Meaning of Nautical Twilight
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 carry out detailed activities 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 took navigational observations during this time, as visibility was ideal.
While many people think of twilight as a rough time between when it’s not totally dark and when it’s clearly clear outside, this period of the day falls into several different categories.
Civil twilight occurs when the sun is less than six degrees below the horizon, allowing people to see things clearly, even though the sun is obviously going up or down.
Next comes nautical twilight, followed by astronomical twilight between 12 and 18 degrees, and when the sun is more than 18 degrees below the horizon, it is considered night.
In the morning, people may refer to nautical twilight as “first light,” the period when light first begins to be visible and the sky begins to slowly redden as the sun rises.
Fog and other obstructions to visibility tend to be low during this period, although cloud cover and fog can interfere with the perception of first light. People generally need headlights to drive and artificial light to work outdoors during nautical twilight.
At night, nautical twilight is sometimes called “dusk,” referring to the fact that the sky begins to darken rapidly, 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 really is and therefore may refrain from using headlights and other visibility aids, which can lead to accidents.
How long you spend at twilight varies, depending on where in the world you are and what time of year it is. Twilight tends to be longest at the poles and shortest around the equator;
In Alaska, for example, twilight can last for several hours during some periods of the year, while in parts of Africa, light levels go from daylight to night in 20 minutes.
Specific information about the time and duration of twilight is usually provided on sunrise and sunset charts issued by organizations that provide weather information.