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Polar night
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==Polar Sun cycle== [[File:Optical effect march sunset - NOAA.jpg|thumb|upright=1.3|At [[Earth's poles]] the Sun appears at the horizon only and all day around [[equinox]], marking the change between the half year long polar night and [[polar day]]. The picture shows the [[South Pole]] right before March equinox, with the Sun appearing through [[refraction]] despite being still below the horizon.]] If an observer located on either the North Pole or the South Pole were to define a "day" as the time from the maximal elevation of the Sun above the horizon during one period of daylight, until the maximal elevation of the Sun above the horizon of the next period of daylight, then a "polar day" as experienced by such an observer would be one Earth-year long.<ref>[https://www-istp.gsfc.nasa.gov/stargaze/StarFAQ10.htm#q164 NASA: The Sun and Seasons] NASA. (See last paragraph, section 164.) By David Stern. Last updated Sept. 17, 2004. Downloaded Feb. 17, 2017.</ref>
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