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Polar night

Polar night is a natural astronomical phenomenon in which the Sun remains below the horizon continuously for more than 24 hours, leading to extended periods of darkness in Earth's polar regions.^[1] This occurs due to the planet's axial tilt of approximately 23.5 degrees relative to its orbital plane around the Sun, which causes the polar areas to be oriented away from solar illumination during the winter half of the year.^[2] The phenomenon affects all locations north of the Arctic Circle (about 66°34' N latitude) and south of the Antarctic Circle (66°34' S latitude), where the tilt ensures the Sun does not rise at least once annually.^[3] The duration of polar night varies by latitude and hemisphere, typically beginning around the autumnal equinox and ending near the vernal equinox, though atmospheric refraction slightly shortens the exact period at the poles. Near the polar circles, it lasts roughly one month (e.g., about 30 days at 68° N), extending to 3.5 months at 78° N, 5.5 months at 88° N, and a maximum of six months at the North and South Poles themselves.^[1] At the poles, continuous darkness prevails from early October to early March in the Northern Hemisphere, with no direct sunlight or even twilight during the deepest phase around the winter solstice.^[4] This extended absence of sunlight profoundly influences ecosystems, human societies, and atmospheric conditions in these regions, driving adaptations in wildlife such as altered circadian rhythms and enhanced bioluminescence, while challenging inhabitants with seasonal affective disorders and reliance on artificial lighting.^[1] Polar night contrasts sharply with the midnight sun of polar summer, highlighting the extreme seasonal variations unique to high latitudes.^[2]

Definition and Astronomy

Phenomenon Overview

The polar night is a natural astronomical phenomenon occurring in Earth's high-latitude regions, defined as the period when the Sun remains below the horizon for more than 24 hours.^[5] This event takes place north of the Arctic Circle or south of the Antarctic Circle, where the geometric conditions prevent the Sun from rising above the horizon for at least one continuous day, potentially extending to prolonged periods of uninterrupted darkness lasting several months at the poles.^[6] This annual occurrence stems from Earth's axial tilt of approximately 23.5 degrees relative to its orbital plane around the Sun, which causes extreme seasonal variations in sunlight exposure at the poles.^[6] In contrast to the polar day—commonly known as the midnight sun, during which the Sun circles the sky without setting and provides continuous daylight—polar night envelops these regions in extended darkness.^[6] Visually, the experience ranges from a perpetual dim twilight near the polar circles, where faint indirect sunlight scatters through the atmosphere, to near-total blackness at higher latitudes, broken only by stars, auroras, or artificial lights.^[4] Its etymology draws from "polar," referring to the Earth's poles (from the Greek polos meaning axis or hinge), combined with "night," rooted in Indo-European languages denoting darkness, akin to the Greek nyx for night.^[7]

Solar Geometry and Causes

The Earth's rotational axis is tilted at an angle of approximately 23.44° relative to the plane of its orbit around the Sun, a configuration that results in seasonal variations in the amount of sunlight received at different latitudes.^[8] This obliquity ensures that, as the planet orbits the Sun over the course of a year, one hemisphere is progressively tilted away from the Sun during winter, reducing solar illumination in polar regions to zero for extended periods.^[9] The solar declination (δ), defined as the angular position of the Sun north or south of the celestial equator, quantifies this tilt's effect and ranges from +23.44° at the June solstice to -23.44° at the December solstice.^[10] Polar night arises when an observer's latitude (φ) satisfies φ > 90° + δ during the period of negative declination, as the Sun's position remains entirely below the horizon even at its highest point in the sky.^[11] This condition is most pronounced around the winter solstices—December for the Arctic and June for the Antarctic—when the absolute value of δ peaks, maximizing the tilt away from the Sun.^[9] The altitude (h) of the Sun above the horizon at any given location and time is determined by the spherical astronomy formula:

