Twilight is the illumination of the Earth's lower atmosphere when the Sun itself isn't 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 additionally 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, the twilight's brightness is nearly zero, and evening twilight becomes nighttime. When the Sun again reaches 18 degrees 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 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 shouldn't 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 additionally at times used metaphorically, to imply that something is losing strength and approaching its end. For example, quite old people might be said to be "in the twilight of their lives". The collateral adjective for twilight is crepuscular which is at times applied to insects, fish, and mammals that are most active throughout that time.
Definitions by geometry
Twilight is defined according to the solar elevation angle θs, which is the position of the geometric centre 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).
Morning civil twilight begins (civil dawn) when the geometric centre of the sun is 6° below the horizon and ends at sunrise or when the geometric centre of the sun is 0°50′ below the horizon. Evening civil twilight begins at sunset or when the geometric centre of the sun is 0°50′ below the horizon, and ends (civil dusk) when the geometric centre 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 these periods.
Nautical dawn and dusk
Morning nautical twilight begins (nautical dawn) and Evening nautical twilight ends (nautical dusk) when the geometric centre of the sun reaches 12° below the horizon.
Definitional variations for ending throughout morning and for beginning throughout evening
According to a few definitions, morning nautical twilight ends when the geometric centre of the sun is 0°50′ below the horizon, according to others, when the geometric centre of the sun is 6° below the horizon. According to a few definitions, evening nautical twilight begins when the geometric centre of the sun is 0°50′ below the horizon, according to others, when the geometric centre of the sun is 6° below the horizon.
Astronomical dawn and dusk
Morning astronomical twilight begins (astronomical dawn) and Evening astronomical twilight ends (astronomical dusk) when the geometric centre of the Sun reaches 18° below the horizon.
Definitional variations for ending throughout morning and for beginning throughout evening
According to a few definitions, morning astronomical twilight ends and evening astronomical twilight begins when the geometric centre of the sun reaches 0°50′ below the horizon, while according to others, when the geometric centre of the sun reaches 12° below the horizon.
Definitions by illumination
There are three established and widely accepted subcategories of twilight: civil twilight (brightest), nautical twilight, and astronomical twilight (darkest).
Under clear weather conditions, civil twilight approximates the limit at which solar illumination suffices for the human eye to clearly distinguish terrestrial objects. Enough sunlight renders artificial sources unnecessary for most outdoor activities. At civil dawn, the beginning of morning civil twilight, and at civil dusk, the end of evening civil twilight, 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 throughout civil twilight.
Law-makers 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 a large number of 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 a few jurisdictions.
The period might affect when pilots require a night rating to fly aircraft. 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.
Before nautical dawn and after nautical dusk, sailors can't navigate via the horizon at sea. 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, throughout the stage of nautical twilight when they can distinguish a visible horizon for reference. Under good atmospheric conditions with the absence of additional illumination, throughout nautical twilight, the human eye might distinguish general outlines of ground objects but can't 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 might 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.
In a few places (away from urban light pollution, moonlight, auroras, and additional 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 throughout and after astronomical twilight in the evening and both before and throughout astronomical twilight in the morning. Notwithstanding a few critical observations, such as of faint diffuse items such as nebulae and galaxies, might require observation beyond the limit of astronomical twilight. Theoretically, the dimmest stars ever visible to the naked eye (those of the sixth magnitude) will become visible in the evening at astronomical dusk), and become invisible at astronomical dawn).
However, in additional places, especially those with skyglow, astronomical twilight might 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, a few localities, generally those in large cities, might never have the opportunity to view even fourth-magnitude stars, irrespective of the presence of any twilight at all, and to experience truly dark skies.
Times of occurrence
Twilight between day and night
Observers within 48.5 degrees of the Equator can view twilight twice each day, between astronomical dawn, nautical dawn, or civil dawn, on the one hand, and sunrise on the other, as well as between sunset on the one hand, and civil dusk, nautical dusk, or astronomical dusk on the other. These occur for most observers at higher latitudes on a large number of dates. Notwithstanding at latitudes closer than 9 degrees to either Pole, the Sun can't rise above the horizon nor sink more than 18 degrees below it on the same day on any date, so this type of twilight can't occur because the angular difference between solar noon and solar midnight elevates less than 18 degrees.
