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Topic: Astronomical twilight

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	Twilight - Wikipedia, the free encyclopedia
		Twilight is the time before sunrise and after sunset when sunlight scattered in the upper atmosphere illuminates the lower atmosphere and the surface of the earth.
		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.
		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.
	en.wikipedia.org /wiki/Astronomical_twilight (579 words)

	Polar night - Wikipedia, the free encyclopedia
		Because of twilight, this is not the case.
		Finally, the astronomical polar night is the period that no trace of light can be seen anywhere and no astronomical twilight occurs.
		Astronomical twilight happens when the sun is between twelve and eighteen degrees below the horizon.
	en.wikipedia.org /wiki/Polar_night (450 words)

	Rise, Set, and Twilight Definitions (Site not responding. Last check: 2007-10-08)
		The major determinants of the amount of natural light during twilight are the state of the atmosphere generally and local weather conditions in particular.
		Before the beginning of astronomical twilight in the morning and after the end of astronomical twilight in the evening the Sun does not contribute to sky illumination; for a considerable interval after the beginning of morning twilight and before the end of evening twilight, sky illumination is so faint that it is practically imperceptible.
		The corresponding solar zenith distances for nautical and astronomical twilight are 102 and 108 degrees, respectively.
	aa.usno.navy.mil /faq/docs/RST_defs.html (1486 words)

	Twilight - Encyclopedia, History, Geography and Biography (Site not responding. Last check: 2007-10-08)
		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.
		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°.
	www.arikah.net /encyclopedia/Astronomical_twilight (546 words)

	Twilight -- Facts, Info, and Encyclopedia article (Site not responding. Last check: 2007-10-08)
		The brightest stars appear during civil twilight, as well as planets, which may be described as the 'morning star' or 'evening star'.
		Nautical twilight is defined as the time when the centre of the refracted disk of the sun is more than 6 degrees degrees below the horizon but less than 12 degrees.
		Astronomical twilight is defined as the time when the centre of the refracted disk of the sun is more than 12 degrees below the horizon but less than 18 degrees.
	www.absoluteastronomy.com /encyclopedia/t/tw/twilight.htm (324 words)

	WWU Planetarium - Twilight Definitions
		Civil Twilight is defined to begin in the morning when the center of the Sun is geometrically 6 degrees below the eastern horizon, and to end in the evening when the center of the Sun is geometrically 6 degrees below the western horizon.
		This is the limit at which twilight illumination is generally considered sufficient, under good weather conditions, for terrestrial objects to be clearly distinguished.
		At the beginning or end of nautical twilight, under good atmospheric conditions and in the absence of other illumination, general outlines of ground objects may be distinguishable, but most detailed outdoor operations are not possible, and the horizon is indistinct.
	www.wwu.edu /depts/skywise/twilight.html (212 words)

	The Columbia Encyclopedia, Sixth Edition: twilight @ HighBeam Research (Site not responding. Last check: 2007-10-08)
		TWILIGHT [twilight] period between sunset and total darkness or between total darkness and sunrise.
		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.
		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.
	www.highbeam.com /library/doc0.asp?DOCID=1E1:twilight&refid=ip_encyclopedia_hf (272 words)

	.: The Galway Sky: Terrestrial Images :. (Site not responding. Last check: 2007-10-08)
		Twilight, as you all know, is the period in the evening after sunset or in the morning before sunrise when the sky is not completely dark because of scattering of sunlight in the atmosphere.
		An example of civil twilight is shown above, it is defined as beginning at sunset and ending when the sun is more than 6 degress below the horizon (vice versa for sunrise).
		Astronomical twilight is when the sun is 18 degrees below the horizon, when the fainter stars can be seen.
	www.thegalwaysky.com /terre03.htm (178 words)

	* Astronomical Twilight - (Astronomy): Definition
		Astronomical twilight: The time when the Sun is 12-18 degrees below the horizon.
		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...
		Astronomical twilight ends when the sun's center is 18° below the horizon; by this time even the faintest stars overhead can be seen.
	en.mimi.hu /astronomy/astronomical_twilight.html (190 words)

