Dusk or civil dusk is the time at which the sun is 6 degrees below the horizon in the evening.
At this time objects are distinguishable but there is no longer enough light to perform any outdoor activities.
Nautical dusk is the time at which the sun is 12 degrees below the horizon in the evening.
At this time, objects are no longer distinguishable, and the horizon is no longer visible to the naked eye.
Astronomical dusk is the time at which the sun is 18 degrees below the horizon in the evening.
At this time the sun no longer illuminates the sky.
Dusk should not be confused with sunset, which is the moment when the trailing edge of the Sun itself sinks below the horizon.
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