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Twilight on Earth is the illumination of the lower atmosphere when the Sun is not directly visible because it is below the horizon. Twilight is produced by sunlight scattering in the upper atmosphere, illuminating the lower atmosphere so that Earth's surface is neither completely lit nor completely dark. The word twilight is also used to denote the periods of time when this illumination occurs. The lower the Sun is beneath the horizon, the dimmer the twilight other factors such as atmospheric conditions being equal. Owing to its distinctive quality, primarily the absence of shadows and the appearance of objects silhouetted against the lit sky, twilight has long been popular with photographers, who sometimes refer to it as "sweet light", and painters, who often refer to it as the blue hourafter the French expression l'heure bleue. Twilight should not be confused with auroraswhich can have a similar appearance in the night The Kingdoms Of Day And Night at high latitudes.

By analogy with evening twilight, the word twilight is also sometimes used metaphoricallyto imply that something is losing strength and approaching its end. For example, very old people may be said to be "in the twilight of their lives". The collateral adjective for twilight is crepuscularwhich may be used to Tge the behavior of animals that are most active during this period. There are three established and widely accepted subcategories of twilight: civil twilight nearest the horizonnautical twilight, and astronomical twilight farthest from the horizon.

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Three subcategories of twilight are established and widely accepted: civil twilight brightestnautical twilight, and astronomical twilight darkest. Civil twilight is the period when enough natural light remains that artificial light is not needed. Civil dawn is preceded by morning nautical twilight and civil dusk is followed by evening nautical twilight.

Under clear weather conditions, civil twilight approximates the limit at which solar illumination suffices for the human eye to clearly distinguish terrestrial objects. Enough illumination renders artificial sources unnecessary for most outdoor activities. At civil dawn and at civil dusk sunlight clearly defines the Nigh while the brightest stars and planets can appear. As observed from the Earth see apparent magnitudesky-gazers know Venusthe brightest planet, as the "morning star" or "evening star" because they can see it during here twilight.

Lawmakers have enshrined the concept of civil twilight.

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Such statutes typically use a fixed period after sunset or before sunrise most commonly 20—30 minutesrather than how many degrees the sun is below the horizon. Examples include the following periods: when drivers of automobiles must turn on their headlights called lighting-up time in the UK ; when hunting is restricted; when the crime of burglary is to be treated as nighttime burglary, which carries stiffer penalties in some jurisdictions. The period may affect when extra equipment, such as Niggt lights, is required for aircraft to operate.

Morning nautical twilight nautical dawn begins when the geometric center of the sun is 12 degrees below the horizon in the morning and ends when the geometric center of the sun is 6 degrees below the horizon in the morning. Evening nautical twilight begins when Anv geometric center of the sun is 6 degrees The Kingdoms Of Day And Night the horizon in the evening and ends nautical dusk when the geometric center of the sun is 12 degrees below the horizon in the evening.

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Nautical dawn is the moment when the geometric center of the Sun is 12 degrees below the horizon in the morning. It is preceded by morning astronomical twilight and followed by morning civil twilight. Nautical dusk is the moment when the geometric center of the Sun is 12 degrees below the horizon in the evening. It marks the beginning of Nighr astronomical twilight and the end of evening civil twilight. Before nautical dawn and after nautical dusk, sailors cannot navigate via the horizon at sea as they cannot clearly see the horizon. Sailors can take reliable star sightings of well-known stars, during the stage of nautical twilight when they can distinguish a visible horizon for reference i. Under good atmospheric conditions with the absence of other illumination, during nautical twilight, the human eye may distinguish general outlines of ground objects but cannot participate in detailed Dqy operations. Nautical twilight has The Kingdoms Of Day And Night considerations as well.

The initialisms BMNT begin morning nautical twilight, i. This is partially due to tactics dating back to the French and Indian War part of the Seven Years' War of —when combatants on both sides would launch attacks at nautical dawn or dusk. Astronomical dawn is the moment when the geometric center of the Sun is 18 degrees below the horizon in the morning. Astronomical dusk is the moment when the geometric center of the Sun is 18 degrees below the horizon in the evening.]