Sunset is the moment when the sun disappears behind the horizon.
The sun has not completely set until the entire solar disk is no longer visible, so the daytime duration is on average slightly longer than the night; the night begins when the sun is completely down and ends already at the moment that the first piece of the sun is visible. Due to reflection in the atmosphere, it remains light for some time, so that sunset is not the same as the moment when it becomes dark.
The time between the moment when the sun has already set while it is not yet completely dark is called the evening twilight: twilight is therefore a transitional phase and sunset is a moment. It takes 3 to 4 minutes from the moment the first edge of the sun disappears behind the horizon until sunset, due to the sun's diameter (0.53°). This duration depends on season and latitude of observation; from the equator it takes just over 2 minutes. The moment the sun sets is often considered the end of the day.
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