There are multiple ways to define twilight, the gradual transition to and from darkness when the Sun is below the horizon.[2] "Civil" twilight occurs when the Sun is between 0 and 6 degrees below the horizon. Nearby planets likeVenus and bright stars likeSirius are visible during this period.[3] "Nautical" twilight continues until the Sun is 12 degrees below the horizon.[4] During nautical twilight, the horizon is visible enough for navigation.[5] "Astronomical" twilight continues until the Sun has sunk 18 degrees below the horizon.[3][6] Beyond 18 degrees, refracted sunlight is no longer visible.[6] True night is defined as the period when the sun is 18 or more degrees below either horizon.[4]
Since the atmosphere refracts sunlight, polar day is longer than polar night, and the area that experiences polar night is slightly smaller than the area that experiences polar day. The polar circles are located atlatitudes between these two areas, at approximately 66.5°. While it is day in theArctic Circle, it is night in theAntarctic Circle, and vice versa.
Any planet or moon with a sufficientaxial tilt thatrotates with respect to its star significantly more frequently than itorbits the star (and with notidal locking between the two) will experience the same phenomenon (a nighttime lasting more than one rotation period).
Civil polar twilight inRuka, Finland at noon in December
The length of polar night varies by latitude from 24 hours just inside the polar circles to 179 days at the poles. As there are various kinds oftwilight, there also exist various kinds of polar twilight that progress towards true polar night. Each kind of polar night is defined as when it is darker than the corresponding kind of twilight. The descriptions below are based on relatively clear skies, so the sky will be darker in the presence of dense clouds.
As mentioned, a location experiencing polar night does not mean that the location will be in full darkness; in most cases, due to sunlight beingrefracted over the horizon, a location experiencing polar night will actually be in one of the various phases of polar twilight. As in locations experiencing daylight, the middle of the day will typically be the brightest time in locations experiencing polar twilight.[7]
For example, a typical day during civil polar twilight inVadsø, Norway will begin with night, astronomical twilight, nautical twilight, and civil twilight in that order (with each successive phase including more light than the last). Following civil twilight, the day will progress through the other phases in the opposite order (nautical twilight, then astronomical twilight, then night to end the day).[8]
Civil polar twilight occurs at latitudes above 72°34' North or South, where the Sun will be below the horizon all day on thewinter solstice, but by less than 6° atsolar noon.[9] There is then no truedaylight at thesolar culmination, onlycivil twilight. During civil polar twilight, there is still enough light for most normal outdoor activities at midday because of light scattering by the upper atmosphere and refraction. However, during dense cloud cover, places like the coast ofFinnmark (about 70°) inNorway will experience a "day" that is darker than usual.Street lamps may therefore remain on even at midday, and a person looking at a window from within a brightly lit room might still be able to see their reflection, as the level of outdoorilluminance will be below that of many illuminated indoor spaces.
Northern Hemisphere:
68° North: about December 9 to January 2
69° North: about December 1 to January 10
70° North: about November 26 to January 16
71° North: about November 21 to January 21
72° North: about November 16 to January 25
Southern Hemisphere:
68° South: about June 7 to July 3
69° South: about May 30 to July 11
70° South: about May 24 to July 18
71° South: about May 19 to July 23
72° South: about May 14 to July 27
Sufferers ofseasonal affective disorder tend to seek out therapy with artificial light, as the psychological benefits of daylight require relatively high levels of ambient light (up to 10,000 lux) which are not present in any stage of twilight; thus, the midday twilights experienced anywhere inside the polar circles are still "polar nights" for this purpose.
Nautical polar twilight occurs at latitudes between about 72° 34' and 78° 34' North or South, which is exactly 6° to 12° inside the polar circles. There is then nocivil twilight at thesolar culmination, onlynautical twilight. During nautical polar twilight, the human eye may distinguish general outlines of ground objects at midday but cannot participate in detailed outdoor operations.[10]
Civil twilight happens when the Sun is between 0 and 6° below the horizon, so this phenomenon can also be referred to as civil polar night. Nowhere on mainland Europe is this definition met. The Norwegian town ofLongyearbyen,Svalbard, experiences nautical polar twilight from about 11 November until 30 January.Dikson, in Russia, experiences nautical polar twilight from about 6 December to 6 January. On the Canadian territory ofPond Inlet, Nunavut, nautical polar twilight lasts from about 16 December until 26 December.
