The polar night is a phenomenon where the

night Night (also described as night time, unconventionally spelled as "nite") is the period of ambient darkness from sunset to sunrise during each 24-hour day, when the Sun is below the horizon. The exact time when night begins and ends depends ...

time lasts for more than 24 hours that occurs in the northernmost and southernmost regions of Earth. This occurs only inside the polar circles. The opposite phenomenon, the polar day, or

midnight sun The midnight sun is a natural phenomenon that occurs in the summer months in places north of the Arctic Circle or south of the Antarctic Circle, when the Sun remains visible at the local midnight. When the midnight sun is seen in the Arctic, ...

, occurs when the Sun remains above the horizon for more than 24 hours. "Night" is understood as the center of the Sun being below a free horizon. Since the atmosphere

refracts In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenomeno ...

sunlight, the polar day is longer than the polar night, and the area that is affected by polar night is somewhat smaller than the area of midnight sun. The polar circle is located at a

latitude In geography, latitude is a coordinate that specifies the north– south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north ...

between these two areas, at approximately 66.5°. While it is day in the

Arctic Circle The Arctic Circle is one of the two polar circles, and the most northerly of the five major circles of latitude as shown on maps of Earth. Its southern equivalent is the Antarctic Circle. The Arctic Circle marks the southernmost latitude at ...

, it is night in the

Antarctic Circle The Antarctic Circle is the most southerly of the five major circles of latitude that mark maps of Earth. The region south of this circle is known as the Antarctic, and the zone immediately to the north is called the Southern Temperate Zone. So ...

, and vice versa. Any planet or moon with a sufficient

axial tilt In astronomy, axial tilt, also known as obliquity, is the angle between an object's rotational axis and its orbital axis, which is the line perpendicular to its orbital plane; equivalently, it is the angle between its equatorial plane and orb ...

that rotates with respect to its star significantly more frequently than it orbits the star (no

tidal locking Tidal locking between a pair of co- orbiting astronomical bodies occurs when one of the objects reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit. In the case where a tidally locked b ...

between the two) will experience the same phenomenon (a nighttime lasting more than one rotation period).

Description

The polar shortest day is not totally dark everywhere inside the polar circle, but only in places within about 5.5° of the poles, and only when the moon is well below the horizon. Regions located at the inner border of the polar circles experience polar

twilight Twilight is light produced by sunlight scattering in the upper atmosphere, when the Sun is below the horizon, which illuminates the lower atmosphere and the Earth's surface. The word twilight can also refer to the periods of time when this i ...

instead of polar night. In fact,

polar region The polar regions, also called the frigid zones or polar zones, of Earth are the regions of the planet that surround its geographical poles (the North and South Poles), lying within the polar circles. These high latitudes are dominated by flo ...

s typically get more twilight throughout the year than equatorial regions. For regions inside the polar circles, the maximum lengths of the time that the Sun is completely below the horizon varies from zero to a few days beyond the

and

to 179 days at the

Poles Poles,, ; singular masculine: ''Polak'', singular feminine: ''Polka'' or Polish people, are a West Slavic nation and ethnic group, who share a common history, culture, the Polish language and are identified with the country of Poland in ...

. However, not all this time is classified as polar night since sunlight may be visible because of

refraction In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenomen ...

. The time when ''any'' part of the Sun is above the horizon at the poles is 186 days. The preceding numbers are average numbers: the ellipticity of Earth's orbit makes the South Pole receive a week more of Sun-below-horizon than the North Pole (see equinox).

Types of polar night

As there are various kinds of

, there also exist various kinds of 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.

Polar twilight

Polar twilight occurs in areas that are located at the inner border of the polar circles, where the Sun will be on or below the horizon all day on the

winter solstice The winter solstice, also called the hibernal solstice, occurs when either of Earth's poles reaches its maximum tilt away from the Sun. This happens twice yearly, once in each hemisphere (Northern and Southern). For that hemisphere, the winter ...

. There is then no true

daylight Daylight is the combination of all direct and indirect sunlight during the daytime. This includes direct sunlight, diffuse sky radiation, and (often) both of these reflected by Earth and terrestrial objects, like landforms and buildings. Sunligh ...

at the

solar culmination In observational astronomy, culmination is the passage of a celestial object (such as the Sun, the Moon, a planet, a star, constellation or a deep-sky object) across the observer's local meridian. These events were also known as meridian transit ...

, only civil twilight. This means that the Sun is below the horizon, but by less than 6°. During civil twilight, there may still be enough light for most normal outdoor activities because of light scattering by the upper atmosphere and refraction. Street lamps may remain on and a person looking at a window from within a brightly lit room may see their reflection even at noon, as the level of outdoor illuminance will be below that of many illuminated indoor spaces. It occurs at latitudes between 67°24’ and 72°34’ North or South, when the Sun does not rise, only civil twilight visible. Northern Hemisphere: *68° North: December 9 to January 2 *69° North: December 1 to January 10 *70° North: November 26 to January 16 *71° North: November 21 to January 21 *72° North: November 16 to January 25 Southern Hemisphere: *68° South: June 7 to July 3 *69° South: May 30 to July 11 *70° South: May 24 to July 18 *71° South: May 19 to July 23 *72° South: May 14 to July 27 Sufferers of seasonal 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 The lux (symbol: lx) is the unit of illuminance, or luminous flux per unit area, in the International System of Units (SI). It is equal to one lumen per square metre. In photometry, this is used as a measure of the intensity, as perceived by ...

