Wiktionary
nautical twilight
n. The period in which the sun is 6° - 12° below the horizon in the morning and evening.
Usage examples of "nautical twilight".
The light level was what meteorologists and sailors called nautical twilight.
The entry for 15 February had begun in much the same way as other days: BMNT [Beginning of Morning Nautical Twilight] 0632 hrs.
This time, Before Morning Nautical Twilight, had been considered the most dangerous time of all for millennia.
It was already pre-dawn, Before Morning Nautical Twilight as they'd say in the military, with the air a ghostly blue.
No Army helicopters will be launched until End of Evening Nautical Twilight.