From Day to Night: What time is considered to be “dusk”?
Last Updated: April 4, 2023
The dichotomy of day and night is fairly obvious in our daily lives, but when does one end and the other begin? And why are humans so fascinated by these periods of transition that go by many names including dusk and twilight?
What causes the ethereal natural displays every morning and evening? Let’s explore the concept of changing from day to night and vice versa including the science behind it as well as the influences on culture and art.
What is Dusk, Twilight, Dawn, and More: Definitions to Describe Transition
As the Earth rotates around on its axis, the Sun appears to rise in the east and set in the west. The Sun provides us with light creating the dichotomy of light and day. However, the transition between the two is not instantaneous, meaning that we have times when the light from the Sun is fading or rising. Outside of sunrise and sunset, what are these moments of transition?
In general, we have dawn, dusk, and twilight to indicate the times when the Sun enters a new phase, throughout the phase, and at the end of a phase respectively. These occur in the morning and the evening in three separate phases. In the morning we transition from Night to Astronomical twilight to Nautical Twilight to Civil Twilight with a dawn starting each phase and a dusk ending each. The process occurs in reverse in the evening from day to civil to nautical to astronomical and finally to night.
While dusk colloquially means the transition from day to night or even broadly as the evening twilight hours, it has a precise scientific definition related to the solar elevation level, or where the Sun is in the sky in relation to the horizon. There are actually three stages of dusk that occur at the end of the three phases of evening twilight:
Civil dusk
When the geometrical center of the sun is 6 degrees below the horizon, marking the end of civil twilight which begins at sunset. Different shades of orange and red permeate the sky as the atmosphere still reflects a large portion of light from the Sun. Civil dusk marks the point when artificial light is generally needed to carry out most outdoor activities and the brightest stars and planets (i.e. Venus and Jupiter) are visible to the naked eye as more celestial bodies appear.
Nautical dusk
When the geometrical center of the Sun is 12 degrees below the horizon, marking the end of nautical twilight which begins at civil dusk. Distinguishing the sky from land or water becomes difficult, especially in clear weather conditions. Most stars and constellations are visible to the naked eye.
Astronomical dusk
When the geometrical center of the Sun is 18 degrees below the horizon and the last bit of daylight has left the sky, ending the period of astronomical twilight which began at nautical dusk, and marking the official beginning of nighttime. During astronomical twilight, the sky may appear to be completely dark, but the final portions of the sunlight are still scattered across the night sky, potentially hindering night sky
The Physics/ Science of Dusk
When discussing light, what we see is only one section of the radiation emitted. The Sun releases radiation of varying wavelengths across the electromagnetic spectrum from longer wavelengths including radio, microwaves and infrared to our visible spectrum starting with red and moving through to violet (ROYGBIV) as well as shorter wavelengths including ultraviolet, x-rays, and gamma rays.
While this radiation travels consistently through the imperfect vacuum of space, molecules will affect it such as when it passes through our atmosphere, scattering the light in random directions. These effects on the light will be determined by how big the particle is to the wavelength. If the particle is smaller than the wavelength (such as nitrogen and oxygen molecules), the light is scattered in all directions, known as Rayleigh Scattering which causes shadows on sunny days to have a cooler tone.
If the particle is roughly equal in size to the wavelength (such as smoke particles), the scattering is less pronounced. If the particle is bigger than the wavelength (such as dust particles), the light is scattered evenly, but primarily forward, and all of the colors in the visible spectrum are scattered equally in contrast with the previous two where blue was scattered more than longer wavelengths.
The scattering occurs most in shorter wavelengths meaning that since the light carries far more blue light than violet (since it is scattered), the sky appears blue. The Sun is directly overhead at midday meaning the light has only a short distance to travel through the atmosphere, but the angle changes as the day progresses, increasing the distance the light has to travel through the atmosphere.
When the Sun is just above the horizon at sunrise or sunset, its rays hit the Earth at a low angle and have to travel through more of the atmosphere before they reach us, encountering particles including gas molecules, dust, and water droplets which filters the light, dimming it and adding more indirect light which softens contours and reduces contrast.
