Twilight is the period of daylight illumination produced by diffuse sky radiation when the Sun is below the horizon, as sunlight from the upper atmosphere is scattered, providing partial light before sunrise and after sunset.[1] It is classified into three types—civil twilight (Sun 0–6° below horizon), nautical twilight (6–12° below), and astronomical twilight (12–18° below)—with durations varying from about 20–30 minutes near the equator to several hours at mid-latitudes, and extending for weeks or months in polar regions where the Sun remains close to the horizon during solstices.[2][3] This phenomenon also occurs on other celestial bodies, such as planets with atmospheres (e.g., Mars) and moons, influenced by their geometry, rotation, and atmospheric composition. Twilight has held cultural and symbolic significance across societies, often representing transition, ambiguity, or spiritual moments in religion, literature, art, and modern media.
Definitions and Types
Geometric Basis
Twilight refers to the period of partial illumination of the Earth's surface following sunset or preceding sunrise, resulting from sunlight scattered by molecules in the upper atmosphere while the Sun's disk remains below the horizon.[1] This scattering occurs because direct sunlight no longer reaches the observer's location, but the upper atmospheric layers continue to be illuminated, redirecting diffuse light toward the ground.[1]The geometric basis of twilight is defined by the solar depression angle \theta_s, which measures the angular position of the Sun's geometric center below the observer's local horizon, equivalent to the negative of the solar elevation angle or the zenith distance minus 90°.[1] Atmospheric refraction alters the apparent position of the Sun, effectively raising it by approximately 0.57° near the horizon due to the bending of light rays through denser lower air layers, which must be accounted for in precise calculations.[1]Rayleigh scattering serves as the primary mechanism for the diffuse illumination of the twilight sky, where sunlight interacts with air molecules much smaller than the wavelength of visible light, preferentially scattering shorter blue wavelengths while allowing longer red wavelengths to penetrate deeper. This process produces the characteristic gradient of sky brightness from the horizon toward the zenith, with the intensity decreasing as the Sun's depression increases and fewer atmospheric paths contribute to scattering.Twilight transitions end when \theta_s reaches specific geometric thresholds, such as -18° for astronomical twilight, beyond which residual sky glow becomes negligible for most observations; these angles incorporate the refraction correction to define the Sun's true geometric position.[1] Civil, nautical, and astronomical twilight represent applications of progressively deeper depression angles (-6°, -12°, and -18°, respectively).[1]An illustrative diagram of twilight geometry typically depicts the observer at ground level, with the horizon line, the Sun's center at a depression angle \theta_s below it, and layered atmospheric strata showing incoming sunlight grazing the upper layers for scattering toward the observer; arrows indicate ray paths bent by refraction and scattered light paths contributing to sky illumination.[1]
Civil Twilight
Civil twilight is the brightest phase of twilight, occurring when the geometric center of the Sun is between 0° and 6° below the horizon. This period marks the transition from full daylight to night, with morning civil twilight—also known as civil dawn—beginning when the Sun reaches 6° below the horizon and ending at sunrise, while evening civil twilight, or civil dusk, starting at sunset and concluding when the Sun descends to 6° below the horizon. The 6° threshold is geometrically defined to account for the Sun's angular diameter and atmospheric refraction, ensuring the solar disk is fully obscured while maintaining adequate surface illumination.[4][5]During civil twilight, natural illumination remains sufficient for most outdoor activities without artificial lighting, allowing clear visibility of the horizon and surrounding landscape features. Distant objects are discernible, and brighter celestial bodies such as Venus become observable against the fading sky. This light level supports everyday tasks like walking or reading outdoors, as the overall brightness is comparable to a well-lit overcast day.[2][6][7]Practically, civil twilight delineates the boundaries for civil dawn and civil dusk, serving key roles in sectors like aviation, where aircraft operations do not require position lights due to the ambient visibility. In photography, it provides optimal soft lighting for capturing landscapes and portraits, enhancing color contrasts without harsh shadows. Daily scheduling in agriculture, transportation, and urban planning also relies on this phase for timing activities that bridge day and night.[8][9]The sky's appearance evolves during civil twilight, shifting from a light blue overhead to deeper orange and red hues near the horizon, a result of Rayleigh scattering preferentially removing shorter blue wavelengths from the sunlight passing through a longer atmospheric path. Under clear skies with scattered clouds, crepuscular rays—beams of sunlight piercing through gaps—may appear, adding dramatic contrast to the scene. At mid-latitudes, this phase typically lasts 20 to 40 minutes, shorter in summer and longer in winter due to the Sun's path.[10][11]
