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Skyglow (or sky glow) is the illumination of the night sky or parts of it. The most common cause of skyglow is artificial light that emits light pollution, which accumulates into a vast glow that can be seen from miles away and from high in the sky. Skyglow from artificial lights is common throughout the world and can be observed over most cities and towns as a glowing dome of the populated area. Skyglow's light domes can be large, as in that over a city, or small, as in that over an over-illuminated shopping center or a stadium. Although often referring to artificial light, skyglow also includes natural sources of diffuse nighttime light like the zodiacal light, starlight, and airglow emitted high in the upper atmosphere.〔 〕 Skyglow can also be caused by natural occurrences, such as the 1908 Tunguska event, in which a meteoroid spanning a few meters in mean radius exploded 5–10 kilometers above the Podkamennaya Tunguska River in the Krasnoyarsk Krai region of Russia. The explosion is estimated to have had released more or less 15 megatons of energy, which is around 1,000 times as powerful as the atomic bomb that exploded over Hiroshima, Japan in 1945 and about one third as powerful as the thermonuclear bomb Tsar Bomba, the most powerful nuclear bomb ever detonated. The light emitted from the Tunguska explosion was so great that it created skyglow as far away as England, where the population experienced a number of weeks of intermittent "bright nights" (a term that is now synonymous with ''skyglow''). ==Dependence on distance from source== For relatively small distances between the light source and observer, the intensity of the skyglow contribution from a single source of light is inversely proportional to the distance between the source and the observer (falls off as 1/r). This can be understood as follows: The illumination of any portion of atmosphere visible by the observer falls as 1/r2, but the path length of illuminated air along any given line of sight which is illuminated comparably to the brightest illumination grows linearly with distance; together, this gives a 1/r dependence. This is valid when the distance between the source and observer is smaller than the scale height of the atmosphere. At distances much larger, where the path length of illuminated air is limited by the height of the atmosphere itself, this becomes an inverse-square (1/r2) dependence. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「skyglow」の詳細全文を読む スポンサード リンク
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