	H2G2 (Site not responding. Last check: 2007-10-08)
		In the morning astronomical twilight begins when the sun is 18 degrees below the horizon at sunrise; in the evening it lasts after sunset until the sun is 18 degrees below the horizon.
		The time between the end and the start of astronomical twilight is important to astronomers as this is the time when the sky is completely dark which is required for observation of faint stellar objects.
		Astronomical twilight differs slightly from the other two as it is the time of darkness between the end and the start of twilight when there is no light that interests astronomers.
	www.bbc.co.uk /dna/h2g2/pda/A4149452?s_id=4 (198 words)

	Almanac (Site not responding. Last check: 2007-10-08)
		This group of controls allows you to select astronomical objects as well as the dates, time zones, and geographic coordinates from which the objects are to be observed.
		Distances to celestial objects, except the Moon, are given in Astronomical Units (AU) which is defined as the mean distance between the Earth and the Sun.
		At the dark limit of nautical twilight, the center of the Sun is 12° below the horizon; For the limits of astronomical twilight, the center of the Sun is 18° below the horizon.
	home.att.net /~srschmitt/almanac_0.html (1861 words)

	Heavenly Shades of Nighttime Falling: It's Twilight Time
		Astronomers and time-keepers define three twilight periods: civil twilight, nautical twilight and astronomical twilight.
		Civil twilight is the period from sunset when the solar disk has just left the horizon until the centre of the sun's disc is 6 degrees below the horizon.
		Astronomical twilight fills the time interval when the sun is between 12 and 18 degrees below the horizon during this time only the gross outlines of objects can be discerned.
	www.suite101.com /article.cfm/13646/95310 (509 words)

	al-Fajr as-Sadiq (the True Dawn)
		According to the astronomers, the morning Astronomical Twilight is the end of night and beginning of day; and the evening Astronomical Twilight is the beginning of night.
		Since the Astronomical Twilight is "too indistinct for sight-taking" by civil population, it cannot qualify as the true dawn.
		However the time between the Astronomical Twilight and the Nautical Twilight is still unknown to us: that is, we have not been able to exactly pin-point the beginning of the true dawn.
	www.al-islam.org /beliefs/practices/fajr.html (3162 words)

	Zoom Astronomy Glossary: A
		Astronomers can determine the composition of gases in stars by looking for their characteristic frequencies (this is called spectroscopy).
		D'ARREST, H.L. Heinrich Louis d'Arrest (1822-1875) was a Danish astronomer and the co-discoverer of Neptune (in 1846), with Galle.
		The Astronomer Royal is appointed by the Queen (or King) of England to be custodian of the Greenwich Observatory.
	www.enchantedlearning.com /subjects/astronomy/glossary/index.shtml (4918 words)

	Salat Times (Site not responding. Last check: 2007-10-08)
		In astronomy the twilight period is divided into civil, nautical and astronomical twilight corresponding to solar depressions of 6, 12 and 18 degrees respectively.
		Nautical twilight - the horizon at sea ceases to be clearly visible and it is impossible to determine altitudes with reference to the horizon.
		King has confirmed that although Muslim astronomers widely used 18 degrees/l8 degrees symmetrical values or a slight variation to 19 degrees/17 degrees (morning/evening)-and in a few (earlier) cases even 20 degrees/16 degrees values were adopted-no record has been found of the use of a value as small as 15 degrees.
	www.ummah.net /astronomy/saltime (2170 words)

	Glossary of Astronomical Terms
		Astronomical Twilight When the centre of the Sun is between 12º and 18º below the horizon; faint stars become visible.
		Astronomical Unit (AU) The mean distance from the Earth to the Sun, i.e.
		Twilight The period of decreasing sky brightness after sunset, or of increasing sky brightness before sunrise.
	www.astunit.com /tutorials/glossary.htm (4622 words)

	Definitions for SunriseSunset.com (Site not responding. Last check: 2007-10-08)
		Civil twilight is defined when the sun is 6 degrees below the horizon.
		Nautical twilight is defined when the sun is 12 degrees below the horizon.
		Astronomical twilight is defined when the sun is 18 degrees below the horizon.
	www.sunrisesunset.com /definitions.html (189 words)

