Twilight is the time between dawn Dawn is the time that marks the beginning of the twilight before sunrise. It is recognized by the presence of weak sunlight, while the sun itself is still below the horizon. Dawn should not be confused with sunrise, which is the moment when the leading edge of the sun itself appears above the horizon and sunrise Sunrise is the instant at which the upper edge of the Sun appears above the horizon in the east. Sunrise should not be confused with dawn, which is the point at which the sky begins to lighten, some time before the sun itself appears, ending twilight. Because atmospheric refraction causes the sun to be seen while it is still below the horizon,, and between sunset Sunset or sundown is the daily disappearance of the sun below the horizon as a result of the Earth's rotation. The atmospheric conditions created by the setting of the sun, occurring before and after it disappears below the horizon, are also commonly referred to as "sunset" and dusk Dusk is the beginning of darkness in the evening, and occurs after twilight, when the sky generally remains bright and blue. Civil dusk is when the center of the sun reaches 6° below the horizon. This marks the end of the evening civil twilight, the point where artificial illumination is required to read outside. It can be confused with sunset,. Sunlight Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon scattered Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of reflected radiation from the angle predicted by the in the upper atmosphere The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night. Dry air contains roughly (by volume) 78% nitrogen, 21% illuminates the lower atmosphere, and the surface of the Earth Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets. It is sometimes referred to as the World, the Blue Planet,[note 6] or by its Latin name, Terra.[note 7] is neither completely lit nor completely dark. The sun itself is not actually visible because it is below the horizon The horizon is the apparent line that separates earth from sky, the line that divides all visible directions into two categories: those that intersect the Earth's surface, and those that do not. At many locations, the true horizon is obscured by trees, buildings, mountains, etc., and the resulting intersection of earth and sky is called the. Owing to the unusual quality of the ambient light In photography and cinematography, available light or ambient light refers to any source of light that is not explicitly supplied by the photographer for the purpose of taking photos. The term usually refers to sources of light that are already available naturally or artificial light already being used (e.g. to light a room).. It generally at this time, twilight has long been popular with photographers and painters, who refer to it as the "blue hour The blue hour comes from a French expression, l'heure bleue, which refers to the period of twilight each morning and evening where there is neither full daylight nor complete darkness. The time is considered special because of the quality of the light at this time of day. The blue hour is considered especially flattering for people with blond hair", after the French French is a Romance language spoken as a first language by about 136 million people worldwide. Around 190 million people speak French as a second language, and an additional 200 million speak it as an acquired foreign language. French speaking communities are present in 57 countries and territories. Most native speakers of the language live in expression l'heure bleue^{[citation needed]}. Twilight is technically defined as the period before sunrise and after sunset during which there is natural light provided by the upper atmosphere, which receives direct sunlight and scatters part of it towards the Earth's surface.^[1]

The collateral adjective A collateral adjective is an adjective with a similar meaning to a given noun, but derived from a different root. For example, lunar serves as an adjective to describe attributes of the Moon; moon comes from the Old English mōna and lunar from the Latin luna. Collateral adjectives contrast with derived adjectives; for "father", for of twilight is crepuscular Crepuscular is a term used to describe some animals that are primarily active during twilight, that is at dawn and at dusk. The word is derived from the Latin word crepusculum, meaning "twilight." Crepuscular is thus in contrast with diurnal and nocturnal behavior. Crepuscular animals may also be active on a bright moonlit night. Many (for daylight it is diurnal Diurnality is an animal behavior characterized by activity during the day and sleeping at night. Animals that are not diurnal might be nocturnal or crepuscular (active primarily during twilight, i.e., at dusk and dawn). Many animal species are diurnal, including many mammals, insects, reptiles and birds. The diurnal pattern is often controlled and for night, nocturnal Nocturnality is an animal behavior characterized by activity during the night and sleeping during the day. The opposite is diurnality. The intermediate crepuscular schedule is also common. Some species are active both in daytime and at night. Living at night can be seen as a form of niche differentiation, where a species' niche is partitioned not). The term is most frequently encountered when applied to certain species of insects Insects are a class within the arthropods that have a chitinous exoskeleton, a three-part body (head, thorax, and abdomen), three pairs of jointed legs, compound eyes, and two antennae. They are among the most diverse group of animals on the planet and include more than a million described species and represent more than half of all known living and mammals Mammals are a class of vertebrate, air-breathing animals whose females are characterized by the possession of mammary glands while both males and females are characterized by hair and/or fur, three middle ear bones used in hearing, and a neocortex region in the brain. Some mammals have sweat glands, but most do not that are most active during that time.