Twilight lasting from one day to the next
At latitudes greater than about 48.5 degrees North or South, on dates near the summer solstice, twilight can last from sunset to sunrise, after the Sun doesn't go more than 18 degrees below the horizon, so complete darkness doesn't occur even at midnight. These latitudes include a large number of densely populated regions of the Earth, including the entire United Kingdom and additional countries in northern Europe.
Twilight between one night and the next
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 close to the winter solstice. At all additional latitudes and dates, the polar night includes a daily period of twilight, when the Sun isn't far below the horizon. Around winter solstice, when the solar declination changes slowly, complete darkness lasts several weeks at the Pole itself, e.g., from May 14 to July 28 at Amundsen–Scott South Pole Station.
Twilight lasting for 24 hours
At latitudes within 9 degrees of either Pole, as the sun's angular elevation difference is less than 18 degrees, twilight can last for the entire 24 hours. This occurs for one day at latitudes near 9 degrees from the Pole and extends up to several weeks the further towards the Pole one goes. The only permanent settlement to experience this condition is Alert, Nunavut, Canada, where it occurs for a week in late February, and again in late October.
The obvious 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 obvious 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's 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 disc 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 quite 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:
- Civil twilight from sunset to sunrise: Tampere, Umeå, Trondheim, Tórshavn, Reykjavík, Nuuk, Whitehorse, Anchorage, and Baltasound.
- Nautical twilight from civil dusk to civil dawn: Moscow, Vitebsk, Vilnius, Riga, Tallinn, Wejherowo, Flensburg, Helsinki, Stockholm, Copenhagen, Oslo, Newcastle upon Tyne, Glasgow, Belfast, Letterkenny, Petropavl, Grande Prairie, Juneau, Ushuaia, and Puerto Williams.
- Astronomical twilight from nautical dusk to nautical dawn: Hulun Buir, Erdenet, Astana, Samara, Kiev, Minsk, Warsaw, Košice, Paris, Dublin, Zwettl, Prague, Stanley (Falkland Islands), Berlin, Luxembourg City, Brussels, Amsterdam, London, Cardiff, Calgary, Unalaska (Bellingham, largest in the continental USA), Rio Gallegos, and Punta Arenas.
- Major cities that near astronomical twilight from nautical dusk to nautical dawn: Saguenay (48°25′0"N), Brest (48°23′26"N), Thunder Bay (48°22′56″N), Vienna (48°12′30″N), Bratislava (48°8′38″N), Munich (48°8'0″N)
Although Helsinki, Oslo, Stockholm, Tallinn, and Saint Petersburg additionally enter into nautical twilight after sunset, they do have noticeably lighter skies at night (known as white nights) around the summer solstice than additional locations mentioned in their category above, because they don't go far into nautical twilight.
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.
On additional 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 Christian practice, "vigil" observances often occur throughout twilight on the evening before major feast days or holidays. For example, the Easter Vigil is held in the hours of darkness between sunset on Holy Saturday and sunrise on Easter Day — most commonly in the evening of Holy Saturday or midnight — and is the first celebration of Easter, days traditionally being considered to start at sunset.
Twilight is sacred in Hinduism. It is called गोधूळिवेळ gōdhūḷivēḷ in Marathi or गोधूलिवेला godhūlivelā in Hindi, గొధూళివేళ "godhoolivela" in Telugu, literally "cow dust time". Many rituals, including Sandhyavandanam and Puja, are performed at twilight hour. Eating of food isn't advised throughout this time. Sometimes it is referred to as Asurasandhya vela. It is believed that Asuras are active throughout these hours. One of the avatars of Lord Vishnu, Narasimha, is closely associated with the twilight period. According to Hindu scriptures, a daemonic king, Hiranakashipa, performed penance and obtained a boon from Brahma that he couldn't be killed throughout day or night and neither by human nor animal. Lord Vishnu appeared in a half-man half-lion form (neither human nor animal), ended the life of Hiranakashipa throughout twilight (neither day nor night).
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). Suhoor (early morning meal) time for fasting (during Ramadan and additional times) ends at morning twilight (at dawn). Fasting is from the beginning of dawn to sunset. There is additionally an important discussion in Islamic jurisprudence between "true dawn" and "false dawn".
In Judaism, twilight is considered neither day nor night; consequently it is treated as a safeguard against encroachment upon either. For example, the twilight of Friday is reckoned as Sabbath eve, and that of Saturday as Sabbath day; and the same rule applies to festival days.