People often need headlights to drive and artificial light to work outdoors during nautical twilight.
Nautical twilight is a period of morning and afternoon 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 blurred during this period. People refer to this time of day as “nautical twilight” to refer to the fact that sailors used to make sailing observations during this time as visibility was ideal.
While many people think of twilight as a period roughly between when it’s not completely dark and when it’s clearly bright outside, this time of day is divided 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 though the sun is obviously rising or falling. 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 starts to be visible and the sky slowly starts to turn colorful 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 often 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 is starting to darken rapidly and will soon get so dark that artificial light will be needed. Visibility can be tricky during this time as people may think it is lighter than it actually is and therefore may avoid using headlights and other visibility features, which could result in accidents.
The time spent in twilight varies, depending on where a person is in the world and the time of year. Twilight tends to be longest at the poles and shortest around the equator; in Alaska, for example, twilight can last for several hours during certain times of the year, while in parts of Africa, light levels range from daylight to night by 20 minutes. Specific information about the times and duration of twilight is usually provided in sunrise charts issued by organizations that provide meteorological information.