night

Night is the portion of the diurnal cycle when the Sun is below the horizon, a consequence of the Earth's rotation. Its length varies with latitude and season: higher latitudes have longer nights in winter and shorter nights in summer, while near the equator night length is more constant. Twilight describes the transition to night: civil twilight occurs when the Sun is 0 to 6 degrees below the horizon; nautical twilight when it is 6 to 12 degrees below; astronomical twilight when it is 12 to 18 degrees below. Night proper begins when the Sun is more than about 18 degrees below the horizon.

In astronomy, night provides darkness suitable for observing celestial objects. The Moon, planets, and stars become visible, and fixed stars reveal fainter deep-sky objects under dark skies. Light pollution from urban areas can limit visibility, so genuine darkness is sought for serious observation and astrophotography. Some natural phenomena, such as auroras, occur at night in certain latitudes.

Biologically, night affects living organisms through circadian rhythms that alternate with day and night. Nocturnal species are active after dusk and before dawn, while others rest. Temperature, humidity, and dew reflect night-time conditions and influence ecosystems. Humans typically sleep at night, though artificial lighting and schedule changes can modify this pattern.

Culturally, night has shaped navigation, ritual practice, and storytelling. Across societies, the night sky has inspired calendars, myths, and art, and continues to support scientific inquiry, astronomy education, and photography.