Nautical Twilight
Nautical twilight is the intermediate phase of evening or morning twilight that occurs when the geometric center of the Sun is between 6° and 12° below the horizon.[12][3] This period follows civil twilight and precedes astronomical twilight, providing a transitional illumination level suitable for specific observational tasks. During this time, the sky darkens further, but sufficient light remains for distinguishing key features against the horizon.[13]Visibility during nautical twilight allows for the horizon to remain discernible, though it becomes less distinct than in civil twilight, posing challenges for precise navigation. Brighter stars, known as nautical stars—primarily those of magnitude 1 to 3 used in celestial navigation—become visible, enabling sailors to take sextant sights by aligning these stars with the horizon. Fainter stars up to approximately magnitude 4 can also be observed toward the end of this phase under clear conditions, facilitating the identification of constellations for orientation.[2][14][15]This twilight phase holds significant practical applications in maritime and military contexts. It defines nautical dawn, the beginning of morning nautical twilight when the Sun reaches 12° below the horizon, and nautical dusk, the end of evening nautical twilight at the same depression. Critical for sailors, it provides the window to identify the horizon line before full darkness, allowing safe passage and accurate positioning via celestial methods. In military operations, terms like Begin Morning Nautical Twilight (BMNT)—the start of morning nautical twilight at 12° solar depression—and End Evening Nautical Twilight (EENT) mark periods for low-light activities such as reconnaissance.[1][16]Atmospheric scattering during nautical twilight results in a darker indigo sky, as shorter blue wavelengths dominate the remaining sunlight refracted through the upper atmosphere. This effect contributes to the "blue hour," a brief interval prized in photography for its deep blue hues and soft lighting, often overlapping with the early or late stages of nautical twilight.[17][18]The term "nautical twilight" originates from historical naval practices, where almanacs like The Nautical Almanac provided tables of twilight times to aid in timing safe voyages and celestial observations at sea.[19][20]
Astronomical Twilight
Astronomical twilight is the darkest phase of twilight, occurring when the geometric center of the Sun is between 12° and 18° below the horizon.[1] This period follows nautical twilight and marks the transition to full astronomical night, during which the sky illumination from scattered sunlight diminishes to levels comparable to or below starlight.[1] At this stage, the horizon becomes indiscernible to the naked eye, and the overall sky brightness approaches that of true night under non-light-polluted conditions.[2]During astronomical twilight, visibility conditions allow for the observation of fainter celestial objects, with stars up to the 6th magnitude becoming discernible as the phase progresses. This makes it an ideal time for astronomical observations of dim sources, such as distant galaxies or nebulae, particularly in locations free from artificial lightinterference, as the remaining solar glow no longer significantly hinders telescope performance.[10] The zodiacal light, a faint diffuse glow from interplanetary dust reflecting sunlight, may become faintly visible toward the end of this phase in clear, dark skies.[21]In practical terms, astronomical twilight defines the boundaries of astronomical dusk in the evening and dawn in the morning, serving as the operational threshold for professional observatories to commence or conclude nighttime observing sessions.[22] For instance, telescopes at sites like Palomar Observatory typically open their domes at the start of astronomical twilight to maximize dark-sky time.[22] This phase ends when the Sun reaches 18° below the horizon, at which point sunlight no longer substantially illuminates the upper atmosphere, and any residual sky glow arises primarily from natural phenomena such as airglow or aurorae.[1] Technically, with the Sun's center at -18°, the upper limb of the solar disk is fully obscured below the horizon, further minimizing direct interference with observations.[1]
Occurrence and Duration
Daily and Seasonal Timing
