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| Twilight |
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Did You Know that twilight can be several types? The Twilight - a gap of time between moment of sunset and coming the full darkness (the evening twilight), as well as between the end of the full darkness and sunrise (the morning twilight). When Sun sits for horizon, a part of firmament continues is illuminated due to diffusing of the sunshine in high layer of atmosphere. Luminosity decreases first quickly, but afterwards slowly. The length of twilight depends on geographical latitude (as high latitude as enduring twilight), time of the year and weather conditions. The most long twilight - at days of the summer & winter solstice (on June 22 and December 22), the most short - at days spring and autumn equinox (on March 21 and September 23). During twilight luminosity value (is measured in lux) changes in milliard once nearly. The Civil twilight begins at moment sunset (luminosity approximately 1000 lux) and end, when Sun turns out to be for horizon under angle 6-8° (luminosity drops to 1...4 lux). By the end of civil twilight illumination such that it is difficult to read the text - in this time majority civil population to go to bed so twilight and is named civil. Sea (navigational) twilight begin at moment of the completion civil and end when leaving Sun for horizon on 12° (luminosity drops to 0.006 lux), when illuminations it is not enough to distinguish seaborne unlighted objects ashore, but possible distinguish only coast line. The Astronomical twilight begin after sea and end at submersion Sun for line of the horizon on 18 (luminosity drops to 0.0001-0.00001 lux) and full disappearance blue sky. This type of twilight is named astronomical whereas, in this time luminosity drops to that level, when astronomers can begin to execute its work on study night sky, however best time for astronomical observations is a full darkness. On latitudes 60...66° depth of the submersion Sun in determined time of the year does not reach 18° and evening twilight move over to morning; the most known example of such phenomena - a “white nights” Saint Petersburg (here period of the phenomena – from June 11 to July 2, but in Arkhangelsk - from 13 May to July 30). On latitudes more than 66°33' during arctic day Sun is not lowered under horizon so twilight does not can be. Aside from solar, there is else and moon twilight, but they much light. The Illumination of the Moon influences upon color sky before moment of its submersion for horizon on 5...7°. |
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