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day vs. night "Night," as regards showing nav lights is concerned, is defined as sunset to sunrise (See Rule 20b). Official sunrise and set times are listed in Tide Tables or the Nautical Almanac. Day shapes must be shown "by day" (see Rule 20d). This phrase "by day" is most likely (most safely) to mean from morning nautical twilight until evening nautical twilight (see Nautical Almanac). Note by contrast, automobile lights are required to be on from evening civil twilight until morning civil twilight. At civil twilight the center of the sun is 6° below the horizon and at nautical twilight the sun is 12° below the horizon. In normal atmospheric conditions, at civil twilight it is dark enough to see the brightest stars and at nautical twilight it is too dark to see the horizon.

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