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Twilight
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==Definitions by geometry== Twilight occurs according to the [[solar elevation angle]] ''ΞΈ<sub>s</sub>'', 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 (nearest the horizon), nautical twilight, and astronomical twilight (farthest from the horizon).<ref name=USNO>{{cite web |archive-url = https://web.archive.org/web/20190927072432/http://aa.usno.navy.mil/faq/docs/RST_defs.php |archive-date = 2019-09-27 |url=http://aa.usno.navy.mil/faq/docs/RST_defs.php |title=Definitions from the US Astronomical Applications Dept |publisher=[[United States Naval Observatory|USNO]] |access-date=2011-07-22}}</ref>
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