Evening twilight commences immediately at sunset and advances through the phases of civil dusk, nautical dusk, and astronomical dusk until the onset of full astronomical night, when the Sun is more than 18° below the horizon.[1] This progression reflects the gradual dimming of scattered sunlight in the atmosphere as the Sun descends further beneath the horizon.[1]Morning twilight, in contrast, concludes at sunrise and originates from the reverse sequence, beginning with astronomical dawn when the Sun is 18° below the horizon, followed by nautical dawn and civil dawn.[1] These phases mark the increasing illumination prior to direct sunlight, with civil, nautical, and astronomical twilight serving as distinct intervals based on the Sun's angular depression.[1]Seasonal variations in twilight timing arise from Earth's orbital position and axial tilt, resulting in shorter twilight periods near the equinoxes—when the Sun's path is more perpendicular to the horizon—and longer periods near the solstices, when the path is shallower.[23] At the equator, twilight occurs twice daily with minimal seasonal fluctuation, as day and night remain approximately equal throughout the year.[24]Latitude significantly influences twilight timing, with higher latitudes experiencing extended transitions due to the Sun's more oblique approach to the horizon; around solstices, evening twilight at these locations can merge seamlessly with morning twilight, producing continuous dim illumination.[6] The timing of these events is fundamentally derived from solar position calculations, which account for Earth's axial tilt of 23.44° and its effects on the Sun's declination throughout the year.[5]
Duration Factors
The duration of twilight phases is primarily influenced by an observer's latitude, the time of year, and the constant rate of Earth's rotation. At higher latitudes, twilight lasts longer because the Sun's path across the sky is more parallel to the horizon, requiring a greater angular displacement for the Sun to traverse the necessary depression angles defining each phase.[1] Near the equator, the Sun descends nearly vertically, resulting in shorter durations. Seasonally, at mid-latitudes, twilight extends longer during summer months due to the Sun's higher declination, which increases the obliquity of its path relative to the horizon, while winter durations are shorter as the path is steeper.[12] Earth's rotation at a steady rate of 15° per hour provides the baseline temporal scale for these changes.[25]Mathematically, the approximate duration of a twilight phase can be estimated as the angular distance spanned by the Sun's center (e.g., 6° for civil twilight from horizon to -6° depression) divided by Earth's rotation rate: duration ≈ (angular distance / 15° per hour). This yields about 24 minutes for civil twilight under equatorial conditions, but the effective angular path lengthens with latitude and seasonal declination, extending the time accordingly.[26]Representative examples illustrate these variations: at the equator, civil twilight typically lasts 20-30 minutes year-round, while at 50° latitude, it can extend to 1-2 hours during summer solstice periods.[27]Atmospheric conditions also play a secondary role in modifying durations. Refraction bends sunlight, effectively lifting the apparent position of the Sun and slightly prolonging twilight by 1-2 minutes on average, with greater variability at high latitudes where the Sun skims the horizon.[1] Aerosols in the atmosphere, such as those from natural dust or pollution, enhance scattering and can subtly extend perceived twilight by maintaining illumination longer; major volcanic eruptions, like the 1883 Krakatoa event, inject stratospheric sulfates that increase scattering, leading to brighter and potentially longer-lasting twilights observable globally for months.[28]Global patterns of average civil twilight durations vary systematically by latitude bands, as summarized below (values represent typical ranges across seasons, with summer maxima noted where applicable):
Latitude Band
Average Duration (minutes)
Notes
0°-20° (Equatorial)
20-30
Minimal seasonal variation
20°-40° (Tropical/Subtropical)
25-40
Slightly longer in summer
40°-60° (Mid/High)
30-90
Up to 1-2 hours in summer at higher end
60°+ (Polar)
60-180+
Merges with daylight in summer; shorter in winter
These patterns are derived from astronomical computations accounting for geometric and rotational factors.[29][12]
Polar Twilight Phenomena
