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Sunset or sundown, is the daily disappearance of the Sun below the western horizon as a result of Earth's rotation. The time of sunset is defined in astronomy as the moment when the trailing edge of the Sun's disk disappears below the horizon. Near to the horizon, atmospheric refraction causes the ray path of light from the Sun to be distorted to such an extent that the Sun’s disk is already about one diameter below the horizon when sunset is observed. Sunset is distinct from dusk, which is the time at which the sky becomes completely dark, which occurs when the Sun is approximately eighteen degrees below the horizon. The period between sunset and dusk is called twilight. Locations north of the Arctic Circle and south of the Antarctic Circle experience no sunset or sunrise at least one day of the year, when the polar day or the polar night persists continuously for 24 hours. Sunset creates unique atmospheric conditions such as the often intense orange and red colors of the Sun and the surrounding sky. ==Terminology== : ''For the terms "civil twilight" and "astronomical twilight", see Twilight#Definitions Sunset is the point at which the Sun is first completely below the horizon marking the start of twilight. It should not be confused with dusk, which occurs at the end of twilight. 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「sunset」の詳細全文を読む スポンサード リンク
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