Twilight is the period of incomplete darkness before sunrise and after the sunset. So the morning twilight ends at sunrise, when the upper limb of the sun just appears above the visible horizon, and evening twilight starts at sunset, when the upper limb of the sun just disappears below the visible horizon.
The period of twilight is very important to the navigator, because in this period the visible horizon can still be seen and the celestial bodies on the sky are bright enough to be observed by the sextant.
There are three stages of twilight:
- Civil twilight is the interval of time between visible sunrise or sunset and the time when the centre of the sun is 6° below the rational horizon.
In other words we can say:
Civil twilight is defined to begin in the morning, and to end in the evening when the center of the Sun is geometrically 6 degrees below the horizon.
- Nautical twilight is the interval of time between visible sunrise or sunset and the time when the centre of the sun is 12° below the rational horizon.
In other words we can say:
Nautical twilight is defined to begin in the morning, and to end in the evening, when the center of the sun is geometrically 12 degrees below the horizon.
During nautical twilight the illumination level is such that the horizon is still visible even on a Moonless night allowing mariners to take reliable star sights for navigational purposes, hence the name.