Definitions

Twilight is defined according to the solar elevation angle θ_s, which is the position of the geometric center of the sun The Sun is the star at the center of the Solar System. It has a diameter of about 1,392,000 kilometers , about 109 times that of Earth, and its mass (about 2 × 1030 kilograms, 330,000 times that of Earth) accounts for about 99.86% of the total mass of the Solar System. About three quarters of the Sun's mass consists of hydrogen, while the rest is relative to the horizon. There are three established and widely accepted subcategories of twilight: civil twilight (brightest), nautical twilight, and astronomical twilight (darkest). ^[2]

(For these definitions, an ideal horizon The horizon is the apparent line that separates earth from sky, the line that divides all visible directions into two categories: those that intersect the Earth's surface, and those that do not. At many locations, the true horizon is obscured by trees, buildings, mountains, etc., and the resulting intersection of earth and sky is called the 90° from the zenith In general terms, the zenith is the direction pointing directly "above" a particular location; that is, it is one of two vertical directions at the location, orthogonal to a horizontal flat surface there. The concept of "above" is more specifically defined in astronomy, geophysics and related sciences as the vertical direction is used.)

Civil twilight

Morning civil twilight begins when the geometric center of the sun The Sun is the star at the center of the Solar System. It has a diameter of about 1,392,000 kilometers , about 109 times that of Earth, and its mass (about 2 × 1030 kilograms, 330,000 times that of Earth) accounts for about 99.86% of the total mass of the Solar System. About three quarters of the Sun's mass consists of hydrogen, while the rest is is 6° below the horizon The horizon is the apparent line that separates earth from sky, the line that divides all visible directions into two categories: those that intersect the Earth's surface, and those that do not. At many locations, the true horizon is obscured by trees, buildings, mountains, etc., and the resulting intersection of earth and sky is called the (the point of civil dawn), and ends at sunrise. Evening civil twilight begins at sunset and ends when the center of the sun The Sun is the star at the center of the Solar System. It has a diameter of about 1,392,000 kilometers , about 109 times that of Earth, and its mass (about 2 × 1030 kilograms, 330,000 times that of Earth) accounts for about 99.86% of the total mass of the Solar System. About three quarters of the Sun's mass consists of hydrogen, while the rest is reaches 6° below the horizon (the point of civil dusk). In general, civil twilight is the point where artificial illumination is required to read outside.^[3]

The brightest stars appear during the civil twilight, as well as planets, such as Venus Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows. Because Venus is an inferior planet from Earth, it never, which is known as the 'morning star' and/or 'evening star'. During this period there is enough light from the sun 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, when pilots may exercise the rights to fly aircraft, or if the crime of burglary Burglary is a crime, the essence of which is entry into a building for the purposes of committing an offence. Usually that offence will be theft, but most jurisdictions specify others which fall within the ambit of burglary. To commit a burglary is to burgle (in British English) or burglarize (in American English) is to be treated as nighttime burglary, which carries stiffer penalties in some jurisdictions. A fixed period (most commonly 30 minutes after sunset or before sunrise) is typically used in such statutes, rather than how many degrees the sun is below the horizon. Civil twilight can also be described as the limit at which twilight illumination is sufficient, under good 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 good atmospheric conditions.

