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Dusk
Dusk
Dusk
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Landers, California at nautical dusk (20-second exposure)

Dusk occurs at the darkest stage of twilight, or at the very end of astronomical twilight after sunset and just before nightfall.[1] At predusk, during early to intermediate stages of twilight, enough light in the sky under clear conditions may occur to read outdoors without artificial illumination; however, at the end of civil twilight (when Earth rotates to a point at which the center of the Sun's disk is 6° below the local horizon), such lighting is required to read outside.[2] The term dusk usually refers to astronomical dusk, or the darkest part of twilight before night begins.

Technical definitions

[edit]
Civil, nautical, and astronomical twilight.[3] Dusk is the darkest part of evening twilight.

The time of dusk is the moment at the very end of astronomical twilight, just before the minimum brightness of the night sky sets in, or may be thought of as the darkest part of evening twilight.[4] However, technically, the three stages of dusk are as follows:

  • At civil dusk, the center of the Sun's disc goes 6° below the horizon in the evening. It marks the end of civil twilight, which begins at sunset. At this time objects are still distinguishable and depending on weather conditions some stars and planets may start to become visible to the naked eye. The sky has many colors at this time, such as orange and red. Beyond this point artificial light may be needed to carry out outdoor activities, depending on atmospheric conditions and location.
  • At nautical dusk, the Sun moves to 12° below the horizon in the evening. It marks the end of nautical twilight, which begins at civil dusk. At this time, objects are less distinguishable, and stars and planets appear to brighten.
  • At astronomical dusk, the Sun's position is 18° below the horizon in the evening. It marks the end of astronomical twilight, which begins at nautical dusk. After this time the Sun no longer illuminates the sky, and thus no longer interferes with astronomical observations.[2][5]

Factors

[edit]

Dusk timing varies depending on geography, climate, and season. Locations at higher latitudes experience prolonged dusk during summer because the Sun descends at a shallow angle below the horizon, while tropical regions—such as the Philippines—have much shorter dusk periods due to the Sun’s steep vertical path. Atmospheric factors like humidity, dust, and pollution can intensify dusk colors, producing vivid reds, oranges, and purples. Urban areas may appear brighter during dusk because of artificial light scattering, while rural and coastal regions often show clearer skies and greater visibility of stars during the later stages of twilight.

[edit]
  • Scots pine at early dusk in New Forest, England
    Scots pine at early dusk in New Forest, England
  • Dusk at the shore
    Dusk at the shore
  • Looking eastwards at dusk
    Looking eastwards at dusk
  • Sandakan, Malaysia at dusk
    Sandakan, Malaysia at dusk
  • Beached boats at dusk
    Beached boats at dusk
  • Curitiba at dusk, Brazil
    Curitiba at dusk, Brazil
  • Nautical dusk in Marina Bay, Singapore
    Nautical dusk in Marina Bay, Singapore
  • Astronomical twilight as seen from a plane window
    Astronomical twilight as seen from a plane window

See also

[edit]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Dusk

View on Grokipedia
from Grokipedia
Dusk is the period of partial darkness that occurs after sunset and before complete nightfall, marking the darkest of evening twilight when the sun is below the horizon but scattered still illuminates the . This transition typically lasts from a few minutes to over an hour, depending on , , and atmospheric conditions, and it plays a key role in daily cycles for both human activities and natural phenomena. Astronomically, dusk is categorized into three distinct types based on the sun's angular depression below the horizon, each defined by practical uses in , , and civil life. Civil dusk begins when the sun reaches 6 degrees below the horizon, at which point outdoor activities like driving without headlights become challenging due to diminishing . Nautical dusk follows at 12 degrees, a threshold historically used by mariners to identify the horizon for using stars, at which point the sea and sky fully blend and objects are no longer distinguishable. Finally, astronomical dusk occurs at 18 degrees, when the geometric center of the sun is fully obscured for stargazing purposes, allowing most celestial objects to become observable with minimal interference from twilight glow. The term "dusk" originates from Old English dox, evolving through to denote obscurity or shadowy dimness, reflecting its etymological roots in concepts of partial between and night. In broader contexts, dusk influences ecological patterns, such as crepuscular ' activity peaks, and cultural practices, including religious observances timed to this transitional phase, underscoring its significance beyond mere astronomy.