\sin h = \sin \phi \sin \delta + \cos \phi \cos \delta \cos H

where φ is the latitude, δ is the solar declination, and H is the hour angle measuring the Sun's angular distance from the local meridian (H = 0° at solar noon, increasing by 15° per hour due to Earth's rotation).^[11] At solar noon (H = 0°), the maximum altitude simplifies to \sin h = \sin \phi \sin \delta + \cos \phi \cos \delta = \cos(\phi - \delta). Polar night occurs throughout a full day when this maximum h is negative (h < 0°), meaning \cos(\phi - \delta) < 0, or equivalently, the Sun never rises above the horizon.^[11] The duration of polar night depends on how long the declination remains sufficiently negative to satisfy the altitude condition, which lengthens with increasing latitude as the observer moves farther from the polar circles (defined at approximately 66.56° N and S, where the phenomenon lasts exactly one day at the winter solstice). This duration is roughly proportional to the latitudinal distance from the polar circle, reflecting the gradual variation in δ over the orbital cycle; for instance, at 70° N, polar night persists for about 50 days centered on the December solstice.

Types and Variations

Twilight Phases

During the polar night, twilight phases represent intermediate periods of indirect sunlight illumination when the Sun remains below the horizon but scatters light through the atmosphere, providing varying degrees of visibility based on the Sun's depression angle. Civil twilight occurs when the geometric center of the Sun is between 0° and 6° below the horizon, offering sufficient illumination for most outdoor activities without artificial light, as the brighter stars and planets are not yet visible. Nautical twilight follows, with the Sun between 6° and 12° below the horizon, during which the horizon remains discernible even on moonless nights, allowing mariners to perform accurate celestial navigation using star sightings. Astronomical twilight is the darkest phase, spanning the Sun's position from 12° to 18° below the horizon, where the sky becomes dark enough for observing faint celestial objects, as solar illumination falls below the intensity of starlight and other natural sources.^[12] In polar regions, the progression of these twilight phases during polar night unfolds gradually due to the Sun's shallow path circling just below the horizon, rather than the rapid transitions seen at lower latitudes. At the onset of polar night, civil twilight may persist continuously for days or weeks as the Sun's maximum elevation hovers near -6°, creating a prolonged dim period before transitioning to nautical twilight as the solar depression deepens to 12° over subsequent weeks. This shift continues to astronomical twilight when the Sun reaches 18° below the horizon, marking the boundary where twilight ceases and complete darkness begins, with no significant atmospheric scattering from the Sun. At the North Pole, for example, civil twilight persists until about October 8, nautical twilight until October 25, and astronomical twilight until November 13, with symmetric periods occurring in the spring. The solar geometry underlying these angles arises from Earth's axial tilt and orbital position, determining the Sun's lowest daily altitude during winter months.^[13] Visually, twilight in polar night is dominated by a dim blue light, resulting from the preferential scattering of shorter blue wavelengths in the upper atmosphere when the Sun is low, creating an ethereal, monochromatic glow that tints the landscape and sky. This blue-dominated spectrum, peaking around 450 nm, persists throughout the twilight phases and influences natural adaptations in local wildlife, such as enhanced sensitivity in reindeer retinas to low blue light levels. Practically, these phases impact aviation and shipping: the extended civil and nautical twilights enable visual flight rules for aircraft in polar areas by providing enough horizon definition for safe operations, while nautical twilight supports ship navigation through visible stars and outlines, reducing reliance on instruments during the otherwise dark season.^[14]

Complete Polar Night

The complete polar night refers to the most intense phase of the polar night phenomenon, characterized by the Sun remaining continuously more than 18° below the horizon, producing true astronomical darkness without any perceptible twilight for periods exceeding 24 hours. This condition marks a transition from the preceding twilight phases, where faint indirect sunlight still scatters in the atmosphere, to total night where the sky exhibits no solar glow.^[15] At the geographic poles, this phase reaches its maximum duration, lasting approximately 77 days, from mid-November to late January at the North Pole. In contrast, locations nearer the polar circles experience shorter complete polar nights, often dominated by extended twilight rather than full darkness, highlighting the phenomenon's latitudinal dependence.^[13] During complete polar night, natural atmospheric factors become the sole providers of illumination, with auroras and moonlight serving as primary light sources alongside enhanced visibility of stars and the Milky Way. The absence of sunlight allows even the faintest celestial objects to appear vividly, as atmospheric scattering from the Sun is entirely eliminated.^[16]^[17] Measurement of complete polar night relies on monitoring the Sun's depression angle, which must exceed the 18° threshold defining astronomical twilight; this ensures no civil (0°-6°), nautical (6°-12°), or astronomical twilight is possible, confirming perpetual night conditions.^[15]

