Astronomical Twilight – Astronomical Dawn & Dusk
Astronomical twilight is the darkest of the 3 twilight phases. It is the earliest stage of dawn in the morning and the last stage of dusk in the evening.
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 astronomical twilight, the geometric center of the Sun's disk is between 12 and 18 degrees below the horizon.
To the naked eye, and especially in areas with light pollution, it may be difficult to distinguish astronomical twilight from night time. Most stars and other celestial objects can be seen during this phase.
However, astronomers may be unable to observe some of the fainter stars and galaxies as long as the Sun is less than 18 degrees below the horizon – hence the name of this twilight phase.
Astronomical Dawn and Astronomical Dusk
The twilight phases in the morning are often 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 moments during the transitions between day and night.
Astronomical dawn is the moment when the geometric center of the Sun is 18 degrees below the horizon in the morning. It is preceded by night time.
Similarly, astronomical dusk is the instant when the geometric center of the Sun is 18 degrees below the horizon in the evening. It marks the beginning of night time and the disappearance of the last shimmer of natural daylight.
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°, making for swift transitions between night and day and relatively short twilight phases. For example, in Quito, Ecuador, which is very close to the Equator, astronomical twilight begins only about 70 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° North) and Wellington (about 40° South), during the equinoxes, it takes about 1 hour and 30 minutes from the beginning of astronomical twilight until the Sun rises.
- In Oslo (about 60° North) and the northernmost tip of Antarctica (about 60° South), the same process takes roughly 2 hours and 30 minutes.
Twilight Around the Poles
At high latitudes and around the summer solstice, the Sun does not move lower than 18° below the horizon, so twilight can last from sunset to sunrise. The area experiencing all-night astronomical twilight around the summer solstice lies between about 48°33′ and 54°33′ North and South. In the northern hemisphere, this roughly correlates with the area between locations just south of the US-Canadian border and Canadian cities like Edmonton, Alberta. In Europe, it covers much of Germany.
An all-night period of astronomical 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 12-18° below the horizon around midday, causing a short daily period of astronomical twilight, a temporary break from the complete and permanent darkness that envelops polar regions in the winter.