In polar regions within the Arctic and Antarctic Circles, approximately 66.5° north and southlatitude, the Earth's axial tilt leads to extreme variations in solar illumination, where twilight phenomena deviate significantly from equatorial patterns. During the summer midnight sun, the Sun remains above the horizon for 24 hours or more, but transitional zones experience prolonged twilight that can extend from apparent sunset to sunrise, creating periods of continuous dim light rather than full darkness. Conversely, in winter polar night, the Sun stays below the horizon for extended durations, yet civil, nautical, or astronomical twilight may persist around midday, providing faint illumination from atmospheric scattering of sunlight. These effects intensify closer to the poles, where twilight can last the entire day or night cycle during solstices.[30][31][32]White nights exemplify continuous civil twilight at high latitudes south of the Arctic Circle, such as in Scandinavia, where the Sun dips just below the horizon during summer, preventing full darkness and maintaining a soft, pervasive glow for weeks. This phenomenon occurs between roughly late May and mid-July in locations like northern Norway or Sweden, with the Sun's maximum depression limited to less than 6° below the horizon at the summer solstice, ensuring the sky never achieves nautical or astronomical darkness. In these regions, the extended twilight supports outdoor activities into the "night" hours but disrupts traditional day-night cycles.[33][34]Twilight circles define latitudinal boundaries where persistent twilight dominates around solstices, as the Sun's path skims near the horizon without fully setting or rising. The civil twilight circle, around 60°34' N/S, marks where the Sun remains above -6° elevation all night during the summer solstice, producing unending civil twilight; further south, the nautical twilight circle at about 54° N/S allows continuous nautical twilight, and the astronomical twilight circle near 48°33' N/S ensures the sky never reaches full astronomical darkness. These circles shift slightly with the Earth's precession but create prolonged dim light in polar-adjacent zones, enhancing the ethereal quality of high-latitude summers.[33]The persistent glow of polar twilight poses challenges for astronomical observations, as scattered sunlight elevates sky brightness, reducing contrast for faint celestial objects and limiting deep-sky imaging or spectroscopy to brief windows during polar night. However, this same dim illumination enhances aurora viewing by providing a subtle backdrop that highlights the auroral curtains without overwhelming their colors, particularly during strong geomagnetic activity when auroras remain visible against the twilight.[31][35][36]Historical polar explorers frequently documented encounters with endless twilight, underscoring its psychological and navigational impacts. During Roald Amundsen's 1910–1912 Antarctic expedition, his team described the "grey twilight" persisting for weeks as they approached the South Pole, with the sudden return of direct sunlight dazzling after acclimation to the dim, unending light. Similarly, Ernest Shackleton's 1914–1917 Imperial Trans-Antarctic Expedition recorded nine hours of twilight daily during winter months aboard the Endurance, providing "good light at noon" amid the pack ice but evoking isolation in the perpetual half-light. Fridtjof Nansen's 1893–1896 Fram expedition accounts in Farthest North detail the "endless twilight" during Arctic drifts, where faint horizon glows blurred day-night boundaries, testing endurance in the transitional seasons.
Twilight on Other Celestial Bodies
Twilight on Planets
Twilight on other planets differs significantly from Earth's due to variations in atmospheric composition, density, and planetary rotation rates, which alter the scattering and duration of sunlight during the transition from day to night.[37]On Mars, twilight periods are notably longer than on Earth, often extending up to two hours after sunset or before sunrise, primarily because the planet's dust-laden atmosphere scatters sunlight over greater distances. This dust, suspended high in the atmosphere, creates a persistent glow similar to conditions on Earth following major volcanic eruptions that loft aerosols into the stratosphere.[37][38][39]Venus experiences prolonged and hazy twilight effects owing to its thick carbon dioxide atmosphere, which diffuses sunlight and shrouds the surface in a dim, reddish illumination resembling perpetual twilight. Surface observations from Soviet Venera landers in the 1970s and 1980s revealed this orange-red glow, resulting from the atmosphere's filtering of shorter wavelengths and the planet's extreme surface pressures and temperatures. The slow rotation of Venus further extends these phases, with a sidereal day lasting 243 Earth days, causing the Sun to traverse the sky gradually and prolonging the civil twilight period dramatically compared to Earth's 24-hour cycle.[40][41][42]For gas giants like Jupiter, twilight analogs occur in the upper cloud layers rather than on a solid surface, where sunlight scatters through ammonia and other aerosols. However, the planet's rapid rotation—completing a day in about 10 hours—results in brief transition phases, with twilight lasting only minutes as the terminator sweeps across the atmosphere at high speeds. Similar dynamics apply to other gas giants such as Saturn, Uranus, and Neptune, where fast rotations (ranging from 10 to 16 hours) minimize the duration of these illuminated boundary regions in the outer atmospheric layers.