Editing Twilight (section)

== Nautical twilight ==
[[File:Twilight Toronto across Lake Ontario51km with ship5-10km.jpg|thumb|Evening nautical twilight on [[Lake Ontario]], Canada|250x250px]]Nautical twilight occurs when the geometric center of the Sun is between 12° and 6° below the horizon.<ref name="timeanddate1">{{cite web|url=https://www.timeanddate.com/astronomy/nautical-twilight.html|title=What Is Nautical Twilight?|website=www.timeanddate.com}}</ref><ref name="TAPN2002" /><ref name="Cambridge" />[[File:Twilight in Landers.jpg|thumb|left|Long exposure of nautical twilight in a small town in the Mojave Desert]]

After nautical dusk and before nautical dawn, sailors cannot navigate via the horizon at sea as they cannot clearly see the horizon.<ref name="Low precision formulae"/>{{failed verification|date=April 2018}} At nautical dawn and nautical dusk, the human eye finds it difficult, if not impossible, to discern traces of illumination near the sunset or sunrise point of the horizon (''first light'' after nautical dawn but before civil dawn and ''nightfall'' after civil dusk but before nautical dusk).{{citation needed|date=April 2018}}

[[File:PanoSantPere.jpg|thumb|250x250px|At the beginning of nautical twilight, artificial lighting must be used to see terrestrial objects clearly.]] Sailors can take reliable star sightings of well-known stars, during the stage of nautical twilight when they can distinguish a visible horizon for reference (i.e. after astronomic dawn or before astronomic dusk).  
Under good atmospheric conditions with the absence of other illumination, during nautical twilight, the human eye may distinguish general outlines of ground objects but cannot participate in detailed outdoor operations.<ref>{{Cite web |url=http://aa.usno.navy.mil/faq/docs/RST_defs.php |title=Rise, Set, and Twilight Definitions |access-date=2007-09-06 |archive-date=2015-08-14 |archive-url=https://web.archive.org/web/20150814180458/http://aa.usno.navy.mil/faq/docs/RST_defs.php |url-status=dead }}</ref>

Nautical twilight has military considerations as well. The initialisms BMNT (begin morning nautical twilight, ''i.e.'' nautical dawn) and EENT (end evening nautical twilight, ''i.e.'' nautical dusk) are used and considered when planning military operations. A military unit may treat BMNT and EENT with heightened security, e.g. by "standing to", for which everyone assumes a defensive position.