Twilight

Twilight is the period of illumination when the sun is below the horizon but sunlight scattered through the upper atmosphere still lights the sky and ground, covering both the dawn and dusk halves of the day. It is caused by diffuse sky radiation, sunlight scattered through the atmosphere after the sun has dipped below the horizon, and it ends once the sun reaches 18 degrees below the horizon and full night begins. Meteorologists and astronomers divide it into three phases by how far the sun sits below the horizon: civil twilight (0 to 6 degrees, bright enough that outdoor artificial light is not yet needed), nautical twilight (6 to 12 degrees, where sailors can still make out the horizon for navigation), and astronomical twilight (12 to 18 degrees, dark enough for observing stars though light pollution can still interfere).