Civil twilight is the period when the sun sits just below the horizon (up to 6° down) but the sky still holds enough ambient light to see clearly outdoors without artificial lighting.
Civil twilight is the period when the sun is up to 6 degrees below the horizon and the sky still carries enough light to work outdoors without a torch. For photographers it does two jobs. It overlaps closely with blue hour, so it is the practical marker for when balanced city and landscape light is available. It is also the legal boundary many countries use to define when drone flight is allowed, or when headlights become mandatory.
A landscape shooter finishing a sunset session can keep working through civil twilight and usually packs up around the time it ends, when the foreground goes properly dark. The exact length shifts with latitude and season: near the equator civil twilight lasts about twenty minutes year-round, while in northern summer it can stretch past an hour, which changes how much shooting time a single sunset actually buys.
A drone photographer in the United States wants to catch a valley at last light. The FAA allows civil-twilight operations with proper anti-collision lighting, so the pilot checks that civil twilight ends at 8:47 pm for that date and place, giving a legal flight window that runs about 26 minutes past the 8:21 sunset. Ground-based photographers shooting the same valley get a matching creative window: the sky holds printable colour and the landscape keeps some shadow detail right up to about 8:45, then falls away fast.
Twenty minutes earlier, at sunset itself, the sky was too bright for the valley's lights to show. Twenty minutes after civil twilight ends, the scene needs 30-second exposures and the colour in the sky is gone. The pilot notes both the sunset and the civil-twilight-end time before every dusk flight, because the legal window and the good-light window close within a few minutes of each other.
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