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Twilight is the illumination of the Earth's lower atmosphere when the Sun itself is not directly visible because it is below the horizon. Twilight is produced by sunlight scattering in the upper atmosphere, illuminating the lower atmosphere so that the surface of the Earth is neither completely lit nor completely dark. The word "twilight" is also used to denote the periods of time when this illumination occurs.〔(【引用サイトリンク】title=Definitions from the US Astronomical Applications Dept (USNO) )〕 The further the Sun is below the horizon, the dimmer the twilight (other things such as atmospheric conditions being equal). When the Sun reaches 18 degrees below the horizon, the twilight's brightness is nearly zero, and evening twilight becomes nighttime. When the Sun again reaches 18 degrees below the horizon, nighttime becomes morning twilight. Owing to its distinctive quality, primarily the absence of shadows and the appearance of objects silhouetted against the bright sky, twilight has long been popular with photographers, who refer to it as 'sweet light', and painters, who refer to it as the blue hour, after the French expression: ''l'heure bleue''. Twilight should not be confused with auroras, which can have a similar appearance in the night sky at high latitudes. By analogy with evening twilight, the word "twilight" is also sometimes used metaphorically, to imply that something is losing strength and approaching its end. For example, a very old woman may be said to be ''in the twilight of her life''. The collateral adjective for ''twilight'' is ''crepuscular;'' it is most frequently encountered when applied to certain insects, fish and mammals that are most active during that time. ==Definitions== Twilight is defined according to the solar elevation angle ''θs'', which is the position of the geometric center of the sun relative to the horizon. There are three established and widely accepted ''subcategories'' of twilight: civil twilight (brightest), nautical twilight, and astronomical twilight (darkest).〔 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「twilight」の詳細全文を読む スポンサード リンク
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