Dusk generally refers to the transition from day to night, and the term is often used as another word for evening twilight. However, there is a formal definition that is more specific. As with dawn, scientists define dusk as specific moments in time based on the solar elevation angle, which is the position of the Sun in relation to the horizon.
There are 3 stages of dusk, each marking the end of 1 of the 3 phases of evening twilight:
- Civil dusk – the geometric center of the Sun’s disk is 6° below the horizon.
- Nautical dusk – the geometric center of the Sun’s disk is 12° below the horizon.
- Astronomical dusk – the geometric center of the Sun’s disk is 18° below the horizon.
Civil dusk is when the geometric center of the Sun’s disk goes 6° below the horizon. It marks the end of civil twilight, which begins at sunset. At this moment, the Earth’s atmosphere still reflects a large portion of sunlight, coloring a clear sky in different shades of orange and red. This is a good time for photographers to capture the sky and the soft glow of the sunlight. Beyond this point, artificial light is generally needed to carry out most outdoor activities. The brightest stars and planets, like Venus and Jupiter, are now visible to the naked eye and more celestial bodies begin to appear in the sky.
At nautical dusk, the Sun is 12° below the horizon. It marks the end of nautical twilight. In clear weather conditions, it is now becoming difficult to distinguish the sky from land or water, making it impossible for sailors to use the horizon as a reference point when navigating by the stars. Most stars and constellations are now visible to the naked eye.
This moment is the official beginning of night time. The Sun is now 18° below the horizon and the last shimmer of daylight has left the sky. Even before astronomical dusk, during the period of astronomical twilight, the sky may look completely dark. However, even the small portion of sunlight that is still scattered across the sky before the moment of astronomical dusk may hinder astronomers from observing some of the fainter stars and galaxies.
While it is common for Christians and adherents of other faiths to offer a prayer in the evening, Islam specifically defines dusk as the proper moment for the Maghrib evening prayer. It is one of the five obligatory daily prayers, which together constitute one of the Five Pillars of Islam.
During the Islamic month of Ramadan, Muslims are required to fast from dawn to the break of night. The iftar is an evening meal that occurs at dusk, when fasters are allowed to eat again.
What About Twilight?
While it’s common to confuse twilight with dusk and dawn, they are different events. Dusk and dawn occur when the Sun’s center is at the exact point below the horizon; twilight occurs between these points. For example, when the Sun is exactly 18° below the horizon – it’s called astronomical dusk (or dawn), but the interval from 12° to 18° is astronomical twilight.
So, twilight is a time of the day when the Sun is below the horizon and not visible directly but partially illuminates the sky. We can see twilight because the Earth has an atmosphere where the light partially scatters and illuminates our planet even when the Sun is gone.
The twilight duration depends on the latitude, time of year, and weather conditions. The shortest twilight is observed at the equator, where darkness falls 24 minutes after sunset; the longest one can last about six weeks at the poles.
There are three types of twilight – civil, nautical, and astronomical. They follow each other and occur in the same order all over the Earth. The twilight types vary depending on the Sun’s center position below the horizon:
- Civil twilight: 0 – 6°
- Nautical twilight: 6 – 12°
- Astronomical twilight: 12 – 18°
The time between astronomical twilight, when the Sun is 18° or further down below the horizon, is called the night.
The civil twilight is the brightest phase; most people refer to this type when talking about twilight. It starts right before sunrise or after sunset when the Sun is just below the horizon. The horizon should be well seen during this time, and only the brightest stars and planets are visible. Right after sunset, the sky is very bright and full of rapid colors.
The nautical twilight is the second phase when the horizon becomes challenging to distinguish. The term itself came from the times when sailors used the stars to navigate at sea. During nautical twilight, many of the stars are visible, making it possible to navigate according to the position of the stars.
When the darkness is almost complete, and the horizon isn’t discernible, it means that the astronomical twilight is here. The Milky Way starts to appear, and the faintest stars and planets can be observed with the naked eye unless the Moon illuminates the sky. However, galaxies, nebulae, and globular clusters require full darkness when the Sun is more than 18° under the horizon.