[43][44]Orbital and rotational influences play a key role across planets; slower rotations, as on Venus, dramatically lengthen twilight by reducing the angular speed of the Sun's apparent motion, while faster rotations on gas giants compress these periods. Mission data from NASA's Perseverance rover, equipped with the Mars Environmental Dynamics Analyzer (MEDA), has confirmed extended civil twilight visibility on Mars through observations of atmospheric dust and mesospheric clouds during dawn and dusk, spanning multiple sols and providing insights into light scattering behaviors.[42][45][46]
Twilight on Moons and Exoplanets
On Earth's Moon, the absence of an atmosphere results in an abrupt transition from day to night at sunrise and sunset, with no gradual twilight phase as sunlight passes directly from full illumination to complete darkness without scattering.[47] This stark shift occurs because the Moon lacks the air molecules that bend and diffuse light on Earth, leading to immediate darkness once the Sun dips below the horizon.[48] However, during the lunar night, faint illumination from Earthshine—sunlight reflected off Earth's oceans, clouds, and land surfaces onto the Moon's unlit side—can mimic a subtle twilight glow, particularly visible during crescentphases.[49] This reflected light, known as planetshine, provides the primary source of diffuse illumination on the Moon's dark hemisphere, varying in intensity based on Earth's phase as seen from the Moon.[50]Among Jupiter's moons, Io experiences an extended effective twilight due to the persistent glow from its hundreds of active volcanoes, which emit thermal radiation and sulfurous plumes that illuminate the surface beyond direct sunlight.[51] These volcanic eruptions, driven by tidal heating from Jupiter's gravity, produce lava flows and gas emissions that create a hazy, glowing atmosphere of sulfur dioxide, faintly scatteringlight and prolonging dim conditions at the terminator.[52] In contrast, Europa's highly reflective icy surface, with an albedo of approximately 0.64, causes sharp contrasts between lit and shadowed regions, effectively shortening any transitional twilight phase as sunlight bounces directly off the frozen terrain without significant atmospheric diffusion.[53] Europa's tenuous oxygen exosphere offers minimal scattering, resulting in abrupt light changes similar to the Moon's, though radiation-induced luminescence from Jupiter's magnetosphere can add a faint greenish glow to the nightside.[54]For exoplanets, hot Jupiters—gas giants orbiting very close to their stars—are often tidally locked, with one hemisphere in perpetual daylight and the other in endless night, creating a narrow, intense twilight zone where temperatures drop rapidly over short distances.[55] This locking synchronizes rotation with orbital period, leading to extreme atmospheric circulation that compresses the twilight region into a band of high winds and heat transfer, with durations on the order of minutes rather than hours.[56] In habitable zone exoplanets, models suggest twilight durations akin to Earth's, influenced by axial tilt, which drives seasonal variations in illumination and scattering; for instance, a 23-degree tilt like Earth's can extend twilight by up to 20-30 minutes through obliquity-induced day length changes.[57]Theoretical simulations of twilight zones on systems like TRAPPIST-1, a compact multi-planet setup around an ultracool dwarf, predict extended dim periods on inner worlds, where synchronous rotation and thin atmospheres could create perpetual twilight bands lasting hours due to the star's low luminosity.[58] As of September 2025, James Webb Space Telescope (JWST) observations indicate that TRAPPIST-1e is unlikely to have a thick Venus- or Mars-like atmosphere but may possess a thinner one or water vapor, potentially affecting scattering in its terminator zone.[59] These models incorporate orbital resonances and albedo effects to forecast terminator regions on planets like TRAPPIST-1e, potentially habitable with surface temperatures allowing liquid water in transitional zones. Observational challenges in studying exoplanet twilights stem from the rarity of direct imaging, relying instead on phase curve analyses of light variations, where stellar spots and instrumental noise obscure subtle dimming signals indicative of atmospheric scattering.[60] Inferred twilight effects appear as gradual flux changes in transit light curves, but contamination from the host star's activity complicates detection, limiting precision to systems with high signal-to-noise ratios like hot Jupiters.[61]
Cultural and Symbolic Significance
In Religion
