twilight

Definition
 

There are three types of twilight; Astronomical, Nautical and Civil.

From a scientific perspective, twilight is defined according to the position of the Sun

The Sun , a G V star, is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 98.6% of the Solar System’s mass….
 (its centre) relative to the horizon. There are three established and widely accepted subcategories of twilight: civil twilight (brightest), nautical twilight and astronomical twilight (darkes

Definition Position of sun
  degrees below the horizon
Night more than 18°
Astronomical twilight 12° – 18°
Nautical twilight 6° – 12°
Civil twilight less than 6°
Day (sun above the horizon)

For comparison, the angular diameter of the Sun is 0.5°.

 

Note that if the Sun is 8 1/2 degrees below the horizon, it provides the same level of illumination to the surface of the Earth as a full moon directly overhead.

(For these definitions, an ideal horizon 90° from the zenith

Zenith

In broad terms, the zenith is the direction pointing directly above a particular location . Since the concept of being above is itself somewhat vague, scientists define the zenith in more rigorous terms….
 is used. The altitudes of the sun below the horizon are "true geometric" altitudes, that is, refraction

Refraction

Refraction is the change in direction of a wave due to a change in its speed. This is most commonly observed when a wave passes from one optical medium to another….
 by the atmosphere and other small factors influencing the observed position of the Sun are not to be accounted for.)

Civil twilight

This begins in the morning when the geometric center of the Sun

The Sun , a G V star, is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 98.6% of the Solar System’s mass….
 is 6° below the Horizon

The horizon is the apparent line that separates earth from sky.More precisely, it is the line that divides all of the directions one can possibly look into two categories: those which intersect the Earth’s surface, and those which do not….
 (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 , a G V star, is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 98.6% of the Solar System’s mass….
 reaches 6° below the horizon (the point of civil dusk).

The brightest stars appear during civil twilight, as well as planets, such as Venus
Venus is the second-closest planet to the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus , the Roman mythology goddess of love
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, 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….  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

This is defined as the time beginning when the geometric center of the Sun is exactly 6° below the horizon (the end of civil twilight) and ending when the sun’s center is exactly 12° below the horizon.

At this time, 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 everyone pulls security). This is partially due to tactics dating back to the French and Indian War, when combatants on both sides would use BMNT and EENT to launch attacks.

Astronomical twilight

This is defined as the time beginning when the center of the Sun is exactly 12° below the horizon (the end of nautical twilight) and ending when the sun’s center reaches exactly 18° below the horizon.
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 such as Star


A star is a massive, luminous ball of Plasma that is held together by its own gravity. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth
s, but faint diffuse objects such as nebula

Nebula

A nebula is an interstellar cloud of cosmic dust, hydrogen gas and Plasma . Originally nebula was a general name for any extended astronomy astronomical object, including galaxy beyond the Milky Way ….
e and Galaxy

A galaxy is a massive, gravitation system that consists of stars and stellar remnants, an interstellar medium of gas and cosmic dust, and an important but poorly-understood component tentatively dubbed dark matter….
 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 measurement of its brightness as seen by an observer on Earth, normalized to the value it would have in the absence of the Earth’s atmosphere….
— will appear in the evening once the Sun falls more than 18° below the horizon (i.e. when astronomical dusk occurs) and disappear when the Sun moves to within 18° of the horizon in the morning (when astronomical dawn occurs). However, due to Light pollution

Light pollution, also known as photopollution or luminous pollution, is excessive or obtrusive artificial light. The International Dark-Sky Association , "The Light Pollution Authority," defines light pollution as: It obscures the stars in the night sky for city dwellers, interferes with astronomy observatory, and, like and 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.

 

Length

 

The length of twilight after sunset and before sunrise is heavily influenced by the latitude


Latitude, usually denoted symbolically by the Greek letter phi gives the location of a place on Earth north or south of the equator. Lines of Latitude are the horizontal lines shown running east-to-west on maps
of the observer. In the arctic


The Arctic is the region around the Earth’s North Pole, opposite the Antarctica region around the South Pole. The Arctic includes the Arctic Ocean and parts of Canada, Greenland , Russia, the United States , Iceland, Norway, Sweden and Finland….
 and Antarctic regions, twilight (if at all) can last for several hours. There is no twilight at the poles within a month on either side of the winter solstice. At the poles, twilight can be as long as two weeks, while at the equator


The equator is the intersection of the Earth’s surface with the Plane perpendicular to the Earth’s rotation and containing the Earth’s center of mass….
, it can go from day to night in as little as twenty minutes. This is because at low latitudes the sun’s apparent movement is perpendicular to the observer’s horizon, in addition to the fact that the rotational speed of a specific location is highest at the Equator and slower as latitude increases. Thus, a location on the equator will pass through the various twilight zones directly and quickly. As one gets closer to the Arctic and Antarctic circles, the sun’s surface moves toward the observer’s horizon from a lower angle and at a slower rate. The observer’s earthly location will pass through the various twilight zones less directly, taking more time. At temperate-zone latitudes, twilight is shortest at or near both equinoxes, slightly longer around the time of the winter solstice.

