![]() In astronomical twilight, sky illumination is so faint that most casual observers would regard the sky as fully dark, especially under urban or suburban light pollution. Begins in the morning, or ends in the evening, when the geometric center of the sun is 18 degrees below the horizon. ![]() Absent fog or other restrictions, outlines of terrestrial objects may still be discernible, but detailed outdoor activities are likely curtailed without artificial illumination. In general, the term nautical twilight refers to sailors being able to take reliable readings via well known stars because the horizon is still visible, even under moonless conditions.
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