Twilight

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(Redirected from Astronomical twilight)

For the movie called Twilight see Twilight (movie)

Twilight in Denmark, just after sunset
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Twilight in Denmark, just after sunset

Twilight is the time before sunrise and after sunset when sunlight reflected from particles in the upper atmosphere illuminates the lower atmosphere and the surface of the earth.

  • Civil twilight begins in the morning when the centre of the sun, as refracted by the atmosphere, is less than 6° below the horizon and ends at sunrise. Evening civil twilight begins at sunset and ends when the centre of the sun, as refracted by the atmosphere, is more than 6° below the horizon. The brightest stars appear during civil twilight, as well as planets, which may be described as the 'morning star' or 'evening star'. During this period of time there is still enough light from the Sun so that in most cases artificial sources of light are not needed to carry on outdoor activities. This concept is sometimes enshrined in laws, such as when drivers of automobiles must legally turn on their headlights, or if the crime of burglary is to be treated as a nighttime burglary, or in the daytime, with the latter determination resulting in a lesser penalty — although a fixed period of time (most commonly 30 minutes after sunset or before sunrise) will form the basis for the application of these statutes, rather than how many degrees the sun is below the horizon.
  • Nautical twilight is defined as the time when the centre of the refracted disk of the sun is more than 6° below the horizon but less than 12°. This is the time when 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 dawn and "nightfall" after dusk).
  • Astronomical twilight is defined as the time when the centre of the refracted disk of the sun is more than 12° below the horizon but less than 18°. Most casual observers would consider the entire sky already fully dark at the limit of astronomical twilight, and astronomers can easily make observations of point sources such as stars, but faint diffuse objects such as nebulae and galaxies can only be properly observed beyond the limit of astronomical twilight. Conceptually, 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 and disappear when the sun moves to within 18° of the horizon in the morning; however, due to photopollution, 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.

(For these definitions, we use an ideal horizon 90° from the zenith.)

Within the polar circles, 24-hour daylight is encountered in summer. In high latitudes outside the polar circles, 24-hour daylight is not seen, but twilight extends from sunset to sunrise, a phenomenon often referred to as 'white nights'. Above roughly 60°N or S, civil twilight lasts all night at midsummer, while above about 55°N or S, nautical twilight lasts all night at midsummer. Astronomical twilight can last all night for several weeks as far from the poles as 50°N or S.

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 (with none at the poles within a month on either side of the winter solstice), while at the equator, it can go from day to night in as little as 20 minutes. This is due to the fact that at low latitudes the earth spins the fastest and therefore the sun spends less time near the horizon while in polar regions the sun's path in the sky is smaller and thus spends more time near the horizon. 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.

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

External links

ca:Crepuscle de:Dämmerung es:Crepúsculo it:Crepuscolo ja:薄明 fi:Hämärä

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