<P> Before nautical dawn and after nautical dusk, sailors cannot navigate via the horizon at sea . 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). </P> <P> 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 . 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 . </P> <P> 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", in which everyone assumes a defensive position . This is partially due to tactics dating back to the French and Indian War (part of the Seven Years' War of 1756--1763), when combatants on both sides would launch attacks at nautical dawn or dusk . </P> <P> Astronomical dawn is the moment when the geometric center of the Sun is 18 degrees below the horizon in the morning . Astronomical dusk is the moment when the geometric center of the Sun is 18 degrees below the horizon in the evening . After astronomical dusk and before astronomical dawn, the sky is not illuminated by the sun . </P>

What is the time between dusk and dawn