The blues are scattered before the light reaches you, meaning the longer orange and red wavelengths reach your eye and reduce the light’s color temperature, creating the typical sunset colors. The more atmosphere the light travels through, the redder the sunrise or sunset appears and high levels of air pollution will cause more vibrant sunrises and sunsets as well.
Shadows are also impacted in addition to color as they are related to the location and angle of the light. When the Sun is directly overhead, the shadow is minimal, practically disappearing if you are at the equator, but the shadows lengthen as the angle stretches with the Sun’s movement across the sky. During the time around sunset, the shadows are long and distinct. After the sun sets, its light shines evenly, helping to minimize and eliminate shadows.
Twilights in general have a tinting effect on color, often pushing to cooler blues and decreasing in power. During civil twilight, the Sun is at most 6 degrees below the horizon, meaning the upper atmosphere reflects the sunlight, illuminating the lower atmosphere, and bathing the sky in golden and reddish colors. In general, the low angle and long travel time through the atmosphere cause the light from the Sun during the time before and after sunset to be soft, diffused, and with little contrast, creating a warm color temperature and glowing effect without strong shadows or harsh lighting. The western sky will be illuminated with orange-red while the eastern sky remains blue and indigo. This time is also ideal for full moon viewing and photography during its rising and setting.
When the Sun is between 4 and 6 degrees below the horizon, the scattering of the shorter wavelengths is a little less, causing more blues to reach our eyes with a cold color temperature, deep blue hue, and saturated colors. Viewings and photography of the moon are still good at this time as well.
During nautical twilight, the sky begins to darken considerably as the Sun shifts even further under the horizon, but the horizon line is still visible until nautical dusk. More and more celestial objects will be visible and our moon’s silhouettes can be particularly odd and beautiful during this time in contrast with the balanced silhouettes of the previous twilight.
By astronomical twilight, light is dissipating quickly as silhouettes become more prevalent and details fade away due to the ever-disappearing light from the Sun.
Night officially occurs after the sun descends more than 18 degrees below the horizon when light from the Sun will no longer be visible. Artificial light will be prevalent, especially in cities to help accommodate any activities after dark, but in rural areas and even dark sky parks, the night sky is able to shine at its true brilliance and these astronomical objects are the sources of light. New Moon nights are typically the best for stargazing as the light from the moon won’t overpower them and therefore will be the darkest nights immediately at astronomical dusk.
While these summaries are accurate, the exact hues, glows, softening, and shadows will depend on the weather conditions and particle suspension in the atmosphere at your location.
Cultural and Societal Significance of Dusk
These periods of transition have long transfixed humans, beckoning them to see the changes in the sky, from day to night and back again in various ways. Since ancient times, we have often seen these times as indicative of the change in our own lives, using the theme of twilight, dusk, and dawn in various forms of art to indicate or represent changes in the characters/ humanity/ features of the art.
Since the dark is often correlated with danger or fear, dusk can often be used in various forms of art including visual and literary to convey an ominous tone or foreshadowing of darker times or themes to come.
For much of human history, night signaled the end of the day. Our night vision is not spectacular and therefore we typically need artificial light to continue different activities. While some work could not wait until the morning, such as navigating a ship in the night, work, in general, often stopped with the coming of night. Therefore, dusk can also have almost a calming or positive connotation and provide a medium to discuss human activities and work.
Sunsets and sunrises are common targets of artists. The breaking of a new day and the ending of a day provide wonderful themes to work with and the view is often so awe-inspiring, artists can’t help but try to capture it. Dusk can also provide some fantastic moods, themes, and backdrops for art. Claude Monet in particular was fascinated by the idea of dusk, featuring several different interpretations of it. Waterloo Bridge, London, at Dusk is a wonderful example, immersing the bridge and London skyline in a foggy blue that is interspersed with touches of violet for deep shadows.