Astronomical polar twilight occurs at latitudes between about 78° 34' and 84° 34' North or South, which is exactly 12° to 18° inside the polar circles. There is then nonautical twilight at thesolar culmination, onlyastronomical twilight. During astronomical polar twilight, the sky is dark enough at midday to permit astronomical observation of point sources of light such asstars, except in regions with more intenseskyglow due tolight pollution,moonlight,auroras, and other sources of light. There is a location at thehorizon with more light than others around midday due to refraction. Some critical observations, such as of faint diffuse items such asnebulae andgalaxies, may require observations beyond the limit of astronomical twilight.
A true polar night is a period of continuousnight where noastronomical twilight occurs at thesolar culmination. During a true polar night, stars of the sixth magnitude, which are the dimmest stars visible to the naked eye, will be visible throughout the entire 24-hourday. Atsolar noon, the sun will be between exactly 18° and approximately 23° 26' below the horizon. These conditions last for about 11 weeks at the poles.
Astronomical twilight happens when the Sun is between 12 and 18° degrees below the horizon, so this phenomenon can also be referred to as astronomical polar night. True polar night is limited to latitudes above roughly 84° 34' North or South, which is exactly 18° within the polar circles, or approximately five and a half degrees from the poles. The only permanent settlement on Earth at these latitudes is theAmundsen–Scott scientific research station inAntarctica, whose winter personnel are completely isolated from mid-February to late October. TheSouth Pole experiences this from about May 11 to August 1,[14] while theNorth Pole experiences this from about November 12 to January 28.[15][16]
AtEarth's poles the Sun appears at the horizon only and all day aroundequinox, marking the change between the half year long polar night andpolar day. The picture shows theSouth Pole right before March equinox, with the Sun appearing throughrefraction despite being still below the horizon.
If an observer located on either the North Pole or the South Pole were to define a "day" as the time from the maximal elevation of the Sun above the horizon during one period of daylight, until the maximal elevation of the Sun above the horizon of the next period of daylight, then a "polar day" as experienced by such an observer would be one Earth-year long.[17]
Numerous analyses have been conducted to examine the effects of polar night on humans. InTromsø, Norway, a city located at 69 degrees north, there is a 2 month long polar night, lasting from mid-November to mid-January. An analysis was conducted based on 2015-16 data from a health survey that involved residents of the region over age 40, with the goal being to analyze the seasonal variation of sleeping patterns in Tromsø. The study found that there was a higher prevalence ofinsomnia among men in the fall and winter months, but not among women. However, overall, sleep duration varied little to none throughout the year despite the extreme changes in daylight; it is worthwhile to note that a factor in this result may be the significant amount ofartificial light in Tromsø.[18]
A similar study was conducted among men who overwintered atBelgrano II, an Argentine research station inAntarctica. The station is located at 77 degrees south, resulting in a polar night 4 months in length. The study was conducted across 5 different winter campaigns in the 2010s, bringing in a total of 82 participants. The study found that participants generally slept for longer periods of time in the summer months than the winter months. Additionally, greater amounts ofsocial jetlag were observed in the winter months.[19]
A third study aimed to examine the mental health of 88 Korean crew members at two different research stations in Antarctica,King Sejong Station andJang Bogo Station. No crew members had been diagnosed with amental illness prior to the study. While in Antarctica, 7 of the 88 crew members were diagnosed with a mental illness during early winter. The mental illnesses includedinsomnia disorder (3 diagnosed),depressive disorder (1 diagnosed),adjustment disorder (2 diagnosed), andalcohol use disorder (1 diagnosed).[20]
Overall, both Antarctic studies showed a lower amount of sleep beginning at the start of winter, while the study from the Korean bases also showed an onset of mental health problems at that time.[19][20] While the study from Tromsø did not show a similar drop in sleep duration as the Antarctic studies (perhaps due to the high amounts of artificial light), it did show an increased amount of insomnia in men during winter;[18] therefore, the polar night was shown to have sleep and/or mental health effects in all three studies.
.jpg)
.jpg)
