nightfall

Nightfall is the period of the day when daylight ends and darkness begins. In common usage it marks the onset of night; the exact time depends on geographic location, date, and local horizon conditions. The transition follows sunset and varies with season and latitude. Astronomical terms describe related phases: civil twilight ends when the sun is 6 degrees below the horizon, nautical twilight ends at 12 degrees, and astronomical twilight ends at 18 degrees. Night generally begins after astronomical twilight, when the sky appears dark enough to see stars, though societal lighting can extend or shorten perceived night.

Culturally, nightfall is a frequent setting in literature, film, and music, often symbolizing mystery, danger, or introspection. The best-known literary use is Isaac Asimov's 1941 science fiction short story Nightfall, in which a planet with multiple suns experiences darkness for the first time in generations, precipitating a crisis of belief. The term Nightfall has been used as a title in other works across genres.

In natural and urban environments, nightfall affects behavior and ecology. Nocturnal animals become active; temperatures commonly fall after sunset; in cities, artificial lighting can blur the boundary between day and night and alter perception of time.