Overview and Definition

Core Definition

Dusk is the period of evening twilight immediately following sunset, during which ambient light levels decrease gradually as the Sun sinks below the horizon, eventually giving way to full night. This phase is part of the broader twilight phenomenon, where indirect sunlight continues to provide illumination despite the Sun's position out of direct view. Visually, dusk features a darkening often adorned with residual hues of orange, , and , created by the of through the atmosphere. These colors fade as the light diminishes, marking the transition from day to night. The word "dusk" derives from the "dox," signifying dark or swarthy, and evolved in to denote the interval of dim light between day and darkness. The duration of dusk varies widely, typically ranging from about 20 minutes near the to over 2 hours at higher latitudes, depending on the and . Sunset marks the precise instant when the of the Sun's disk disappears below the horizon, signifying the end of direct daylight and the onset of evening twilight. Dusk immediately follows this moment, representing the gradual dimming of the sky due to atmospheric of , rather than the abrupt cessation of the Sun's visibility. This distinction ensures that sunset is a singular geometric event, while dusk encompasses the extended period of fading illumination thereafter. In contrast to dawn, which denotes the morning twilight phase preceding sunrise, dusk functions as its evening equivalent, both characterized by indirect but occurring in opposing solar trajectories. Dawn involves the sky brightening as the Sun approaches the horizon from below, whereas dusk features progressive darkening as the Sun recedes further beneath it. These symmetric yet directionally inverse transitions highlight dusk's role in the diurnal cycle's descent into night. Dusk terminates at the conclusion of astronomical twilight, when the Sun's center reaches 18 degrees below the horizon, at which point scattered becomes negligible, yielding to full astronomical night. This boundary delineates the shift from residual twilight glow to complete darkness, where celestial observations are unimpeded by solar interference. Twilight serves as the broader term encompassing these illumination phases, with dusk specifically the evening segment. In polar regions, the conventional distinctions blur during transitional periods between polar day and , where the Sun's low path can produce extended twilight resembling continuous dusk for days or weeks, without the persistent overhead light of the midnight sun phenomenon. These prolonged dusklike conditions arise from the Earth's and result in multifaceted light cycles unique to high latitudes.

Types of Dusk

Civil Dusk

Civil dusk marks the initial stage of evening twilight, commencing at the moment of sunset and concluding when the geometric center of the Sun reaches 6° below the horizon. This period, known as civil twilight (sometimes referred to as civil dusk), provides the brightest phase of dusk, where scattered by the atmosphere still illuminates the sky sufficiently for general visibility. Visually, the conditions during civil dusk allow the horizon to remain distinctly outlined, and the sky retains enough brightness to support most everyday outdoor pursuits without supplemental lighting. The brightest and begin to emerge against a fading blue sky, but overall illumination is adequate for reading outdoors or navigating familiar terrain. In maritime contexts, the clearly defined horizon aids basic orientation. This stage holds practical importance for human activities, particularly in transportation and urban infrastructure. Many jurisdictions activate streetlights and impose headlight requirements for vehicles precisely at the end of civil dusk to ensure safety as natural light diminishes. It also influences regulations for takeoffs and landings, as well as seasons, where sufficient ambient light is deemed necessary. For example, in mid-latitudes around 40° north during summer, civil dusk typically endures for 25 to 35 minutes, offering a brief but usable window of transitional daylight.

Nautical Dusk

Nautical dusk encompasses the intermediate phase of evening twilight, occurring when the geometric center of the Sun is positioned between 6° and 12° below the horizon, thereby defining the duration of nautical twilight until its conclusion at 12° depression. This stage immediately follows civil dusk, transitioning into dimmer conditions as the Sun continues its descent. Visually, nautical dusk brings partial darkness to the sky, where the horizon grows increasingly indistinct due to fading residual light, yet the brighter stars and planets emerge clearly enough for practical observation. This balance allows for the visibility of celestial objects against a still-discernible but subdued background, distinguishing it from the brighter residual daylight of earlier twilight. The significance of nautical dusk traces back to , when sailors relied on this period to sight and identify key stars and planets for , a critical practice before the widespread adoption of GPS in the late . During this time, the horizon's glow diminishes progressively, eventually blending sea and sky, which prompted the phase's naming for its utility in offshore orientation. In equatorial regions, such as near , , the duration of nautical dusk spans approximately 23 to 26 minutes year-round, reflecting the Sun's near-vertical path and providing a concise window for these navigational tasks.

