Twilight is daylight illumination produced by diffuse sky radiation when the Sun is below the horizon as sunlight from the upper atmosphere is scattered in a way that illuminates both the Earth's lower atmosphere and also the Earth's surface. Twilight also may be any period when this illumination occurs, including dawn and dusk.
The lower the Sun is beneath the horizon, the dimmer the sky (other factors such as atmospheric conditions being equal). When the Sun reaches 18° below the horizon, the illumination emanating from the sky is nearly zero, and evening twilight becomes nighttime. When the Sun approaches re-emergence, reaching 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 .
By analogy with evening twilight, sometimes twilight is 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.
Definitions by geometry
Twilight occurs according to the solar elevation angle, 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
Civil twilight is the period of time for which the geometric center of the Sun is between the horizon and 6° below the horizon.http://msi.nga.mil/MSISiteContent/StaticFiles/NAV_PUBS/APN/Gloss-1.pdf#9 Glossary of Marine Navigationhttp://www.ast.cam.ac.uk/public/ask/2445 University of Cambridge – Institute of Astronomy – Ask an Astronomer [NB: questionable source as its sources refer back to Wikipedia]
Civil twilight is the period when enough natural light remains so that artificial light in towns and cities is not needed. 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.
Although "civil dawn" marks the time of the first appearance of civil twilight before sunrise, and "civil dusk" marks the time of the first disappearance of civil twilight after sunset, civil twilight statutes typically denote 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 when drivers of automobiles must turn on their headlights (called lighting-up time in the UK), when hunting is restricted, or 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.
Nautical twilight Nautical twilight occurs when the geometric center of the Sun is between 12° and 6° below the horizon.
After nautical dusk and before nautical dawn, 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 nautical dawn or before nautical 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", for which everyone assumes a defensive position.
Astronomical twilight
Astronomical twilight occurs when the geometric center of the Sun is between 18° and 12° below the horizon. During astronomical twilight, the sky is dark enough to permit astronomical observation of point sources of light such as stars, except in regions with more intense skyglow due to light pollution, moonlight, auroras, and other sources of light. 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.
Times of occurrence
Between day and night Observers within about 48°34' of the Equator can view twilight twice each day on every date of the year between astronomical dawn, nautical dawn, or civil dawn, and sunrise as well as between sunset and civil dusk, nautical dusk, or astronomical dusk. This also occurs for most observers at higher latitudes on many dates throughout the year, except those around the summer solstice. However, at latitudes closer than 8°35' (between 81°25’ and 90°) to either Pole, the Sun cannot rise above the horizon nor sink more than 18° below it on the same day on any date, so this example of twilight cannot occur because the angular difference between solar noon and solar midnight is less than 17°10’.
Observers within about 63°26' of the Equator can view twilight twice each day on every date between the month of the autumnal equinox and the month of vernal equinox between astronomical dawn, nautical dawn, or civil dawn, and sunrise as well as between sunset and civil dusk, nautical dusk, or astronomical dusk, i.e., from September 1 to March 31 of the following year in the Northern Hemisphere and from March 1 to September 30 in the Southern Hemisphere.
The maximum latitude to view both astronomical dawn to sunrise and from sunset to astronomical dusk in the entire year is 48º33'43". The maximum latitude to view both astronomical dawn to sunrise and from sunset to astronomical dusk in the months that includes autumn and winter is 63º26'07".
The nighttime/twilight boundary solar midnight's latitude varies depending on the month:
In January or July, astronomical dawn to sunrise or sunset to astronomical dusk occurs at latitudes less than 48°50' North or South, because then the Sun's declination is less than 23°10' from the Equator;
In February or August, astronomical dawn to sunrise or sunset to astronomical dusk occurs at latitudes less than 53°47' North or South, because then the Sun's declination is less than 18°13' from the Equator;
In March or September before the equinoxes, astronomical dawn to sunrise or sunset to astronomical dusk occurs at latitudes less than 63°26' North or South, because before the equinoxes the Sun's declination is then less than 8°34' from the Equator;
During the equinoxes, astronomical dawn to sunrise or sunset to astronomical dusk occurs at latitudes less than 72°00' North or South, because during the equinoxes the Sun is crossing the Equator line;
In March or September after the equinoxes, astronomical dawn to sunrise or sunset to astronomical dusk occurs at latitudes less than 67°45' North or South, because after the equinoxes the Sun's declination is then less than 4°15' from the Equator;
In April or October, astronomical dawn to sunrise or sunset to astronomical dusk occurs at latitudes less than 57°09' North or South, because the Sun's declination is then less than 14°51' from the Equator;
In May or November, astronomical dawn to sunrise or sunset to astronomical dusk occurs at latitudes less than 50°03' North or South, because the Sun's declination is then less than 21°57' from the Equator;
In June or December, astronomical dawn to sunrise or sunset to astronomical dusk occurs at latitudes less than 48°34' North or South, because in June the Sun crosses the Tropic of Cancer (about 23°26' North) and in December the Sun crosses the Tropic of Capricorn (about 23°26' South).
Lasting from one day to the next
At latitudes greater than about 48°34' North or South, on dates near the summer solstice (June 21 in the Northern Hemisphere or December 21 in the Southern Hemisphere), twilight can last from sunset to sunrise, since the Sun does not sink more than 18 degrees below the horizon, so complete darkness does not occur even at solar midnight. These latitudes include many densely populated regions of the Earth, including the entire United Kingdom and other countries in northern Europe and even parts of central Europe. This also occurs in the Southern Hemisphere, but occurs on December 21. This type of twilight also occurs between one day and the next at latitudes within the polar circles shortly before and shortly after the period of midnight sun. The summer solstice in the Northern Hemisphere is on June 21st, while the summer solstice in the Southern Hemisphere is on December 21st.
Civil twilight: between about 60°34' and 65°44' north or south. In the northern hemisphere, this includes the center of Alaska, Iceland, Finland, Sweden, Norway, Faroe Islands, and part of Shetland. In the southern hemisphere this includes parts of the Southern Ocean and the northern tip of the Antarctic Peninsula. When civil twilight lasts all night, this is also referred as a white night. Nautical twilight: between about 54°34' and 60°34' north or south. In the northern hemisphere this includes the center of Alaska, Russia, Canada, Estonia, Latvia, Scotland, Norway, Sweden, Finland, Lithuania, and Denmark. In the southern hemisphere this includes the southernmost point of South America, Ushuaia in Argentina, and Puerto Williams in Chile. When nautical twilight lasts all night, this does not constitute a white night, which requires the Sun to remain less than 6 degrees below the horizon all night. This phenomenon is known as the grey night, a night when it does not get dark enough for astronomers to do their observations of the deep sky.