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Winter Solstice

Twilight 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.[2]




The lower the Sun is beneath the horizon, the dimmer the twilight (other factors such as atmospheric conditions being equal). When the Sun reaches 18° below the horizon, the twilight's brightness is nearly zero, and evening twilight becomes nighttime. When the Sun again reaches 18° below the horizon, nighttime becomes morning twilight. 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 and painters, who often refer to it as the blue hour, after the French expression l'heure bleue.




By analogy with evening twilight, the word twilight is also sometimes used metaphorically, to 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 crepuscular, which may be used to describe the behavior of animals that are most active during this period.


Twilight is defined according to the solar elevation angle θs, which is the position of the geometric center of the sun relative to the horizon. There are three established and widely accepted subcategories of twilight: civil twilight (nearest the horizon), nautical twilight, and astronomical twilight (farthest from the horizon).


Civil twilight is the period when enough natural light remains that artificial light is not needed.


Morning civil twilight begins when the geometric center of the sun is 6°


below the horizon and ends at sunrise. Evening civil twilight begins at sunset and ends when the geometric center of the sun reaches 6° below the horizon. In the United States' military, the initialisms BMCT (begin morning civil twilight, i.e. civil dawn) and EECT (end evening civil twilight, i.e. civil dusk) are used to refer to the start of morning civil twilight and the end of evening civil twilight, respectively. 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 horizon while the brightest stars and planets can appear. As observed from the Earth (see apparent magnitude), sky-gazers know Venus, the brightest planet, as the "morning star" or "evening star" because they can see it during civil twilight.


Lawmakers have enshrined the concept of civil twilight. Such statutes typically use a fixed period after sunset or before sunrise (most commonly 20–30 minutes), rather 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 anti-collision lights, is required for aircraft to operate. In the US, civil twilight for aviation is defined in Part 1.1 of the Federal Aviation Regulations (FARs) as the time listed in the American Air Almanac.

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 (nautical dusk) begins when the geometric center of the Sun is 6 degrees below the horizon in the evening and ends when the geometric center of the sun is 12 degrees below the horizon in the evening Before nautical dawn and after nautical dusk, sailors cannot navigate via the horizon at sea as they cannot clearly see the horizon. At nautical dawn and nautical dusk, the human eye finds it difficult, if not impossible, to discern traces of illumination near the sunset or sunrise point of the horizon (first light after nautical dawn but before civil dawn and nightfall after civil dusk but before nautical dusk).

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.e. after astronomic dawn or before astronomic dusk). 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 outdoor operations. Nautical twilight has military considerations as well. The initialisms BMNT (begin morning nautical twilight, i.e. nautical dawn) and EENT (end evening nautical twilight, i.e. nautical dusk) are used and considered when planning military operations. A military unit may treat BMNT and EENT with heightened security, e.g. by "standing to", in which everyone assumes a defensive position.

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. After astronomical dusk and before astronomical dawn, the sky is not illuminated by the sun.

Morning astronomical twilight begins (astronomical dawn) when the geometric center of the sun is 18° below the horizon in the morning and ends when the geometric center of the sun is 12° below the horizon in the morning. Evening astronomical twilight begins when the geometric center of the sun is 12° below the horizon in the evening and ends (astronomical dusk) when the geometric center of the sun is 18° below the horizon in the evening.

In some places (away from urban light pollution, moonlight, auroras, and other sources of light), where the sky is dark enough for nearly all astronomical observations, astronomers can easily make observations of point sources such as stars both during and after astronomical twilight in the evening and both before and during astronomical twilight in the morning. However, some critical observations, such as of faint diffuse items such as nebulae and galaxies, may require observation beyond the limit of astronomical twilight. Theoretically, the faintest stars detectable by the naked eye (those of approximately the sixth magnitude) will become visible in the evening at astronomical dusk, and become invisible at astronomical dawn.

However, in other places, especially those with skyglow, astronomical twilight may be almost indistinguishable from night. In the evening, even when astronomical twilight has yet to end and in the morning when astronomical twilight has already begun, most casual observers would consider the entire sky fully dark. Because of light pollution, observers in some localities, generally in and near large cities, may never have the opportunity to view anything but the brightest stars, irrespective of the presence of any twilight at all, nor to experience anything close to a truly dark sky.




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