the morning and when it is 6° below the horizon.
• Evening nautical twilight. This is the period between the time the sun is 6° below the horizon in the evening and when it is 12° below the horizon.

Finding times of twilight.
The method used to find the times of civil and nautical twilight is exactly the same as for finding the times of sunrise and sunset.