Astronomers define the three stages of twilight on the basis of how far the Sun is below the horizon.
Astronomical terms & definitions
Different degrees of twilight.
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Civil Twilight, Dawn, and Dusk
Civil twilight occurs when the Sun is less than 6 degrees below the horizon. In the morning, civil twilight begins when the Sun is 6 degrees below the horizon and ends at sunrise. In the evening, it begins at sunset and ends when the Sun reaches 6 degrees below the horizon.
Civil dawn is the moment when the center of the Sun is 6 degrees below the horizon in the morning.
Civil dusk is the moment when the center of the Sun is 6 degrees below the horizon in the evening.
Civil twilight is the brightest form of twilight. There is enough natural sunlight during this period that artificial light may not be required to carry out outdoor activities. Only the brightest celestial objects can be observed by the naked eye during this time.
Several countries use this definition of civil twilight to make laws related to aviation, hunting, and the usage of headlights and street lamps.
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Nautical Twilight, Dawn, and Dusk
Nautical twilight occurs when the center of the Sun is between 6 degrees and 12 degrees below the horizon. This twilight period is less bright than civil twilight, and artificial light is generally required for outdoor activities.
Nautical dawn occurs when the Sun is 12 degrees below the horizon during the morning.
Nautical dusk occurs when the Sun is 12 degrees below the horizon in the evening.
The term nautical twilight dates back to the time when sailors used the stars to navigate the seas. During this time, most stars can be easily seen with the naked eye, and the horizon is usually also visible in clear weather conditions.
In addition to being important to navigation on the seas, nautical twilight also has military implications. For example, the military forces of the United States use nautical twilight, as reflected by the terms begin morning nautical twilight (BMNT) and end evening nautical twilight (EENT), to plan tactical operations.
During nautical twilight, bright celestial objects and the horizon are both visible.
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Astronomical Twilight, Dawn, and Dusk
Astronomical twilight occurs when the Sun is between 12 degrees and 18 degrees below the horizon.
Astronomical dawn is the time when the center of the Sun is at 18 degrees below the horizon.
Astronomical dusk is the instant when the center of the Sun is at 18 degrees below the horizon.
During astronomical twilight, most celestial objects can be observed in the sky. However, the atmosphere still scatters and refracts a small amount of sunlight, and that may make it difficult for astronomers to view the faintest objects.
Before astronomical dawn and after astronomical dusk, it is astronomical nighttime, when no indirect sunlight is visible, and even faint celestial objects can be seen, weather permitting.
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How Long Does Twilight Last?
The length of twilight depends on the location and the time of year. The shallower the angle of the Sun’s path in relation to the horizon, the longer the twilight phases last.
That’s why equatorial and tropical regions have shorter twilight than locations on higher latitudes.
Arctic and Antarctic Twilight
At high latitudes, the Sun may never go more than a few degrees below the horizon between sunset and sunrise during summer. When that is the case, technically, these places don’t experience nighttime even though the Sun sets each day. This is sometimes referred to as white nights.
White nights in Tromsø, Norway
Within the Arctic and Antarctic Circles, the Sun doesn’t set at all during parts of the year, a phenomenon commonly called Polar Day or Midnight Sun.
Nyksund lighthouse in Norway under the Midnight Sun.
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Twilight at the Poles
At the North Pole and the South Pole, each day-night cycle spans an entire year. During one half of the year, in the summer, the Midnight Sun is up in the sky for several months; during the other half, it’s Polar Night, and the Sun does not rise for several months.
The transitions between Polar Day and Polar Night are marked by lengthy twilight periods. Once the Sun has disappeared behind the horizon in the fall, it slowly sinks lower each day, resulting in about two weeks of civil twilight, followed by nautical and astronomical twilight periods of roughly the same length. The opposite occurs in the spring, as the Polar Night draws to a close and the Sun begins to illuminate the atmosphere several weeks before it actually rises.