Astronomical Dusk

Astronomical dusk refers to the period when the geometric center of the Sun is between 12° and 18° below the horizon, marking the darkest phase of twilight and the transition to full astronomical night. This stage follows nautical dusk and concludes with the precise moment when the Sun reaches 18° below the horizon, ending all twilight and dusk proper. During astronomical dusk, the sky becomes fully dark, with no discernible natural illumination from the Sun, as the horizon is no longer visible and only faint celestial objects like and can be observed under non-light-polluted conditions. This visual condition arises because the Sun's rays are scattered minimally by the atmosphere at such depths, rendering the overhead sky effectively black. The significance of astronomical dusk lies in its role as the threshold for optimal deep-sky astronomy, where scattered sunlight becomes negligible, allowing faint , galaxies, and nebulae to be seen without solar interference. At this point, astronomers can conduct observations of dim celestial bodies that would otherwise be obscured by residual twilight glow. In high latitudes, such as above 48° N or S, astronomical dusk may not fully occur during summer months because the Sun's maximum depression remains shallower than 18° below the horizon, resulting in perpetual twilight known as the midnight sun effect.

Astronomical Explanation

Physical Mechanisms

Dusk occurs as a consequence of Earth's rotation on its axis, which causes the Sun to appear to descend below the horizon from an observer's perspective. This rotation, occurring at approximately 15 degrees per hour relative to the Sun's position, transitions the local sky from direct sunlight to indirect illumination through the atmosphere. During this phase, known as twilight, the Sun's geometric position relative to the observer places its center below the horizon while its rays continue to interact with Earth's atmospheric layers, scattering light to produce the characteristic dimming and coloration of the evening sky. The primary physical mechanism enabling visibility during dusk is atmospheric scattering of , predominantly through by air molecules such as and oxygen. As the Sun dips below the horizon, sunlight travels through a longer path in the atmosphere, enhancing the scattering effect; shorter wavelengths like are dispersed more efficiently and removed from the direct beam, leaving longer wavelengths—reds, oranges, and yellows—to dominate the sky's hue. This selective scattering intensifies the warm colors observed during dusk, with the effect becoming more pronounced as the solar path lengthens. Atmospheric refraction further influences the perception and duration of dusk by bending incoming sunlight toward the observer, effectively raising the apparent position of the Sun. This refraction, caused by the density gradient in the atmosphere, can delay the apparent sunset by about two minutes and extend the twilight period slightly, as the Sun's image appears elevated by up to 0.5 degrees near the horizon. The bending is most significant at low solar elevations, contributing to phenomena like the elongated or flattened appearance of the setting Sun. The progression of dusk is quantified by the solar depression angle θ\theta, defined as θ=−h\theta = -h where hh is the Sun's altitude (elevation angle) below the horizon. As θ\theta increases from 0° at sunset to 6°–18° across dusk stages, the diminishing direct illumination aligns with the and effects described.

Calculation and Duration

The of dusk times and durations involves determining the precise relative to the observer's horizon using astronomical ephemerides, which provide the Sun's coordinates as a function of date, time, latitude, and longitude. Dusk concludes when the geometric center of the Sun reaches a defined depression angle below the horizon: 6° for civil dusk, 12° for nautical dusk, and 18° for astronomical dusk, accounting for the Sun's apparent radius and . These computations are typically performed with algorithms that solve for the local and the at which the Sun achieves the specified zenith distance. Several factors influence the duration of dusk. plays a primary role, with durations lengthening at higher latitudes due to the shallower of the Sun's path across the sky; near the , the Sun descends more vertically, resulting in shorter twilight periods. Seasonal variations arise from changes in the Sun's , leading to longer durations in winter when the declination is lower relative to the observer's latitude. , including and horizon obstructions like mountains, can alter the effective depression and thus shorten or extend the perceived duration by several minutes. Atmospheric conditions, such as , also introduce minor adjustments, though standard models incorporate an average value. The (USNO) provides authoritative tools and methods for these calculations, drawing from algorithms detailed in the Astronomical Almanac and related publications like Jean Meeus's Astronomical Algorithms. These include iterative solutions for the Sun's position and event times, valid across a wide range of dates and locations. For practical computation, online services from the USNO allow input of specific coordinates and dates to generate tables of twilight begin and end times. An approximate formula for the length of astronomical twilight (from sunset to 18° depression) is t≈18∘15∘ per hour×f(ϕ)t \approx \frac{18^\circ}{15^\circ \text{ per hour}} \times f(\phi), where f(ϕ)f(\phi) is an adjustment factor involving sin⁡ϕ\sin \phi () to account for the obliquity of the and ; more precise versions use the H=arccos⁡(sin⁡(−18∘)−sin⁡ϕsin⁡δcos⁡ϕcos⁡δ)H = \arccos \left( \frac{\sin(-18^\circ) - \sin \phi \sin \delta}{\cos \phi \cos \delta} \right), with duration as (H−90∘)/15(H - 90^\circ)/15 hours from sunset. Similar adaptations apply to civil and nautical dusk by substituting the respective depression angles. As an example, at 40°N on (), the duration of civil dusk is approximately 34 minutes, from sunset to the end of civil twilight; this varies seasonally by 10–50%, being shorter near equinoxes (around 28 minutes) and longer in winter. Such variations highlight the interplay of and solar declination in predictive models.

