Night or nighttime (sp. night-time or night time) is the period of time between sunset and sunrise, when the Sun is below the horizon. Complete darkness or astronomical night is the period between astronomical dusk and astronomical dawn when the Sun is between 18 and 90 degrees below the horizon and does not illuminate the sky. As seen from latitudes between 48.5° and 66.5° north or south (of the Equator), complete darkness does not occur around the summer solstice because although the Sun sets, it is never more than 18° below the horizon at lower culmination. The opposite of night is day (or "daytime", to distinguish it from "day" referring to a 24-hour period). The start and end points of time for a night vary, based on factors such as season, latitude, longitude, and time zone. Twilight is the period of night after sunset or before sunrise when the Sun still illuminates the sky when it is below the horizon. At any given time, one side of Earth is bathed in sunlight (the daytime) while the other side is in the shadow caused by Earth blocking the sunlight. The central part of the shadow is called the umbra. Natural illumination at night is still provided by a combination of moonlight, planetary light, starlight, zodiacal light, gegenschein, and airglow.
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