What Is Dusk?

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.

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Dusk in Kyoto, Japan.


Scientific Definition

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:

  1. Civil dusk – the geometric center of the Sun's disk is 6 degrees below the horizon.
  2. Nautical dusk – the geometric center of the Sun's disk is 12 degrees below the horizon.
  3. Astronomical dusk – the geometric center of the Sun's disk is 18 degrees below the horizon.

Civil Dusk

Civil dusk is when the geometric center of the Sun's disk goes 6 degrees 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.

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The 3 stages of dusk and evening twilight.

Nautical Dusk

At nautical dusk, the Sun is 12 degrees 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.

Astronomical Dusk

This moment is the official beginning of night time. The Sun is now 18 degrees 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.

Popular Usage

In everyday language, the term dusk is commonly used as another word for evening twilight—the period from sunset until nighttime. Other colloquial synonyms include nightfall, sundown, and eventide. In some contexts, dusk is also used to denote the setting of the Sun.

Religious Significance

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.

Topics: Astronomy, Sun, Atmospheric Phenomena