Cultural and Historical Significance

In and

Dusk has long served as a potent in and , representing ambiguity, transition, and the interplay between light and shadow that evokes , romance, or foreboding. In creative works, it often marks a liminal space where day yields to night, mirroring themes of change, melancholy, or impending mystery. This symbolism underscores dusk's role as a for the human condition, bridging and . In , Homer's epics frequently invoke evening dimness to signal the close of daily strife and the onset of rest or peril, as seen in passages where the sun sinks into the ocean, drawing night across the earth, such as in the Iliad's depiction of the "fiery light of day" descending. This recurring motif heightens narrative tension, portraying dusk as a veil between the heroic world and nocturnal dangers. Similarly, in , William Wordsworth's "It Is a Beauteous Evening, Calm and Free" captures a serene dusk by the sea, where the sinking sun and tranquil waves inspire a sense of divine reverence and gentle melancholy, evoking spiritual communion with nature. In , dusk symbolizes foreboding transition, as in , where the fading light heralds the vampire's awakening and the blurring of boundaries between life and undeath, amplifying themes of and the uncanny. Artistically, dusk's shifting hues—often rendered in warm oranges fading to cool blues due to atmospheric —have inspired painters to explore light's ephemerality. J.M.W. Turner's "" (1839) dramatizes this through a vivid sunset over the Thames, where the obsolete is towed to its end, symbolizing the Romantic lament for a vanishing heroic age amid industrial progress. Impressionists further elevated dusk's aesthetic, using loose brushstrokes to capture twilight's subtle glow; Alfred Sisley's landscapes, for instance, depict the sun's last rays softening fields into subdued tones, emphasizing fleeting beauty and atmospheric depth. This portrayal evolved historically from Homer's mythic evenings to Romantic introspection and into modern media, where dusk enhances mood in . In Billy Wilder's (1950), scenes evoke faded glamour and existential decay over , with the boulevard's name itself connoting the dimming of Hollywood's golden era, blending visual poetry with narrative irony.

