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Twilight is the illumination of the Earth's lower atmosphere when the Sun itself 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 the surface of the Earth is neither completely lit nor completely dark. The word "twilight" is also used to denote the periods of time when this illumination occurs.
The further the Sun is below the horizon, the dimmer the twilight (other things such as atmospheric conditions being equal). When the Sun reaches 18 degrees below the horizon, twilight's brightness is nearly zero, thus evening twilight ends, and night begins. When the Sun again reaches 18 degrees below the horizon, night ends and morning twilight begins. Owing to its distinctive quality, primarily the absence of shadows and the appearance of objects silhouetted against the bright sky, twilight has long been popular with photographers, who refer to it as 'sweet light', and painters, who refer to it as the blue hour, after the French expression l'heure bleue.
Twilight should not be confused with auroras, which can have a similar appearance in the night sky at high latitudes.
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, a very old man may be said to be in the twilight of his life. The collateral adjective for twilight is crepuscular; it is most frequently encountered when applied to certain insects, fishes and mammals that are most active during that time.
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 (brightest), nautical twilight, and astronomical twilight (darkest).
Morning civil twilight begins when the geometric center of the sun is 6° below the horizon (civil dawn) and ends at sunrise. Evening civil twilight begins at sunset and ends when the geometric center of the sun reaches 6° below the horizon (civil dusk). Civil twilight can also be described as the limit at which twilight illumination is sufficient, under clear weather conditions, for terrestrial objects to be clearly distinguished; at the beginning of morning civil twilight, or end of evening civil twilight, the horizon is clearly defined and the brightest stars are visible under clear atmospheric conditions.
The brightest planets can appear during this time. Venus, the brightest planet as observed from the Earth (see Apparent magnitude), is known as the "morning star" or "evening star" due to its visibility during civil twilight. There is enough light from the sun during this period that artificial sources of light may not be needed to carry on outdoor activities. This concept is sometimes enshrined in laws, for example, when drivers of automobiles must turn on their headlights (called lighting-up time in the UK); when pilots may exercise the rights to fly aircraft; restrictions on hunting; or if the crime of burglary is to be treated as nighttime burglary, which carries stiffer penalties in some jurisdictions. A fixed period (most commonly 20–30 minutes after sunset or before sunrise) is typically used in such statutes, rather than how many degrees the sun is below the horizon. The military 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 these periods.
Nautical twilight is defined to begin in the morning, and to end in the evening, when the center of the sun is geometrically 12 degrees below the horizon. In general, nautical twilight ends when navigation via the horizon at sea is no longer possible.
During nautical twilight, sailors can take reliable star sightings of well-known stars, using a visible horizon for reference. The end of this period in the evening, or its beginning in the morning, is also the time at which traces of illumination near the sunset or sunrise point of the horizon are very difficult, if not impossible, to discern (this often being referred to as "first light" before civil dawn and "nightfall" after civil dusk). At the beginning of nautical twilight in the morning (nautical dawn), or at the end of nautical twilight in the evening (nautical dusk), and under good atmospheric conditions with the absence of other illumination, general outlines of ground objects may be distinguishable; but detailed outdoor operations are not possible, and the horizon is indistinct.
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. This is partially due to tactics dating back to the French and Indian War (part of the Seven Years' War of 1756–1763), when combatants on both sides would launch attacks at nautical dawn or dusk.
Astronomical twilight is defined to begin in the morning, and to end in the evening when the center of the Sun is geometrically 18 degrees below the horizon. From the end of astronomical twilight in the evening to the beginning of astronomical twilight in the morning, the sky (away from urban light pollution) is dark enough for all astronomical observations.
Most casual observers would consider the entire sky fully dark even when astronomical twilight is just beginning in the evening or just ending in the morning, and astronomers can easily make observations of point sources such as stars, but faint diffuse items such as nebulae and galaxies can be properly observed only beyond the limit of astronomical twilight. In some places, especially those with sky glow, astronomical twilight may be almost indistinguishable from night. Theoretically, the dimmest stars ever visible to the naked eye (those of the sixth magnitude) will become visible in the evening once the sun falls more than 18° below the horizon (i.e., at astronomical dusk), and become invisible when the sun moves to within 18° of the horizon in the morning (at astronomical dawn). However, because of light pollution, some localities, generally those in large cities, may never have the opportunity to view even fourth-magnitude stars, irrespective of the presence of any twilight at all.
The most familiar occurrences of twilight are between dawn and sunrise and between sunset and dusk each day. These occur for observers at latitudes within 48.5 degrees of the Equator on all dates of the year, and also for most observers at higher latitudes on many dates. However, at latitudes closer than 9 degrees to either Pole, the Sun cannot rise above the horizon and go more than 18 degrees below it on the same day on any date, so this type of twilight cannot occur.
