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Twilight

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For other uses, see Twilight (disambiguation).
Twilight at Baker Beach

Twilight is the time between dawn and sunrise, and the time between sunset and dusk. The sun itself is not actually visible because it is below the horizon. Due to the unusual, romantic quality of the ambient light at this time, twilight has long been popular with photographers and painters, who refer to it as "sweet light" or the "blue hour", after the French expression l'heure bleue.

Twilight is technically defined as the period before sunrise and again after sunset during which there is natural light provided by the upper atmosphere, which does receive direct sunlight and reflects part of it toward the Earth's surface.[1]

The collateral adjective of "twilight" is crepuscular (for daylight it is diurnal and for night, nocturnal). The term is most frequently encountered when applied to certain species of insects and mammals that are most active during that time.

Definitions

Various definitions of twilight.

Twilight is defined according to the position of the Sun (its centre) relative to the horizon. There are three established and widely accepted subcategories of twilight: civil twilight (brightest), nautical twilight and astronomical twilight (darkest).

Definition Position of sun
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 8½ 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 is used. The altitudes of the Sun below the horizon are "true geometric" altitudes; that is, refraction by the atmosphere and other small factors influencing the experiential position of the Sun are not to be accounted for.)

Civil twilight

Under civil twilight circumstances, the horizon is clearly visible, and terrestrial objects are easily perceptible, without artificial light.

This starts in the morning when the geometric center of the Sun is 6° below the horizon (the point of civil dawn), and ends at sunrise. Evening civil twilight begins at sunset and ends when the center of the Sun reaches 6° below the horizon (the point of civil dusk).

The brightest stars appear during civil twilight, as well as planets, such as Venus, 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 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

File:Atardecer campechano.jpg
Twilight at Campeche, Mexico.

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 stars, but faint diffuse items such as nebulae and galaxies 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— 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, 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

Twilight at Lyme Regis, UK.


The duration of twilight depends on the latitude and time of year. Note the brief times in March and September where continuous civil twilight exists at locations near either pole.

The length of twilight after sunset and before sunrise is heavily influenced by the latitude of the observer. In the Arctic and Antarctic 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, 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. 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. At temperate-zone latitudes, twilight is shortest at or near both equinoxes, slightly longer around the time of the winter solstice, and much longer in late spring and early summer.

Within the polar circles, twenty-four hour daylight is encountered in summer, and twilight literally lasts for weeks (in the polar fall and spring). Outside the polar circles, where the distance from the polar circle is less than the angle which defines twilight (see above), twilight can last around the clock near the summer solstice (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 last for twenty-four hours, varies slightly from year to year with Earth's axial tilt. The lowest latitudes at which twilight can last from sunset to sunrise 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.[2] [3]

These are the largest cities, of their respective countries, where 24-hour twilight can occur:

Although Helsinki, Oslo, Stockholm, Tallinn and Saint Petersburg do not actually get civil twilight all night, in mid summer they do have noticeably lighter skies at night (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.[4]

See also

References

  1. ^ a b "Definitions from the US Astronomical Applications Dept (USNO)". Retrieved 2009-03-03. Cite error: The named reference "USNO" was defined multiple times with different content (see the help page).
  2. ^ http://herbert.gandraxa.com/herbert/lod.asp
  3. ^ Herbert Glarner's website, reference 2. "Civil Twilight" "6°", "Nautical Twilight" "12°". "90°-Axis(23.439°)-12°=54.561°
  4. ^ NASA-Jet Propulsion Laboratory: Winter Solstice on Mars: Rovers Look Forward to A Birtha Williams Sanford Crisanthemum Barbra Layota Martian Spring, August 90, 2006

Bibliography

  • Mateshvili, Nina (2005). "Twilight sky brightness measurements as a useful tool for stratospheric aerosol investigations". Journal of Geophysical Research. 110 (D09209): D09209. doi:10.1029/2004JD005512. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
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