Religious and Mythological Contexts

In , dusk is associated with , the primordial god of darkness, whose enveloping mists herald the onset of night as the consort of , the goddess of night. , born from Chaos according to Hesiod's , embodies the shadowy veil that transitions the world from daylight to obscurity, filling the earth's hollows and encircling mortal realms. In , twilight serves as a symbolic bridge between day and night, most prominently evoked in , known as the "twilight of the gods," which foretells the apocalyptic transition from the current cosmic order to renewal through cataclysmic events. This liminal phase reflects the cyclical interplay of light and darkness in the Norse cosmos, where deities like (day) and (night) perpetuate the eternal rhythm, with twilight marking the precarious balance before night's dominance. Religious practices across traditions often align with dusk as a sacred temporal boundary. In , the commences immediately after sunset, serving as the fourth of the five daily obligatory salat and signaling the transition from day to evening repose. This timing, rooted in Quranic injunctions and prophetic tradition, emphasizes gratitude for the day's end and preparation for night. Similarly, in , the service, also called Arvit, is recited in the evening twilight, beginning with the blessing Ma'ariv Aravim ("who brings on evenings"), which extols for orchestrating the blending of day into night and maintaining cosmic harmony. In , Sandhya rituals—meaning "junction" or "twilight"—are performed at dusk (as well as dawn and noon) by initiates, involving purification through ablutions, recitation of Vedic mantras like the , and offerings to invoke during this transitional hour. In , is an evening prayer service traditionally held at dusk, marking the transition to night with hymns and scripture readings focused on thanksgiving and reflection. Symbolically, dusk represents a liminal phase for spiritual purification and foreboding omens in ancient beliefs. In Egyptian funerary traditions, as detailed in the , the evening descent parallels the soul's passage into the (underworld), where spells guide the deceased through the night's trials, mirroring Ra's solar journey from sunset to rebirth at dawn and symbolizing renewal amid peril. This transition evokes purification rites to ensure safe navigation of the , while the fading light often portends omens of mortality and . Historical festivals underscore dusk's ritualistic role in honoring the spiritual realm. Japan's Obon, a Buddhist-derived observance typically held in mid-August, involves dusk ceremonies where families light and float lanterns on water to welcome and guide ancestral spirits back to the living world, culminating in farewell rituals at twilight to ensure their peaceful return to the beyond.

Biological and Environmental Impacts

Effects on Wildlife Behavior

Dusk triggers significant behavioral shifts in , marking the transition from diurnal to nocturnal activity patterns. Nocturnal species, such as bats, typically emerge from roosts around civil dusk, when is reduced but ambient remains sufficient for initial orientation and . Conversely, diurnal animals like songbirds cease activities and seek roosting sites at this time to avoid heightened predation risks in dimmer conditions. Specific examples illustrate these adaptations in action. Great horned owls often initiate during twilight periods near dusk, leveraging the intermediate levels of nautical dusk—when the sun is 6 to 12 degrees below the horizon—for optimal prey detection against a still-visible horizon. Fireflies synchronize their bioluminescent flashes in the low- environment of early dusk to facilitate mating, with displays peaking shortly after sunset when natural twilight provides a balanced visual backdrop. Many animals possess physiological adaptations that enhance performance during dusk. Predators such as and cats feature a , a reflective layer behind the that amplifies low-light vision by redirecting photons through photoreceptors, enabling effective in twilight conditions. For migratory birds, twilight polarization patterns in the sky serve as critical orientation cues, allowing species like thrushes to calibrate their compasses at sunset for accurate nocturnal . Ecologically, dusk functions as a low-risk window for crepuscular , such as nightjars, which exhibit peak activity intensity during this period when diurnal predators are less vigilant and nocturnal competitors have not yet dominated. However, artificial disrupts these natural behaviors by extending perceived daylight, causing songbirds to initiate dawn and dusk singing earlier in the season and potentially shortening the safe twilight interval for and roosting. This interference can lead to increased energy expenditure and altered predator-prey dynamics in affected habitats.

Human Perception and Physiology

During dusk, the human visual system undergoes a transition from , dominated by cone cells for color and detail , to mesopic and , where rod cells become increasingly active in response to diminishing levels. This shift enhances sensitivity to low-light conditions, allowing detection of faint stimuli, but impairs color discrimination and reduces , leading to a of the environment. Dusk acts as a primary environmental cue, or , for synchronizing the human circadian rhythm, triggering the onset of secretion from the shortly after sunset to promote preparation and regulate the sleep-wake cycle. This natural dimming of light facilitates a gradual physiological wind-down, but artificial blue light exposure from screens during this transitional period suppresses production, delaying circadian phase and contributing to sleep disturbances. The changing illumination during dusk can influence psychological states, often inducing feelings of calm and through the awe-inspiring visual spectacle of twilight, which studies link to stress reduction and mood enhancement via increased connection to natural cycles. Conversely, the ambiguous lighting may heighten anxiety in some individuals, particularly as deepen and visibility wanes, though this effect varies with personal and environmental factors. From a health perspective, civil dusk poses elevated risks for accidents, such as vehicular collisions, due to the interplay of residual sun glare on the horizon and rapidly declining ambient light, which impairs and reaction times. Furthermore, cutaneous synthesis, dependent on ultraviolet B , terminates post-sunset as the solar angle precludes effective UVB penetration, marking the end of daily photochemical production in the skin.

References

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