Twilight is the transitional lighting phase between day and night. It occurs because the atmosphere scatters sunlight even when the sun is below the horizon. Three types are defined by the sun's angle:
Types
Civil (sun between 0° and −6°): enough light for outdoor activities without artificial illumination. Nautical (−6° to −12°): the marine horizon is visible for navigation. Astronomical (−12° to −18°): sunlight interferes with observation of faint stars. In Madrid, civil twilight lasts about 30 minutes; at high latitudes it can extend for hours in summer (Scandinavian white nights).