Winter solstice may refer to:* Winter solstice* Winter Solstice and much longer in late spring and early summer.

Within the polar circle

A polar circle is either the Arctic Circle or the Antarctic Circle. On Earth, the Arctic Circle is located at a latitude of 66? 33′ 38" N, and the Antarctic Circle is located at a latitude of 66? 33′ 38" S
, twenty-four hour daylight


Daylight or the light of day is the combination of all direct and indirect sunlight outdoors during the Daytime . This includes direct sunlight, diffuse sky radiation, and both of these reflected from the Earth and terrestrial objects….
 is encountered in summer, and twilight literally lasts for weeks (in the polar fall and spring). The Arctic Circle


The Arctic Circle is one of the five major circle of latitude that mark maps of the Earth. It is the parallel of latitude that runs 66degree 33’39? north of the Equator
on a particular day in early March 2008, was at 66° 33.7060’ N (66° 33’ 42.36” N or 66.56177° N). In high latitude


Latitude, usually denoted symbolically by the Greek letter phi gives the location of a place on Earth north or south of the equator. Lines of Latitude are the horizontal lines shown running east-to-west on maps ….
s outside the polar circles, 24-hour daylight is not seen, but twilight can extend from sunset to sunrise, a phenomenon often referred to as ‘white nights’. The furthest south in the Northern Hemisphere


The Northern Hemisphere is the half of a planet that is north of the equator?the word sphere literally means ‘half sphere’. It is also that half of the celestial sphere north of the celestial equator
or north in the Southern Hemisphere

 

The Southern Hemisphere is the half of a planet that is south of the equator?the word sphere literally means ‘half ball’. It is also that half of the celestial sphere south of the celestial equator that Civil, Nautical and Astronomical twilight all night can occur are, respectively, at approximately 60° 33’ 42” (60.56177°), 54° 33’ 42” (54.56177°) and 48° 33’ 42” (48.56177°). These are the largest cities of their respective countries, that twilight all night can occur: Civil twilight all night:

Arkhangelsk , formerly called Archangel in English language, is a types of inhabited localities in Russia and the administrative center of Arkhangelsk Oblast, Russia.


Tampere is a city in southern Finland located between two lakes, N?sij?rvi and Pyh?j?rvi . Since the two lakes differ in level by , the rapids linking them, Tammerkoski, have been an important power source throughout history, most recently for generating electricity
Umea,

 
Trondheim

is a city and Municipalities of Norway in S?r-Tr?ndelag Counties of Norway, Norway. The city of Trondheim was established as a municipality on 1 January 1838 ….

Mid Yell is a coastal settlement on the Yell , the second-largest of the Shetland Islands, Scotland.Mid Yell, the largest settlement on the island, is at the head of Mid Yell Voe on the B9081 road about a mile from its junction with the A968 road….

T
órshavn is the Capital and largest city of the Faroe Islands. It is located in the southern part on the east coast of Streymoy. To the north west of the town lies the high mountain H?sareyn, and to the southwest, iis the Capital and largest city of Iceland. Its latitude at 64?08′ N makes it the world’s most northern national capital city. It is located in southwestern Iceland, on the southern shore of Faxafl?i Bay….

Nuuk is the Capital and largest city of Greenland, and the seat of government for the Sermersooq municipality. It has a population of 15,047 , of whom 11,862 were born in Greenland….

Whitehorse is the Capital of the Yukon, Canada. Whitehorse accounts for more than 75% of the territory’s population and is the largest city in the three Canadian territories….

Anchorage is a consolidated city-Borough in the U.S. state of Alaska. With an estimated 279,671 municipal residents in 2007 , it is Alaska’s largest city and constitutes more than 40 percent of the state’s total population….
. Nautical twilight all night:


Petropavl , also known as Petropavlovsk is a city by Ishim River in North Kazakhstan Province of Kazakhstan close to the border with Russia, about 350km west of Omsk along the Trans-Siberian Railway
.


Moscow is the capital and the largest types of inhabited localities in Russia of the Russian Federation. It is also the largest European cities and metropolitan areas, with the Moscow metropolitan area ranking among the largest urban areas in the world….
, Vicebsk


Vilnius is the largest city and the Capital of Lithuania, with a population of 555,613 as of 2008. It is the seat of the Vilnius city municipality and of the Vilnius district municipality….
, Riga

Riga

Riga the Capital of Latvia, is situated on the Baltic Sea coast on the mouth of the river Daugava River. Riga is the largest city in the Baltic states….
, Tallinn

Tallinn

Tallinn is the capital and largest city in the Republic of Estonia and of Harju County. It occupies a surface of 159.2 km? in which 397,617 inhabitants live….
, Wejherowo

Wejherowo

Wejherowo [] is a town in Gdansk Pomerania, northern Poland, with 47,000 inhabitants . It has been the capital of Wejherowo County in Pomeranian Voivodeship since 1999; previously, it was a town in Gdansk Voivodeship ….
, Flensburg

Flensburg

Flensburg is an independent city in the North of the States of Germany Schleswig-Holstein. Flensburg is the centre of the region Southern Schleswig….
, Helsinki

Helsinki

Helsinki is the Capital and largest List of cities and towns in Finland of Finland. It is in the southern part of Finland, on the shore of the Gulf of Finland, by the Baltic Sea Stockholm

 

is the capital and largest city of Sweden. It is the site of the national Swedish Government of Sweden, the Parliament of Sweden, and the official residence of the Swedish Monarchy of Sweden….