Geographic and Temporal Extent

Arctic Regions

The polar night phenomenon in the Arctic occurs in regions north of the Arctic Circle, a parallel of latitude situated at approximately 66°33′N.^[3] This boundary marks the southernmost point where the sun can remain continuously below the horizon for at least one full day around the winter solstice on December 21.^[18] Northward from this line, the duration of continuous darkness increases progressively with latitude due to Earth's axial tilt, extending the period when no direct sunlight reaches the surface.^[19] Specific locations within the Arctic experience varying lengths of polar night tied to their positions. In Tromsø, Norway, at roughly 69.6°N, the polar night lasts about two months, typically from November 27 to January 15, though surrounding mountains can extend the effective darkness slightly earlier.^[20] Utqiaġvik, Alaska, the northernmost community in the United States at 71.3°N, endures 65 days of polar night, beginning around November 18 and ending in late January.^[21] Farther north, in Svalbard, Norway—particularly Longyearbyen at 78°N—the period stretches to approximately four months, from October 26 to February 16.^[22] At the North Pole itself, the polar night persists for six months, spanning from the autumnal equinox in late September to the vernal equinox in March.^[18] The temporal patterns of polar night in the Arctic show that it commences earlier at higher latitudes and later at those closer to the Arctic Circle. For instance, at 70°N, the period generally begins in late November, around November 23, and concludes in mid-January, with the transition to daylight marked by a gradual sunrise as the sun's disk begins to appear above the horizon.^[23] This progression reflects the geometric shift in solar elevation, where lower-latitude Arctic sites experience shorter durations overall. Modern satellite observations, such as those from NASA's VIIRS instrument, confirm these durations by monitoring light levels and environmental conditions during the polar darkness, providing data that also inform impacts on indigenous communities like the Inuit, who have long observed these cycles in their traditional knowledge systems.^[24]

Antarctic Regions

The polar night in Antarctic regions occurs south of the Antarctic Circle, located at approximately 66°33′ S, where the Sun does not rise above the horizon for more than 24 hours during the June solstice around June 21.^[25] This phenomenon symmetrically mirrors the Arctic Circle at the same latitude in the Northern Hemisphere, resulting in comparable durations of continuous darkness in both polar zones. Due to Antarctica's geography as a remote, ice-covered continent with no indigenous populations or permanent settlements, polar night primarily impacts temporary research outposts rather than communities. Key sites include McMurdo Station, situated at 77°51′ S on Ross Island, which experiences polar night for about four months, typically from late April to mid-August.^[26] At the Amundsen-Scott South Pole Station, located at 90° S on the polar plateau, the period extends to six months of uninterrupted darkness, beginning with the final sunset in late March and ending around the September equinox.^[27] These durations increase progressively southward from the Circle, with the South Pole marking the maximum extent.^[28] The temporal progression of polar night coincides with the austral winter, reaching its midpoint around June 21, when stations face peak isolation exacerbated by the formation of extensive sea ice that encases coastal areas and restricts resupply until spring.^[29] This sea ice buildup, typical from April onward, heightens logistical challenges for the approximately 200 personnel overwintering at McMurdo Station.^[30] The period concludes with the gradual return of sunlight near the September equinox, initiating the transition to the midnight sun phase.^[27] Antarctic polar night has historically been endured at research and exploration bases, such as during Robert Falcon Scott's Terra Nova Expedition (1910–1913), when the team wintered at Cape Evans—near modern McMurdo Station—overcoming the extended darkness to conduct early scientific observations.^[31] Today, these stations serve exclusively for international research under the Antarctic Treaty, underscoring the region's focus on science amid profound inaccessibility compared to the more populated Arctic.

