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The period of ambient darkness from sunset to sunrise throughout each 24-hour day, when the Sun is below the horizon, is known as night or nighttime (sometimes spelt night-time or night time). The precise hour when darkness begins and finishes (as well as evening) changes depending on where you are and throughout the year. When night is defined as the period that follows evening, it generally begins about 8 p.m. and ends around 4 a.m. The arrival of dawn at sunrise marks the end of the night.

The term can also refer to the period of time between going to bed and getting up. The term “night” is commonly used as a goodbye (“good night”) and is occasionally abbreviated to “night” in everyday conversation, especially when someone is going to sleep or departing. Consider the following scenario: “It was great to see you again. Have a good night!” “Good night” (or “goodnight”) is not a greeting like “good morning,” “good afternoon,” or “good evening.”

The interval between astronomical sunset and astronomical dawn when the Sun is between 18 and 90 degrees below the horizon and does not illuminate the sky is known as astronomical night. Complete darkness does not occur around the summer solstice as seen from latitudes between 48.56° and 65.73° north or south of the Equator because, although the Sun sets, it is never more than 18° below the horizon at lower culmination, -90° sun an angles occur at Tropic of Cancer on December Solstice and Tropic of Capricorn on June Solstice, and at equator on equinoxes.

Day is the polar opposite of darkness (or “daytime”, to distinguish it from “day” referring to a 24-hour period). The beginning and conclusion of a night’s time varies depending on factors such as season and latitude. After sunset or before sunrise, twilight is the time of night when the Sun still lights the sky even though it is below the horizon. At any one time, one side of the Earth is drenched in sunshine (during the day), while the other is under the shadow cast by Earth. The umbra is the center portion of the shadow.

Moonlight, planetary light, stars, zodiacal light, gegenschein, and airglow continue to offer natural illumination at night. Aurorae, lightning, and bioluminescence may all provide light under certain situations. Because artificial illumination may interfere with observational astronomy and ecosystems, it is also referred to as light pollution.