Nautical twilight

Nautical twilight is the time when the center of the sun is between 6° and 12° below the horizon. In general, nautical twilight ends when navigation via the horizon at sea is no longer possible.^[3]

During nautical twilight, sailors can take reliable star sights 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), under good atmospheric conditions and in 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) and EENT (end evening nautical twilight) are used and considered when planning military operations. A military unit may treat BMNT and EENT with heightened security (i.e. a process called "stand to" in which every one assumes their defensive positions). This is partially due to tactics dating back to the French and Indian War The French and Indian War is the common U.S. name for the war between Great Britain and France in North America from 1754 to 1763. In 1756 the war erupted into the world-wide conflict known as the Seven Years' War and thus came to be regarded as the North American theater of that war. In Canada, it is usually just referred to as the Seven Years' (part of the Seven Year's War of 1756-1763), when combatants on both sides would use BMNT and EENT to launch attacks.

Astronomical twilight

Astronomical twilight is the time when the center of the Sun is between 12° and 18° below the horizon. In general, the end of astronomical twilight is the point where the sky is no longer illuminated by the sun and is dark enough for all astronomical observations.

Most casual observers would consider the entire sky already 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 A point source is a single identifiable localized source of something. A point source has negligible extent, distinguishing it from other source geometries. Sources are called point sources because in mathematical modeling, these sources can usually be approximated as a mathematical point to simplify analysis such as stars A star is a massive, luminous ball of plasma held together by gravity. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth. Other stars are visible in the night sky, when they are not outshone by the Sun. Historically, the most prominent stars on the celestial sphere were grouped together into constellations, but faint diffuse items such as nebulae A nebula is an interstellar cloud of dust, hydrogen gas, helium gas and other ionized gases. Originally nebula was a general name for any extended astronomical object, including galaxies beyond the Milky Way (some examples of the older usage survive; for example, the Andromeda Galaxy was referred to as the Andromeda Nebula before galaxies were and galaxies A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The name is from the Greek root galaxias [γαλαξίας], meaning "milky," a reference to the Milky Way galaxy can only be properly observed beyond the limit of astronomical twilight. Theoretically, the dimmest stars ever visible to the naked eye—those of the sixth magnitude The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, normalized to the value it would have in the absence of the atmosphere. The brighter the object appears, the lower the value of its magnitude—will appear in the evening once the sun falls more than 18° below the horizon (i.e. when astronomical dusk ends) and disappear when the sun moves to within 18° of the horizon in the morning (when astronomical dawn begins). However, because of light pollution Such an approach confuses the cause and its result, however. Pollution is the adding-of/added light itself, in analogy to added sound, CO2 etc. Adverse consequences are multiple, some of them may be not known yet. Real definitions are to be, therefore, like:, 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.^[1]

