Twilight (redirected from twilighted)

twilight, period between sunset and total darkness or between total darkness and sunrise. Total darkness does not occur immediately when the sun sinks below the horizon because light from the sun that strikes the atmosphere is scattered (both by the air itself and by suspended matter, e.g., dust and smoke). Civil twilight ends when the center of the sun is 6° below the horizon. Although it is still not very dark, it is necessary to use artificial light to carry out most activities. Nautical twilight ends when the sun's center is 12° below the horizon; at about this time the light is too dim for the user of a sextant to see a sharp horizon. Astronomical twilight ends when the sun's center is 18° below the horizon; by this time even the faintest stars overhead can be seen. (Similar definitions apply to morning twilight.) During twilight, Venus or Mercury is often seen as the evening star or morning star. The length of twilight depends on latitude and the time of year. Twilight is generally shorter at the equator, where the sun's path toward the horizon is more nearly vertical than at higher latitudes; typically, astronomical twilight may last for 1 hr at the equator and 1 1-2 hr in New York City.

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twilight [′twī‚līt]

(astronomy)

An intermediate period of illumination of the sky before sunrise and after sunset; the three forms are civil, nautical, and astronomical.

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Twilight 

an optical phenomenon observed in the atmosphere before sunrise and after sunset. The term “twilight” is also applied to the continuous transition from daylight to darkness and vice versa.

Twilight is characterized by a number of colorful phenomena that are observed in the sky before sunrise or after sunset when there are no clouds; in any weather, sunlight scattered from the upper atmosphere after sunset or before sunrise illuminates the earth’s surface. The duration of twilight depends on the angle between the horizon and the direction of the sun’s apparent motion across the sky. Twilight thus varies with latitude; the closer to the equator, the shorter the period of twilight. In addition, the duration of twilight depends on the declination of the sun. In regions of the earth sufficiently far from the equator, the sun sinks only a small angular distance below the horizon in summer, and evening twilight merges with morning twilight; as a result, it does not become dark at night. In Russia, this phenomenon is known as white nights.

Twilight occurs as a consequence of the earth’s spheroidal shape; the sun’s rays illuminate the upper layers of the atmosphere after the sun has set or before it has risen.

For practical purposes, the period of twilight is defined in three ways. Civil twilight is the time during which the sun sinks to 7° (sometimes 6°) below the horizon; in this brightest part of twilight, natural light is sufficiently intense for any kind of work, including reading and writing, to be performed outdoors without artificial illumination. Nautical twilight is the period when the sun sinks to 12° below the horizon. During this time, visibility deteriorates severely owing to insufficient illumination, but ships near a coast can still be navigated by sighting landmarks on the coast. Astronomical twilight is the period when the sun sinks to 18° below the horizon; during astronomical twilight, the earth’s surface becomes dark, but a barely perceptible light remains in the sky. This light is sufficient, however, to impede astronomical observations of faint stars.

Astronomical almanacs, nautical almanacs, and other reference publications report the times at which each of the three twilight periods begins and ends at various latitudes for each day of the year. These times are only approximate, however, since the illumination and visibility of one’s surroundings still depend on such factors as the weather and the presence of snow cover. In the rare instances when meteoric and volcanic phenomena increase the turbidity of the upper atmosphere, the intensity of twilight is enhanced, and its duration is extended.

REFERENCES

Minnaert, M. Svet i tsvet v prirode. Moscow, 1958. (Translated from English.)
Rozenberg, G. V. Sumerki. Moscow, 1963.