At latitudes greater than about 48.5 degrees North or South, on dates near the summer solstice, twilight can last from sunset to sunrise, since the Sun does not go more than 18 degrees below the horizon, so complete darkness does not occur even at midnight. These latitudes include many densely populated regions of the Earth, including the entire United Kingdom and other countries in northern Europe.
In Arctic and Antarctic latitudes in wintertime, the polar night only rarely produces complete darkness for 24 hours each day. This can occur only at locations within 5.5 degrees of latitude of the Pole, and there only on dates very close to the winter solstice. At all other latitudes and dates, the polar night includes a daily period of twilight, when the Sun is not far below the horizon. This period may be any fraction of the day. It may be brief, around noon, or it can last for the entire 24 hours.
The apparent travel of the Sun occurs at the rate of 15 degrees per hour (360° per day), but sunrise and sunset happen typically at oblique angles to the horizon and the actual duration of any twilight period will be a function of that angle, being longer for more oblique angles. This angle of the Sun's motion with respect to the horizon changes with latitude as well as the time of year (affecting the angle of the Earth's axis with respect to the Sun).
At Greenwich, England (51.5°N), the duration of civil twilight will vary from 33 minutes to 48 minutes, depending on the time of year. At the equator, conditions can go from day to night in as little as 20–25 minutes. This is true because at low latitudes the sun's apparent movement is perpendicular to the observer's horizon. But at the poles, civil twilight can be as long as 2–3 weeks. In the Arctic and Antarctic regions, twilight (if there is any) can last for several hours. There is no astronomical twilight at the poles near the winter solstice (for about 74 days at the North Pole and about 80 days at the South Pole). As one gets closer to the Arctic and Antarctic circles, the sun's disk moves toward the observer's horizon at a lower angle. The observer's earthly location will pass through the various twilight zones less directly, taking more time.
Within the polar circles, twenty-four hour daylight is encountered in summer, and in regions very close to the poles, twilight can last for weeks on the winter side of the equinoxes. Outside the polar circles, where the angular distance from the polar circle is less than the angle which defines twilight (see above), twilight can continue through local midnight near the summer solstice. The precise position of the polar circles, and the regions where twilight can continue through local midnight, varies slightly from year to year with Earth's axial tilt. The lowest latitudes at which the various twilights can continue through local midnight are approximately 60.561° (60°33′43″) for civil twilight, 54.561° (54°33′43″) for nautical twilight and 48.561° (48°33′43″) for astronomical twilight.
These are the largest cities, of their respective countries, where the various twilights can continue through local solar midnight:
Although Helsinki, Oslo, Stockholm, Tallinn and Saint Petersburg do not actually receive civil twilight from sunset to sunrise, they do have noticeably lighter skies at night (known as white nights) around the summer solstice.
At the winter solstice within the polar circle, twilight can extend through solar noon at latitudes below 72.561° (72°33′43″) for civil twilight, 78.561° (78°33′43″) for nautical twilight, and 84.561° (84°33′43″) for astronomical twilight.
Twilight on Mars is longer than on Earth, lasting for up to two hours before sunrise or after sunset. Dust high in the atmosphere scatters light to the night side of the planet. Similar twilights are seen on Earth following major volcanic eruptions.
Twilight is sacred in Hinduism. It is called गोधूळिवेळ in Marathi or गोधूलिवेला godhūlivelā in Hindi—literally "cow dust time". Many rituals including Sandhyavandanam and Puja are performed at twilight hour. Eating of food is not advised during this time. Some times it is referred to as Asurasandhya vela. It is believed that Asuras are active during these hours. One of the avatars of Lord Vishnu, Narasimha is closely associated with Twilight period. According to Hindu Scriptures a daemonic king Hiranyakashipa performed penance and obtained a boon from Brahma not to be killed during days or night times and neither by human nor animal. As Godhulivela is not regarded as day or night, lord Vishnu ended the life of Hiranakashipa during these hours.
Twilight is important in Islam as it determines when certain universally obligatory prayers are to be recited. Morning twilight is when morning prayers (Fajr) are done, while evening twilight is the time for evening prayers (Maghrib prayer). There is also an important discussion in Islamic jurisprudence between "true dawn" and "false dawn".
Nautical twilight in Acapulco, with visible stars and horizon
Evening twilight in Joshua Tree, California, US
The Mohave Desert at evening twilight
Figures in silhouette during twilight
Astronomical twilight at Prague Castle
Twilight before Sunrise Landers, California
Clouds begin to glow with colors at nautical dawn, the second stage of twilight, when the Sun is 12 degrees below the horizon.
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