Copenhagen is the capital and largest city of Denmark, with an urban area with a population of 1,153,615 . Copenhagen is situated on the Islands of Zealand and Amager….

is the Capital and largest List of cities in Norway in Norway.Metropolitan Oslo or the Greater Oslo Region makes up the third largest urban area in Scandinavia after Metropolitan Stockholm and Metropolitan Copenhagen….

Newcastle upon Tyne is a City status in the United Kingdom and metropolitan borough of Tyne and Wear, in North East England. Situated on the north bank of the River Tyne, the city developed from a Roman Empire settlement called Pons Aelius, though it owes its name to the Newcastle Castle built in 1080, by Robert Curthose, the eldest son of…

Glasgow is the largest city in Scotland and List of largest United Kingdom settlements by population in the United Kingdom. The city is situated on the River Clyde in the country’s Scottish Lowlands….

Belfast is the capital city of Northern Ireland and the seat of Devolution#United Kingdom Northern Ireland Executive and legislative Northern Ireland Assembly in Northern Ireland….

Ushuaia may refer to the following:*Ushuaia, a city in Argentina.**Ushuaia Department, an administrative division**Ushuaia River**Ushuaia International Airport…

Puerto Williams is a Chilean port, located on Isla Navarino facing the Beagle Channel. It is the Capital of Ant?rtica Chilena Province, one of four Provinces of Chile located in the Magellan and Chilean Antartica Region….
. Astronomical twilight all night: Hulun Buir,

Astana , is the capital and second largest city of Kazakhstan, with an officially estimated population of 600,200. It is located in the north-central portion of Kazakhstan, within Akmola Province, though politically separate from the rest of the province….

Kiev, also known as Kyiv , is the Capital and the largest city of Ukraine, located in the north central part of the country on the Dnieper River….

Minsk is the Capital and largest city in Belarus, situated on the Svislach River and Nemiga rivers. Minsk is also a headquarters of the Commonwealth of Independent States ….

Warsaw is the Capital and World’s largest cities of Poland. It is located on the Vistula River roughly from both the Baltic Sea coast and the Carpathian Mountains….

Ko?ice Being the economic and cultural centre of eastern Slovakia, Ko?ice is the seat of the Ko?ice Region and Ko?ice Self-governing Region, the Slovak Constitutional Court of Slovakia, three universities, various dioceses, and other institutions….

Zwettl is a town and district capital in Lower Austria Austria. It is chiefly known as the location of Zwettl Abbey, first mentioned in October 1139….

Prague is the Capital and World’s largest cities of the Czech Republic. Its official name is Hlavn? mesto Praha, meaning Prague, the Capital City….

Berlin is the Capital of Germany city and one of sixteen States of Germany of Germany. With a population of 3.4 million within its city limits, Berlin is the country’s largest city….

Paris is the Capital of France and the country’s largest city. It is situated on the river Seine, in northern France, at the heart of the ?le-de-France Regions of France ….

Amsterdam is the Capital of the Netherlands and List of cities in the Netherlands with over 100,000 people of the Netherlands, located in the Provinces of the Netherlands of North Holland in the west of the country….

London is the capital of both England and the United Kingdom, and the most populous municipality in the European Union. An important settlement for two millennia, History of London goes back to its founding by the Roman Empire….

Cardiff is the Capital , largest city and most populous Unitary authority#Wales in Wales. The city is Wales’ chief commercial centre, the base for many national cultural and sport institutions, the Welsh national media, and the seat of Welsh Assembly Government ….

Dublin is both the largest city and capital of Republic of Ireland. It is located near the midpoint of Ireland’s east coast, at the mouth of the River Liffey and at the centre of the Dublin Region….

Calgary is the largest city in the province of Alberta, Canada. It is located in the south of the province, in an area of foothills and High Plains, approximately east of the front ranges of the Canadian Rockies….

 

Vancouver is a coastal city and major seaport located in the Lower Mainland of southwestern British Columbia, Canada. It is the largest city in British Columbia and the second largest metropolitan area in the Pacific Northwest region…. largest metropolitan area), Bellingham Washington, Rio Gallegos and Punta Arenas. Although, Helsinki, Oslo, Stockholm, Tallin and Saint Petersburg

Saint Petersburg is a types of inhabited localities in Russia and a federal subjects of Russia of Russia located on the Neva River at the head of the Gulf of Finland on the Baltic Sea…., do not actually get Civil twilight all night, even in mid summer. They do have noticeably lighter skies at night, in mid summer (white nights).

 

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.


In cryptography, MARS is a block cipher that was IBM’s submission to the Advanced Encryption Standard process. MARS was selected as an AES finalist in August 1999, after the AES2 conference in March 1999, where it was voted as the fifth and last finalist algorithm….
 

 

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