Sunrise, sunset and twilight
Defining when daylight and darkness occur
When traveling by boat around the clock—out of sight of land with an unobstructed horizon—you get to observe (in clear weather) the true rising and setting of the sun. The actual time of sunrise and sunset are defined as the time when the upper edge of the sun’s disc lies on the unobstructed horizon. The exact time at which you observe this phenomenon depends on many factors, including atmospheric conditions, your elevation, sea conditions, and when on land, the terrain you look over.
When observing sunrise and sunset at sea, you notice that the sky has already brightened before the sun rises and brightness continues after the sun sets. Twilight is the period between sunset and darkness and between darkness and sunrise when some outdoor activities can continue for a limited time without artificial light.
In marine navigation we use two periods of twilight: civil twilight and nautical twilight. Civil twilight begins in the morning or ends in the evening when the center of the sun lies 6 degrees below the horizon. During this period ordinary outdoor activities can be performed without artificial illumination. Nautical twilight begins in the morning or ends in the evening when the center of the sun lies 12 degrees below the horizon. During this time (and in the absence of other illumination) general outlines of objects may be distinguishable, but outdoor operations aren’t possible.
A third defined period of twilight (not generally used), astronomical twilight, is defined as the period beginning in the morning and ending in the evening when the sun lies 18 degrees below the horizon. Before astronomical twilight begins in the morning and ends in the evening, the sky has no illumination from the sun, making objects practically imperceptible.
To learn more about the sun and stars and how they can be used in marine navigation, consider taking the United States Power Squadrons Junior Navigation and Navigation courses.