nautical twilight
Technical term in astronomy and navigation for the period when the sun is between 6 and 12 degrees below the horizon, providing enough light for sailors to take reliable star sights.
The period of nautical twilight is important for sailors because both stars and the horizon are visible.
Civil twilight (sun 0–6 degrees below horizon); brighter than nautical twilight, used for ordinary outdoor activities.
Astronomical twilight (sun 12–18 degrees below horizon); darker than nautical twilight, used for deep-sky observations.
Compound of 航海 (kōkai, 'navigation, sailing') and 薄明 (hakumei, 'twilight'). The term is a direct calque of English 'nautical twilight'.