Environmental and Biological Impacts

Climate and Ecosystems

During the polar night, the absence of sunlight drastically reduces solar heating in polar regions, leading to significant temperature drops that average around -40°C in the Arctic and -60°C in the Antarctic.^[32]^[27] This prolonged darkness causes the surface to radiate heat to space unchecked, chilling the air above ice sheets and creating dense, cold air masses.^[33] In turn, this density gradient drives katabatic winds—strong downslope flows that can exceed 100 km/h—particularly intensifying during the winter months when solar input is zero.^[34]^[35] The extreme cold of the polar night promotes extensive sea ice expansion, as surface waters cool below freezing and solidify, often reaching maximum extent by late winter.^[36] This ice growth releases brine, increasing ocean salinity and density, which facilitates the sinking of cold water and drives key components of the thermohaline circulation, the global conveyor belt of ocean currents.^[37]^[36] In the Arctic, for instance, brine rejection from shelf sea ice formation enhances deep water ventilation, strengthening the overturning circulation that distributes heat and nutrients worldwide.^[37] Polar night disrupts polar ecosystems by halting surface phytoplankton blooms, as the lack of light suppresses photosynthesis for months, shifting primary production to minimal under-ice levels sustained by faint subsurface irradiance.^[38]^[39] This seasonal cessation alters the carbon cycle, reducing CO2 uptake from the atmosphere and ocean surfaces during winter, which limits organic matter export to deeper waters and influences annual carbon storage dynamics in polar seas.^[40] On a broader scale, the polar night's cooling effects contribute to polar amplification, where regional warming exceeds global averages by a factor of two to four, primarily through ice-albedo feedbacks that persist beyond the dark period.^[41]^[42] Climate change exacerbates this by reducing sea ice duration in the 2020s, allowing more light penetration even during twilight phases and intensifying the contrast between dark-season cooling and summer heat absorption.^[43]

Wildlife Adaptations

During the polar night, many Arctic and Antarctic animals exhibit behavioral adaptations to cope with prolonged darkness and extreme cold. Polar bears (Ursus maritimus) enter a state of denning, where pregnant females excavate snow dens in late autumn and remain largely inactive for up to four months, conserving energy while giving birth and nursing cubs in the absence of light.^[44] This denning behavior aligns with the polar night period, allowing mothers to protect vulnerable offspring from harsh conditions without foraging. Similarly, migratory birds like the Arctic tern (Sterna paradisaea) undertake one of the longest annual migrations, traveling from Arctic breeding grounds to Antarctic waters near the equator during the northern winter, thereby escaping the darkness and following perpetual daylight for feeding.^[45] In marine environments, bioluminescence serves as a key adaptation for nocturnal navigation, predation, and communication; for instance, during high Arctic polar night, a vertical gradient in bioluminescent activity among plankton and fish enhances trophic interactions in the otherwise lightless mesopelagic zone.^[46] Physiological adaptations further enable survival in sub-zero temperatures and vitamin-deficient darkness. Antarctic notothenioid fishes produce antifreeze proteins (AFPs) that bind to ice crystals in their blood, preventing lethal freezing by inhibiting crystal growth despite seawater temperatures dropping to -1.9°C.^[47] Seals, such as ringed seals (Pusa hispida), accumulate high levels of vitamin D in their blubber during summer months of continuous sunlight, providing a stored reserve to maintain calcium metabolism and bone health through the dark winter when UVB exposure is absent.^[48] Arctic tundra plants, including mosses, employ dormancy to endure the polar night, slowing metabolic processes and desiccating to withstand freezing under snow cover for months.^[49] Many vascular plants in the tundra rely on mycorrhizal symbioses with fungi, which facilitate nutrient uptake from frozen soils even during winter, as fungal networks remain active beneath the snow, transferring nitrogen and phosphorus to host plants in exchange for carbohydrates.^[50] Recent 21st-century research highlights the intricacies of these adaptations amid environmental shifts, underscoring the role of internal circadian cues in energy conservation for species like polar bears despite constant darkness.^[51]

