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twilight




The interval that follows sunset and comes before sunrise, during which the sky is partially illuminated by light from the Sun. Twilight lengthens with the distance of the observer from the equator, and is shortest as seen from anywhere on Earth at the equinoxes. Astronomical twilight is defined to begin in the morning, and to end in the evening, when the center of the Sun is geometrically 18° below the horizon. Before the beginning of astronomical twilight in the morning and after the end of astronomical twilight in the evening the Sun does not contribute at all to sky illumination. This corresponds to the older definition, going back to ancient times, that hinged on the ability to discern a sixth magnitude star (at the limit of naked-eye visibility) at the zenith. Civil twilight and nautical twilight are defined in exactly the same way but with 6° and 12°, respectively, in place of the 18° for the astronomical variety.