Twilight, Dawn, and Dusk
Twilight is the time between day and night when there is light outside, but the Sun is below the horizon.
Twilight occurs when Earth's upper atmosphere scatters and refracts sunlight which illuminates the lower atmosphere. A number of atmospheric phenomena and colors can be seen during twilight.
Types of Twilight
There are three types of twilight:
Astronomers define the three stages of twilight on the basis of how far the Sun is below the horizon.
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.
Brightest Form of Twilight
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.
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 naked eyes, 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.
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.
Shorter Twilight at the Equator
The length of twilight depends on the latitude. Equatorial and tropical regions tend to have shorter twilight than locations on higher latitudes.
During summer months at higher latitudes, there may be no distinction between astronomical twilight after sunset and astronomical twilight before sunrise. This happens when the Sun never goes more than 18 degrees below the horizon during the night.
Similarly, higher latitudes may experience an extended period of nautical or civil twilight.
Twilight at the Poles
Around the North Pole and the South Pole, each day-night cycle spans an entire year. In the summer, the poles experience Polar Day or Midnight Sun, when the Sun is up in the sky for several months; in the winter, during the Polar Night, 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.