noun
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.
航海薄明の時間帯は、星と水平線の両方が見えるため、船乗りにとって重要です。
The period of nautical twilight is important for sailors because both stars and the horizon are visible.
Compound of 航海 (kōkai, 'navigation, sailing') and 薄明 (hakumei, 'twilight'). The term is a direct calque of English 'nautical twilight'.