Length

The length of twilight after sunset and before sunrise is heavily influenced by the latitude Latitude, usually denoted by the Greek letter phi gives the location of a place on Earth (or other planetary body) north or south of the equator. Lines of Latitude are the imaginary horizontal lines shown running east-to-west (or west to east) on maps (particularly so in the Mercator projection) that run either north or south of the equator of the observer. In the Arctic The Arctic is the region around the Earth's North Pole, opposite the Antarctic region around the South Pole. The Arctic includes the Arctic Ocean (which overlies the North Pole) and parts of Canada, Greenland (a territory of Denmark), Russia, the United States (Alaska), Iceland, Norway, Sweden and Finland and Antarctic The Antarctic is the region around the Earth's South Pole, opposite the Arctic region around the North Pole. The Antarctic comprises the continent of Antarctica and the ice shelves, waters and island territories in the Southern Ocean situated south of the Antarctic Convergence. The region covers some 20% of the Southern Hemisphere, of which 5.5% (1 regions, twilight (if at all) can last for several hours. There is no civil twilight at the poles within a month on either side of the winter solstice. At the poles, civil twilight can be as long as two weeks, while at the equator An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass. The capitalized term Equator refers to the Earth's equator, it can go from day to night in as little as twenty minutes. This is because at low latitudes The tropics, seated in the equatorial regions of the world, are limited in latitude by the Tropic of Cancer in the northern hemisphere at approximately 23°26' N latitude and the Tropic of Capricorn in the southern hemisphere at 23°26' (23.4°) S latitude. The tropics are also referred to as the tropical zone and the torrid zone (see geographical the sun's apparent movement is perpendicular In geometry, two lines or planes , are considered perpendicular (or orthogonal) to each other if they form congruent adjacent angles (a T-shape). The term may be used as a noun or adjective. Thus, referring to Figure 1, the line AB is the perpendicular to CD through the point B. Note that by definition, a line is infinitely long, and strictly to the observer's horizon. 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 A polar circle is either the Arctic Circle or the Antarctic Circle. On Earth, the Arctic Circle is located at a latitude of 66˚ 34', and the Antarctic Circle is located at a latitude of 66˚ 34' S, twenty-four hour daylight The midnight sun is a natural phenomenon occurring in summer months at latitudes north and nearby to the south of the Arctic Circle, and south and nearby to the north of the Antarctic Circle where the sun remains visible at the local midnight. Given fair weather, the sun is visible for a continuous 24 hours, mostly north of the Arctic Circle and is encountered in summer and in regions very close to the poles, twilight can last for weeks on the winter side of the equinoxes An equinox occurs twice a year, when the tilt of the Earth's axis is inclined neither away from nor towards the Sun, the centre of the Sun being in the same plane as the Earth's equator. The term equinox can also be used in a broader sense, meaning the date when such a passage happens. The name "equinox" is derived from the Latin aequus. 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 Summer Solstice occurs exactly when the Earth's axial tilt is closest to the sun at its maximum of 23° 26'. Though the Summer Solstice is an instant in time, the term is also colloquially used like Midsummer to refer to the day on which it occurs. Except at the equator and in the polar regions (where daylight is continuous for half of the (June in the Northern Hemisphere, December in the Southern Hemisphere). The precise position of the polar circles, and thus of the regions where twilight can continue through local midnight, varies slightly from year to year with Earth's axial tilt In astronomy, axial tilt is the angle between an object's rotational axis, and a line perpendicular to its orbital plane. It differs from inclination. 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.^[4][5]

These are the largest cities, of their respective countries, where the various twilights can continue through local solar midnight:

• Civil twilight from sunset to sunrise: Arkhangelsk Arkhangelsk , or Archangel in English, is a city and the administrative center of Arkhangelsk Oblast, Russia. It lies on both banks of the Northern Dvina River near its exit into the White Sea in the far north of European Russia. City districts spread for over 40 kilometers (25 mi) along the banks of the river and numerous islands of its delta, Tampere Tampere (Finnish pronunciation: [ˈtɑmpere] ; Swedish: Tammerfors [tamərˈfɔrs] or [tamərˈfɔʂ]) is a city in southern Finland. It is the most populous inland city in any of the Nordic countries. The city has a population of 211,691, growing to approximately 300,000 people in the conurbation and over 340,000 in the metropolitan area. Tampere, Umeå Umeå (Swedish pronunciation: [ʉːmе.оː] ; Finnish: Uumaja, Northern Sami: Ubmi) is a university town in Västerbotten, Sweden, Trondheim Trondheim (historically Nidaros and Trondhjem) is a city and municipality in Sør-Trøndelag county, Norway. The city of Trondheim was established as a municipality on 1 January 1838 (see formannskapsdistrikt). The rural municipalities of Byneset, Leinstrand, Strinda, and Tiller were merged with Trondheim on 1 January 1964, Mid Yell, Tórshavn, Reykjavik, Nuuk, Whitehorse and Anchorage.

• Nautical twilight from civil dusk to civil dawn: Petropavl, Moscow, Vicebsk, Vilnius, Riga, Tallinn, Wejherowo, Flensburg, Helsinki, Stockholm, Copenhagen, Oslo, Newcastle upon Tyne, Glasgow, Belfast, Letterkenny, Grande Prairie, Juneau, Ushuaia and Puerto Williams.

• Astronomical twilight from nautical dusk to nautical dawn: Hulun Buir, Erdenet, Astana, Samara, Kiev, Minsk, Warsaw, Košice, Zwettl, Prague, Berlin, Paris, Luxembourg city, Brussels, Amsterdam, London, Cardiff, Dublin, Calgary, Vancouver, International Falls, Bellingham, Rio Gallegos and Punta Arenas.

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.

On other planets

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.^[6]

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