Twilight, Dawn, and Dusk
Twilight is the time between day and night when there is light outside, but the Sun is below the horizon.
Types of Twilight
There are three types of twilight:
Twilight occurs when Earth's upper atmosphere scatters and reflects sunlight which illuminates the lower atmosphere.
Astronomers define the three stages of twilight – civil, nautical, and astronomical – on the basis of the Sun's elevation which is the angle that the geometric center of the Sun makes with 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 geometric center of the Sun is 6 degrees below the horizon in the morning.
Civil dusk is the moment when the geometrical 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 geometrical 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 goes 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.
In addition to being important to navigation on the seas, nautical twilight also has military implications. For example, the United States' military uses nautical twilight, called begin morning nautical twilight (BMNT) and end of 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 geometric center of the Sun is at 18 degrees below the horizon. Before this time, the sky is absolutely dark.
Astronomical dusk is the instant when the geographical center of the Sun is at 18 degrees below the horizon. After this point, the sky is no longer illuminated.
In the morning, the sky is completely dark before the onset of astronomical twilight, and in the evening, the sky becomes completely dark at the end of astronomical twilight. Any celestial bodies that can be viewed by the naked eye can be observed in the sky after the end of this phase.
Shorter Twilight at the Equator
The length of twilight depends on latitude. Equatorial and tropical regions tend to have shorter twilights 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 angle the Sun makes with the horizon – also known as the Solar Elevation Angle – is less than 18 degrees during the local midnight.
Similarly, higher latitudes may experience an extended period of nautical twilight – if the Sun remains less than 12 degrees below the horizon throughout the night.
Twilight at the poles
For a few days before the March equinox – the North Pole does not have nautical or astronomical twilight. Instead, there is a continuous period of civil twilight.
At the equinox, the Sun rises and stays up all day at the North Pole until the September equinox. During this time, the North Pole does not experience any kind of twilight. This phenomenon is called Polar Day or Midnight Sun.
A few days after the September equinox, when the Sun sinks below the horizon, the North Pole has a few continuous days of only civil twilight, followed by days of nautical twilight and then astronomical twilight.
This transition ends sometime in October when the Sun sinks more than 18 degrees below the horizon. When this happens, the pole experiences Polar Night – a continuous period of darkness without twilight.
By early March, astronomical twilight becomes visible to observers on the North Pole. This is followed by a few days of nautical twilight as the Sun moves further up the sky.
The same phenomena can be observed at the South Pole but during opposite times of the year.