Note that the angles are not to scale so as to show the three twilight categories with more clarity. The figure below shows civil, nautical and astronomical twilight. But diffuse light sources such as galaxies, nebula, and globular clusters need to be observed under a totally dark sky, again when the sun is more than 18 degrees below the horizon. Dusk is the period after twilight, when the sky is dark and the stars are easily visible. Dusk in a Spanish Village dusk to dawn from dusk until dawn gathering dusk know this: you will fall before dusk. Point light sources such as stars and planets can be readily studied by astronomers under astronomical twilight. But to test the limits of naked eye observations, the sun needs to be more than 18 degrees below the horizon. Under astronomical twilight, the horizon is not discernible and moderately faint stars or planets can be observed with the naked eye under a non light polluted sky. 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.Nautical Twilight is also known as Nautical Dawn and Nautical Dusk. 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. It refers to the partial darkness following sunset. Dusk is the time after sunset when the sky gradually gets darker. Twilight and dusk mark the transition between day and night. Begins in the morning, or ends in the evening, when the geometric center of the sun is 12 degrees below the horizon. Dusk is the time just before night when the daylight has almost gone but when it is not completely dark. Dusk is only at the end of the day and usually lasts 70 to 100 minutes.Civil Twilight is also known as Civil Dawn and Civil Dusk. 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. The setting of the sun can also be physically observed, whereas dusk is experienced. Evening civil twilight begins at sunset, and ends when the geometric center of the sun is 6 degrees below the horizon. The main difference between sunset and dusk is that dusk is considered the process and transition to the night, whereas sunset is a specific point that proceeds it. 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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