Nautical Twilight – Nautical Dawn & Dusk
Nautical twilight is the second twilight phase. Both the horizon and the brighter stars are usually visible at this time, making it possible to navigate at sea.
Twilight is the time between day and night when the Sun is below the horizon but its rays still light up the sky. Astronomers differentiate between 3 phases:
Each twilight phase is defined by the solar elevation angle, which is the position of the Sun in relation to the horizon. During nautical twilight, the geometric center of the Sun's disk is between 6 and 12 degrees below the horizon.
In clear weather conditions, the horizon is faintly visible during this twilight phase. Many of the brighter stars can also be seen, making it possible to use the position of the stars in relation to the horizon to navigate at sea. This is why it is called nautical twilight.
Nautical Dawn and Nautical Dusk
The twilight phases in the morning are also colloquially called dawn, while the twilight phases in the evening are referred to as dusk. However, unlike the term twilight, which describes a time span, the terms dawn and dusk refer to exact moments during the transitions between day and night.
Nautical dawn is the moment when the geometric center of the Sun is 12 degrees below the horizon in the morning. It is preceded by astronomical twilight.
Similarly, nautical dusk is the instant when the geometric center of the Sun is 12 degrees below the horizon in the evening. It marks the beginning of astronomical twilight.
Timing & Length
The duration of each twilight phase depends on the latitude and the time of the year. In locations where the Sun is directly overhead at noon – for example at the Equator during the equinoxes – the Sun traverses the horizon at an angle of 90 degrees, making for swift transitions between night and day with relatively short twilight phases. For instance, in Quito, Ecuador, which is very close to the Equator, nautical twilight begins only about 45 minutes before sunrise during the equinoxes.
At higher latitudes, in both hemispheres, the Sun's path makes a lower angle with the horizon, so the twilight phases last longer:
- In New York (about 40 degrees North) and Wellington (about 40 degrees South), during the equinoxes, it takes about 1 hour from the beginning of nautical twilight until the Sun rises.
- In Oslo (about 60 degrees North) and the northernmost tip of Antarctica (about 60 degrees South), the same process takes roughly 1 hour and 35 minutes.
Twilight Around the Poles
At high latitudes and around the summer solstice, the Sun does not move lower than 18 degrees below the horizon, so twilight can last from sunset to sunrise. The area experiencing all-night nautical twilight around the summer solstice lies between about 54°33′ and 60°33′ North and South. In the Northern Hemisphere, this roughly correlates with the northern half of the Canadian province of Alberta, beginning just north of Edmonton. In Europe, it covers Denmark and southern Norway.
An all-night period of nautical twilight does not constitute a white night, which requires the Sun to remain less than 6 degrees below the horizon all night, causing civil twilight from sunset to sunrise.
Within the polar circles, the Sun does not set at all in the summer, so there is no twilight during that time of the year. However, in locations around the poles that experience polar night during the winter months, the Sun may reach an angle of 6-18 degrees below the horizon around midday. This causes a short daily period of astronomical and nautical twilight, which provides a temporary break from the complete and permanent darkness that envelops polar regions in the winter.
Nautical twilight may make it more difficult to observe the northern or southern lights. However, this phenomenon can normally still be seen, especially during periods of heightened auroral activity.