The term civil twilight refers to the period between the time when the sun goes down and when the natural illumination is so dim that artificial light is needed to distinguish objects on the ground.
Officially it begins at sunset and it ends when the sun's geometric center reaches 6° below the horizon.
The designation has some legal ramifications for laws that define when headlights are required or that designate a crime as having occurred in the daytime or the nighttime.
Artists are conscious of the big changes that happen during this period of time when the sun no longer shines on objects on the ground, but the light still touches the higher clouds. Maxfield Parrish made a career of painting during this fleeting time period.
Civil twilight is followed by nautical twilight (above), where the sun moves between 6° and 12° below the horizon. When you're at sea during that period of time you can still distinguish the horizon, but the sky is dark enough to discern many stars for navigation.
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Wikipedia on civil twilight
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