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. Begins in the morning, or ends in the evening, when the geometric center of the sun is 12 degrees below the horizon. Under these conditions absent fog or other restrictions, the brightest stars and planets can be seen, the horizon and terrestrial objects can be discerned, and in many cases, artificial lighting is not needed. Evening civil twilight begins at sunset, and ends when the geometric center of the sun is 6 degrees below the horizon. Therefore morning civil twilight begins when the geometric center of the sun is 6 degrees below the horizon, and ends at sunrise. Begins in the morning, or ends in the evening, when the geometric center of the sun is 6 degrees below the horizon. However there are three categories of twilight that are defined by how far the sun is below the horizon. In its most general sense, twilight is the period of time before sunrise and after sunset, in which the atmosphere is partially illuminated by the sun, being neither totally dark or completely lit.
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