His Impression, Sunrise and The Houses of Parliament, Sunset feature darker aspects of this theme from the perspective of the beginning and end of the day, featuring a full Sun near the horizon in the first and streaks of orange and indigo on the horizon in the second. Samuel Palmer’s Old England’s Sunday Evening shows the array of vibrant colors near the sunset and the darker cooler blues further up in the sky as the farmers make their way to evening mass.
Golden/ blue/ magic hours are some of the different terms that are given for these transition periods of twilights and dusks because of the changes to the light and shadows, creating a golden or blue hue. Urban landscapes in particular can be beautiful at dusk hours, creating ominous or hopeful tones. In fact, photographers specifically seek out these hours to take advantage of this unique light which can create some wonderful aesthetic effects. Golden hour is typically defined as the time immediately before and after sunset, when the Sun is up to 6 degrees above the horizon to four degrees below the horizon as well as the time before and after sunrise while the Sun is between 4 degrees under the horizon and up to 6 degrees above the horizon.
The blue hour is typically defined as when the Sun is between 4 and 6 degrees below the horizon, occurring before the golden hour in the morning and after the golden hour in the evening. The term “hour” is not accurate here as it is often not a full hour, but dependent on your latitude and time of year. There are a variety of different resources available to help track when these magic hours will occur in your location and for how long, especially for photographers.
While an evening prayer is common in Christianity and other religions, Islam specifically designates dusk as the appropriate time for the Maghrib evening prayer, one of the five obligatory daily prayers. During Ramadan, Muslims are required to fast from dawn to the break of night and the iftar is an evening meal at dusk, ending the fast.
Nautical twilight, dawn, and dusk refer back to when sailors used the stars as navigation tools and refer to the time of night before the horizon becomes indeterminable in reading the stars. It has been used in military branches to plan tactical operations, which continues to this day.
Since civil twilight is the brightest form of twilight, providing enough natural light that artificial light may not be required, several countries use civil twilight to make laws related to aviation, hunting, and the use of headlights and streetlamps.
Conclusion
Our daily lives are impacted by the movement of the Earth, Sun, and Moon, impacting society and culture. Day and night have provided a regular schedule since the dawn of humanity, impacting our activities for survival and relationship building.
While the arrival of the Sun signals a new day and its retreat under the horizon plunges us into the night until its return, the change is gradual, creating periods of transition that produce awe-inspiring views of the sky and horizon based on the path of light from the Sun through our atmosphere to us here on Earth.
The light from the Sun is scattered in our atmosphere and the longer it has to travel in it, the more light will be scattered in the shorter wavelengths, impacting the colors we see in the sky. Sunrises and sunsets feature brilliant vibrant reds and oranges because when it is near the horizon, it is at the lowest angle and needs to travel the longest through the atmosphere.
We experience three different periods of twilight and dusk as the Sun sinks further and further beneath the horizon: Civil Twilight and Dusk, Nautical Twilight and Dusk, and Astronomical Twilight and Dusk before night officially begins with the sun is more than 18 degrees below the horizon, blanketing its light until it reaches 18 degrees below the opposite horizon before daybreak when it goes through the same phases but in reverse: Astronomical, Nautical, and Civil, gradually brightening instead of darkening.
These transition periods affect our lives and activities as well as speak to our very souls, inspiring themes of change, beginning, and end. These periods of dusk have impacted us culturally and societally for eons, signaling both menial tasks and more ceremonial ones with a focus on change. Various forms of art and culture have embraced these transitions in painting, photography, literature, and more as they attempt to capture these ideas in artistic forms.
Sarah Hoffschwelle is a freelance writer who covers a combination of topics including astronomy, general science and STEM, self-development, art, and societal commentary. In the past, Sarah worked in educational nonprofits providing free-choice learning experiences for audiences ages 2-99. As a lifelong space nerd, she loves sharing the universe with others through her words. She currently writes on Medium at https://medium.com/@sarah-marie and authors self-help and children’s books.
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