Twilight is the time between darkness and sunrise in the morning, and sunset and complete darkness in the evening, when there is light outside, but the Sun is below the horizon. There are 3 types of twilight: civil, nautical, and astronomical
Earth’s atmosphereMorning twilight ends with dawn, while twilight in the evening ends with dusk. A number of atmospheric phenomena and colors can be seen during twilight.
Twilight occurs when the Earth’s upper atmosphere scatters and reflects sunlight and illuminates the lower atmosphere.
Astronomers define twilight in the context of the Sun’s elevation with respect to the horizon (the angle that the geometric center of the Sun makes with the horizon). They also distinguish between 3 types of twilight, dawn and dusk based on this definition.
Civil twilight, civil dawn & civil dusk
Civil twilight occurs when the Sun is betwen 0° and 6° below the horizon. In the morning, civil twilight begins when the Sun is 6 degrees below the horizon and ends at sunrise, while in the evening, it begins at sunset and ends when the Sun reaches 6 degrees below the horizon.
Civil dawn begins when the geometric center of the Sun is 6° below the horizon and ends withsunrise, which is the moment when the Sun’s upper edge touches the horizon.
Similarly, civil dusk begins with sunset and ends when the geometrical center of the Sun goes 6° below the horizon. Sunset is the instant when the trailing edge of the Sun goes below the horizon.
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 human activities. Only the most brightest celestial and space objects can be observed from Earth during civil twilight.
Several countries use this definition of civil twilight and civil dawn to make laws related to aviation, hunting, and the usage of headlights and street lamps.
In the United Kingdom, the time when everyone has to switch on their headlights is known as hours of darkness, which is 30 minutes before sunrise and after sunset.
Nautical twilight, nautical dawn & nautical dusk
Nautical twilight: Nautical twilight occurs when the geometrical center of the Sun is between 6° and 12° below the horizon. This twilight period is less bright than civil twilight and artificial light is generally required for human activities.
The term, nautical twilight, dates back to the time when sailors used the stars to navigate the seas. During this time, observers on Earth can easily see most stars.
Nautical dawn occurs when the Sun is 12° below the horizon during the morning.
Natutical dusk occurs when the Sun goes 12° below the horizon in the evening.
In addition to being important to navigation on the seas, nautical dawn and nautical twilight also has implications for the military in several countries. 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, astronomical dawn & astronomical dusk
Astronomical twilight occurs when the Sun is between 12° and 18° below the horizon.
In the morning, the sky is completely dark before the onset of the 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 sky after the end of astronomical twilight.
Astronomical dawn is the time when the geometric center of the Sun is at 18° 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° below the horizon. After this point, the sky is no longer illumintaed.
Shorter twilight at the Equator
The length of twilight experienced by a location depends on its latitude. Equatorial and topical regions tend to have shorter twilights than locations on higher latitudes.
At higher latitudes, during the summer months, 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 12 degrees below the horizon throughout the night.
Twilight at the North and South Pole
For a few days before the Spring Equinox – the Poles do not have a nautical or astronomical twilight. Instead, the civil twilight in the evening extends up to the civil twilight in the morning. This is because the Sun is never below 6 degrees from the horizon. During this time the sky at the poles is illuminated throughout the night.
During the summer months, especially around the Summer Solstice, the North and South Poles experience extended several days with no twilight at all.
Similarly, a few days before the Fall Equinox the Poles experience no nautical or astronomical twilight and an extended period of civil twilight, that keeps the sky illuminated throughout the night.
As the days move towards the Winter Solstice, the Sun does not rise 6 degrees above the horizon. Because of this, for some days, the Poles do not have civil twilight, only an extended period of nautical twilight. Eventually, the Sun does not rise more than 12 degrees above the horizon, causing the Poles to experience only extended periods of astronomical twilight. Finally, after the Winter Solstice, the Sun does not rise at all i.e. it stays 18 degrees below the horizon leading the Poles to experience several months of darkness.