Human Impacts and Adaptations

Health and Physiology

The absence of sunlight during polar night disrupts human circadian rhythms by removing key zeitgebers, leading to desynchronization of melatonin secretion and cortisol production. In studies of individuals exposed to extreme polar photoperiods, melatonin levels remain elevated longer into the day.^[52] Research in Antarctic stations has shown that polar night conditions are associated with reduced sleep efficiency, increased awakenings, and decreased sleep duration due to delayed chronotypes and social jetlag.^[53] This circadian misalignment exacerbates seasonal affective disorder (SAD), a subtype of major depressive disorder triggered by reduced daylight. In Arctic communities, SAD prevalence reaches up to 6.3%, with subsyndromal SAD affecting an additional 11.7% of residents, manifesting in symptoms such as persistent low mood, fatigue, hypersomnia, and carbohydrate cravings.^[54] Common treatments include bright light therapy, which mimics natural sunlight to suppress melatonin and boost serotonin, often administered for 30-60 minutes daily upon waking, yielding symptom remission in 50-80% of cases within weeks.^[55] Pharmacotherapy with selective serotonin reuptake inhibitors may complement light therapy for severe cases.^[55] Prolonged darkness also impairs cutaneous vitamin D synthesis, elevating risks of deficiency and associated conditions like rickets in children, characterized by skeletal deformities from impaired calcium absorption. Supplementation is recommended for at-risk groups in low-sunlight environments to maintain adequate serum 25(OH)D levels and prevent rickets.^[56] In polar expeditions, 2000-4000 IU daily doses have proven effective in restoring levels during winter isolation.^[56] Twilight phases during partial polar night may partially mitigate these effects by providing dim blue-enriched light that supports partial entrainment.^[57] In Antarctic research stations, such as those during winter-over periods, personnel experience similar circadian disruptions and increased risks of mood disorders, with studies reporting heightened anxiety and depression mid-winter due to isolation and darkness. Adaptations include scheduled light exposure and group activities to maintain psychological well-being.^[58]

Cultural and Societal Responses

Indigenous communities in the Arctic have long incorporated cultural practices to cope with the extended darkness of polar night, fostering social cohesion and spiritual resilience. Among the Sámi people of northern Norway, yoik singing—a traditional vocal art form without words—serves as a means of storytelling and emotional expression, often performed during winter gatherings to recount histories, honor ancestors, and maintain cultural identity amid isolation.^[59] Similarly, Inuit communities rely on the qulliq, a soapstone oil lamp fueled by seal blubber, which provides essential light, heat, and a communal focal point in igloos during the polar night's long darkness, symbolizing survival and women's central role in family life.^[60] These practices not only address practical needs but also reinforce oral traditions and community bonds in environments where daylight is absent for months.^[61] In modern Arctic settlements, technological and communal adaptations have transformed polar night from a period of hardship into one of celebration and functionality. In Longyearbyen, Svalbard, extensive artificial lighting systems illuminate streets and public spaces, designed to minimize light pollution while ensuring safety and visibility during the four-month polar night, allowing daily life to continue uninterrupted.^[62] Communities like Tromsø in Norway host festivals such as the Northern Lights Festival and Tromsø International Film Festival, which since the 2010s have incorporated light installations, aurora-themed events, and screenings to counter the darkness with cultural vibrancy and attract visitors.^[63]^[64] These events emphasize music, film, and artificial illuminations, turning the season into a highlight of regional identity. Economically, polar night influences Arctic industries by shifting activities to seasonal patterns that leverage or mitigate its effects. Tourism surges in the lead-up to and aftermath of polar night, particularly from October to March, as visitors flock to experience the northern lights and winter festivals, boosting local economies in places like Svalbard and Tromsø with revenues from guided tours and accommodations.^[65] This cyclical downtime underscores the interplay between natural conditions and economic sustainability in polar communities. Historical expeditions and recent global events highlight the psychological and logistical strains of polar night on human societies. During the 19th-century Franklin expedition, crews wintering in the Canadian Arctic faced severe morale declines from the interminable darkness, compounded by isolation and failing provisions, contributing to the expedition's tragic end as men struggled with despair and deteriorating group dynamics.^[66] In the 21st century, the COVID-19 pandemic amplified remote work challenges in isolated Arctic outposts like Svalbard, where polar night's darkness already disrupts routines; quarantines and travel bans forced researchers into prolonged virtual collaboration, testing mental resilience amid limited daylight and connectivity issues.^[67] These episodes illustrate how polar night can exacerbate societal vulnerabilities, prompting innovations in communication and support systems.

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