Nautical twilight is a period in the morning and evening when the sun is between six and 12 degrees below the horizon. Some visible light is present, but not enough to conduct 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 reference the fact that sailors often took navigational observations during this time, since the visibility was ideal.
While many people think of twilight as a time roughly between when it is not totally dark and when it is clearly light outside, this period of the day is actually broken into several distinct categories. Civil twilight occurs when the sun is less than six degrees below the horizon, allowing people to see things clearly, although the sun is obviously on the way 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 slowly starts to flush with color as the sun comes up. Haze 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 usually need headlights to drive and artificial light to work outdoors during nautical twilight.
At night, nautical twilight is sometimes called “nightfall,” referencing the fact that the sky is rapidly starting to darken, and it is soon going to be so dark that artificial light will be a necessity. Visibility can be tricky during this time, as people may think that it is lighter than it really is, and they may therefore refrain from using headlights and other aids to visibility, which can result in accidents.
The length of time spent in twilight varies, depending on where in the world one is, and what time of year it is. Twilight tends to be the longest in the poles, and the shortest around the equator; in Alaska, for example, twilight can last several hours during some periods of the year, while in parts of Africa, light levels go from full daylight to night in 20 minutes. Specific information about twilight times and length is usually provided in sunrise-sunset charts issued by organizations which provide weather information.