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Nautical twilight is a period in the morning and evening when the sun is 6-12 degrees below the horizon. It’s named after sailors who navigated during this time. Twilight is broken down into several categories, with civil twilight being the period when the sun is less than 6 degrees below the horizon. The length of twilight varies depending on location and time of year.
Nautical twilight is a period in the morning and evening when the sun is between six and 12 degrees below the horizon. There is some visible light, but not enough to perform detailed tasks without the use of artificial light, and the horizon tends to be somewhat indistinct during this period. People refer to this time of day as “nautical twilight” to refer to the fact that sailors often made navigational observations during this time, as visibility was ideal.
While many people think of twilight as roughly the time between when it’s not completely dark and when it’s clearly light outside, this time of day is actually broken down into several distinct categories. Civil twilight occurs when the sun is less than six degrees below the horizon, allowing people to see things clearly, even if the sun is obviously up or down. Nautical twilight follows, followed by astronomical twilight between 12 and 18 degrees, and when the sun is more than 18 degrees below the horizon, it is called night.
In the morning, people may refer to nautical twilight as “first light,” the time when light begins to show and the sky slowly begins to turn color as the sun rises. Haze and other obstructions to visibility tend to be low during this time, although cloud cover and fog can interfere with first light perception. People usually need headlights to drive and artificial light to work outdoors during nautical twilight.
At night, nautical twilight is sometimes called “nightfall,” referring to the fact that the sky is rapidly starting to darken, and will soon be so dark that artificial light will be a necessity. Visibility can be difficult during this time, as people may think it is lighter than it actually is, and may therefore refrain from using headlamps and other visibility aids, which can lead to accidents.
The length of time spent in twilight varies depending on where you are in the world and what time of year it is. Dusk tends to be longest at the poles and shortest around the equator; in Alaska, for example, twilight can last for several hours at certain times of the year, while in parts of Africa, light levels go from full daylight to night in 20 minutes. Specific information about the times and duration of twilight is usually provided in sunrise-sunset charts issued by meteorological information organizations.
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