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Most casual observers would consider the entire sky already fully dark even when astronomical twilight is just beginning in the evening or just ending in the morning, and astronomers can easily make observations of point sources such as stars, but faint diffuse items such as nebulae and galaxies can only be properly observed beyond the limit of astronomical twilight.Theoretically, the dimmest stars ever visible to the naked eye-those of the sixth magnitude-will appear in the evening once the sun falls more than 18° below the horizon (i.e.Astronomical twilight Astronomical twilight is the time when the center of the sun is between 12° and 18° below the horizon.
However, due to light pollution, some localities-generally those in large cities-may never have the opportunity to view even fourth-magnitude stars, irrespective of the presence of any twilight at all.
During this period there is enough light from the sun that artificial sources of light may not be needed to carry on outdoor activities.
This concept is sometimes enshrined in laws, for example, when drivers of automobiles must turn on their headlights, when pilots may exercise the rights to fly aircraft, or if the crime of burglary is to be treated as nighttime burglary, which carries stiffer penalties in some jurisdictions.
At the beginning of nautical twilight in the morning (nautical dawn), or at the end of nautical twilight in the evening (nautical dusk), under good atmospheric conditions and in the absence of other illumination, general outlines of ground objects may be distinguishable, but detailed outdoor operations are not possible, and the horizon is indistinct.
Nautical twilight has military considerations as well.