	BBC - h2g2 - The Definitions of Twilight
		Many people, when they think of twilight, will think of the period shortly before sunrise when the birds begin to sing, there is a warm glow in the eastern sky and the air has a stillness and serenity about it that cannot be experienced at any other time of day.
		However, twilight has a specific scientific definition which varies depending on whether you are a civilian, a mariner or an astronomer.
		In the morning civil twilight begins when the sun is six degrees below the horizon before sunrise; in the evening it lasts after sunset until the sun is six degrees past the horizon.
	www.bbc.co.uk /dna/ww2/A4149452 (825 words)

	Glossary (Site not responding. Last check: 2007-10-08)
		Astronomical Polar Night - is a period of the arctic winter north of 84°33'N, when there is no trace of light in the southern sky, and even the faintest stars can be seen.
		Astronomical Twilight - is the darkest period of evening twilight, before the only light in the sky is from stars and chemical reactions in the atmosphere.
		Twilight - is light from the sky which we receive each day just before the Sun rises or just after it sets.
	www.collections.gc.ca /simply_science/scirep4h.htm (603 words)

	Ocean Navigator Online - Twilight time explained (Site not responding. Last check: 2007-10-08)
		Nautical twilight extends until the sun has dropped 12 degrees below the horizon and astronomical twilight continues until the sun is 18 degrees below the horizon.
		This is the length of twilight on the equator at the equinox.
		To determine the time of evening civil twilight and nautical twilight for a vessel at 30 degrees N, 33 degrees W on April 14, it is necessary to obtain declination, which is approximately 9 degrees 30' N and insert the values for declination, co-altitude and latitude.
	www.oceannavigator.com /print.php?a=1289 (1350 words)

	twilight (Site not responding. Last check: 2007-10-08)
		At sunset, twilight begins when the disk of the sun is no longer visible, and at sunrise twilight ends when the disk becomes visible.
		Astronomical twilight ends at sunset when the center of the sun reaches an altitude of minus 18°.
		In the morning, civil twilight begins when the center of the sun reaches an altitude of minus 6° and in the evening it ends when the center of the sun reaches an altitude of minus 6°.
	www.sizes.com /time/twilight.htm (312 words)

	Twilight Definitions (Site not responding. Last check: 2007-10-08)
		Before sunrise and again after sunset there are intervals of time, twilight, 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.
		That is, at the dark limit of nautical twilight, the center of the Sun is geometrically 12 degrees below a horizontal plane.
		That is, at the dark limit of astronomical twilight, the center of the Sun is geometrically 18 degrees below a horizontal plane.
	www.notam02.no /motherboard/twilight.html (422 words)

	PHY 445/515: Astronomical Glossary (Site not responding. Last check: 2007-10-08)
		Astronomers use the 1 AU baseline of the Earth's orbit to measure the parallax of nearby stars.
		Astronomical objects transit when their right ascension equals the local sidereal time, or their hour angle = 0.
		Twilight: The time between sunset or sunrise and the when the night sky is dark.
	www.ess.sunysb.edu /fwalter/PHY515/glossary.html (1436 words)

	[No title]
		; -18.0 degrees is the height of the middle of the sun at the ; beginning of morning astronomical twilight and at the end of ; evening astronomical twilight.
		; -12.0 degrees is the height of the middle of the sun at the ; beginning of morning nautical twilight and at the end of ; evening nautical twilight.
		; -6.0 degrees is the height of the middle of the sun at the ; beginning of morning civil twilight and at the end of ; evening civil twilight.
	members.wri.com /billw/astronomy/astronomy.el (1783 words)

	SPACE.com -- Twilight's Last Gleaming
		The onset of nautical twilight occurs when the sun is between 6 and 12 below the horizon (which is transparent in this and the following map).
		Astronomical twilight ensues when the sun is between 12 and ends when it is 18 below the horizon.
		First we have the beginning of astronomical twilight, as 6th-magnitude stars fade from view, then nautical twilight when one can see where Earth meets sky, then civil twilight, when the world awakes and astronomers head for their beds.
	www.space.com /spacewatch/spacewatch_010717.html (788 words)

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