In Christianity, twilight holds significance in liturgical practices, particularly through the Easter Vigil, a night-time ceremony originating in early Church traditions that begins at twilight to symbolize Christ's resurrection and the triumph of light over darkness. This vigil, historically devoted to baptisms and communal prayer, marks the transition from death to new life, with the lighting of the Paschal candle representing the risen Christ dispelling the shadows of sin.[62] In monastic traditions, twilight aligns with Vespers, the evening prayer that serves as a liminal moment for reflection on the day's end and preparation for rest, fostering spiritual introspection during this threshold between day and night.[63]Hinduism regards twilight as a sacred interval for rituals, most notably in the practice of sandhya vandana, a daily devotional act performed at dawn and dusk to honor the divine through mantras and offerings, emphasizing the balance of vital energies during these transitional periods.[64] In Hindu mythology, the deity Narasimha, an avatar of Vishnu, emerges at twilight from a palace pillar to slay the demon king Hiranyakashipu, circumventing the demon's boon of invulnerability by acting in this neither-day-nor-night liminal space, thus protecting devotee Prahlada and restoring cosmic order.[65]In Islam, twilight defines key prayer times, with Fajr commencing at true dawn's twilight when the sun is 18 degrees below the horizon, and Maghrib beginning immediately after sunset, both calculated using astronomical twilight angles to ensure precise observance of the five daily prayers.[66] During Ramadan, fasting concludes at Maghrib with iftar, the breaking of the fast, marking the end of the day's spiritual discipline as the sun dips below the horizon.[67]Judaism associates twilight, termed bein ha-arbayim or "between the evenings," with pivotal rituals, such as the Passover lamb sacrifice on the 14th of Nisan, performed in the early evening shortly after sunset until total darkness sets in, symbolizing redemption and transition.[68] This timing also governs the onset of the Sabbath and festivals, where candles are lit 18 minutes before sunset to welcome the holy day before full twilight arrives, ensuring the sacred period begins without delay.[69]Across these faiths, twilight embodies a sacred threshold between day and night, a liminal space invoking divine intervention, renewal, and precise ritual timing often informed by geometric solar calculations.[63]
In Romantic literature, twilight frequently serves as a symbol of melancholic transition, evoking the impermanence of existence amid shifting light and mood. William Wordsworth captures this in poems like "Mutability," underscoring themes of change and introspection.[70] Similarly, in "The Prelude," twilight "blazed" marks perceptual shifts during evening scenes, blending awe with a sense of fleeting beauty.[71]Gothic novels further associate twilight with eternal dusk in vampire lore, portraying it as the liminal realm where the undead awaken. Bram Stoker's Dracula (1897) depicts the Count active at night, his castle shrouded in dim light that blurs mortality and immortality, heightening the horror of unending night.Impressionist art harnesses twilight's ephemeral colors to explore light's transformative effects, prioritizing atmosphere over detail. Claude Monet's San Giorgio Maggiore at Dusk (1908) renders Venice's church in deepening violets and blues, capturing the hazy interplay of fading sunlight and emerging shadows to convey serenity and transience. In Renaissance works, twilight symbolizes liminality as a threshold between earthly and divine realms, often through celestial motifs that evoke ambiguity and spiritual passage, as seen in paintings integrating dusk skies to represent human vulnerability.Modern media amplifies twilight's motifs for emotional and atmospheric depth. The Twilight saga films (2008–2012) set their narrative in the perpetually overcast Forks, Washington, where endless dusk enables vampires' daylight avoidance while symbolizing romantic limbo and eternal youth.[72] In sci-fi horror, John Carpenter's The Thing (1982) leverages Antarctica's polar twilight—prolonged low light during endless nights—to intensify isolation and paranoia, framing the alien threat against a backdrop of unrelenting dimness.[73] Photography techniques during twilight's "golden hour" (warm post-sunrise or pre-sunset glow) and "blue hour" (cool nautical twilight) exploit diffused light for vibrant, romantic compositions, softening contrasts to evoke intimacy or mystery.[74]Twilight embodies ambiguity as a boundarystate in creative works, fostering romance through its intimate, uncertain glow while hinting at apocalyptic closure in narratives of decline.[75] In Japanese culture, yozakura—cherry blossom viewing at twilight during hanami festivals—celebrates sakura's brief bloom under evening lights, symbolizing life's transience and harmonious reflection.[76]