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Sunset, also called sundown in some American English dialects, is the time at which the Sun disappears below the horizon. It should not be confused with dusk, which is the (variously defined) point at which darkness falls (some time after the Sun itself sets).
As sunrise and sunset are calculated from the leading and trailing edges of the Sun, and not the center; this slightly increases the duration of "day" relative to "night".
The red-hued sky at sunset is explained by the phenomenon of Rayleigh scattering.