) which are not present in any stage of twilight; thus, the midday twilights experienced anywhere inside the polar circles are still "polar night" for this purpose.

Civil polar night

The civil polar night period produces only a faint glow of light visible at midday. It happens when there is no civil twilight and only nautical twilight occurs at the

. Civil twilight happens when the Sun is between 0 and 6° below the horizon, and civil night when it is lower than that. Therefore, the civil polar night is limited to latitudes above 72° 34', which is exactly 6° inside the polar circle. Nowhere on mainland Europe is this definition met. On the Norwegian territory of

Svalbard Svalbard ( , ), also known as Spitsbergen, or Spitzbergen, is a Norwegian archipelago in the Arctic Ocean. North of mainland Europe, it is about midway between the northern coast of Norway and the North Pole. The islands of the group rang ...

, however, civil polar night lasts from about 11 November until 30 January. Dikson, in Russia, experiences civil polar night from December 6 to January 6. During dense cloud cover places like the coast of

Finnmark Finnmark (; se, Finnmárku ; fkv, Finmarku; fi, Ruija ; russian: Финнмарк) was a county in the northern part of Norway, and it is scheduled to become a county again in 2024. On 1 January 2020, Finnmark was merged with the neighbour ...

(about 70°) in

Norway Norway, officially the Kingdom of Norway, is a Nordic countries, Nordic country in Northern Europe, the mainland territory of which comprises the western and northernmost portion of the Scandinavian Peninsula. The remote Arctic island of ...

will get a darker "day". On the Canadian territory of Pond Inlet, Nunavut, however, civil polar night lasts from about 16 December until 26 December.

Nautical polar night

During the nautical polar night period, there is no trace of daylight, except around midday. It happens when there is no nautical twilight and only

astronomical twilight Twilight is light produced by sunlight scattering in the upper atmosphere, when the Sun is below the horizon, which illuminates the lower atmosphere and the Earth's surface. The word twilight can also refer to the periods of time when this i ...

occurs at the

. Nautical twilight happens when the Sun is between six and twelve degrees below the horizon. There is a location at the

horizon The horizon is the apparent line that separates the surface of a celestial body from its sky when viewed from the perspective of an observer on or near the surface of the relevant body. This line divides all viewing directions based on whether i ...

around midday with more light than others because of refraction. During nautical night, the Sun is lower than 12° below the horizon, so nautical polar night is limited to latitudes above 78° 34', which is exactly 12° within the polar circle, or 11.5° from the pole. Alert,

Nunavut Nunavut ( , ; iu, ᓄᓇᕗᑦ , ; ) is the largest and northernmost territory of Canada. It was separated officially from the Northwest Territories on April 1, 1999, via the '' Nunavut Act'' and the '' Nunavut Land Claims Agreement Act'' ...

, the northernmost settlement in

Canada Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by to ...

and the world, experiences this from November 19 to January 22. Oodaaq, a gravel bank at the northern tip of

Greenland Greenland ( kl, Kalaallit Nunaat, ; da, Grønland, ) is an island country in North America that is part of the Kingdom of Denmark. It is located between the Arctic and Atlantic oceans, east of the Canadian Arctic Archipelago. Greenland ...

and a disputed northernmost point of land, experiences this from November 15 to January 27. Its antipode () experiences this from May 13 to July 31. The

Canadian Canadians (french: Canadiens) are people identified with the country of Canada. This connection may be residential, legal, historical or cultural. For most Canadians, many (or all) of these connections exist and are collectively the source of ...

territory of Eureka, Nunavut, experiences this from December 1 to January 10. Its antipode () experiences this from June 1 to July 11. The Norwegian territory of

Ny-Ålesund Ny-Ålesund ("New Ålesund") is a small town in Oscar II Land on the island of Spitsbergen in Svalbard, Norway. It is situated on the Brøgger peninsula (Brøggerhalvøya) and on the shore of the bay of Kongsfjorden. The company town is owned ...

experiences this from December 12 to 30. Its antipode () experiences this from June 12 to July 1. The Russian territory of

Franz Josef Land , native_name = , image_name = Map of Franz Josef Land-en.svg , image_caption = Map of Franz Josef Land , image_size = , map_image = Franz Josef Land location-en.svg , map_caption = Location of Franz Josef ...

experiences this from November 27 to January 15. Its antipode () experiences this from May 25 to July 17.

Astronomical polar night

The astronomical polar night is a period of continuous

where no

occurs. Astronomical twilight happens when the Sun is between twelve and eighteen degrees below the horizon and astronomical night when it is lower than that. Thus, the astronomical polar night is limited to latitudes above 84° 34', which is exactly 18° within the polar circle, or five and a half degrees from the pole. The only permanent settlement on Earth above this latitude is

Amundsen–Scott South Pole Station The Amundsen–Scott South Pole Station is the United States scientific research station at the South Pole of the Earth. It is the southernmost point under the jurisdiction (not sovereignty) of the United States. The station is located on the ...

, the

South Pole The South Pole, also known as the Geographic South Pole, Terrestrial South Pole or 90th Parallel South, is one of the two points where Earth's axis of rotation intersects its surface. It is the southernmost point on Earth and lies antipod ...

scientific research station, whose winter personnel are completely isolated from mid-February to late October. During the astronomical polar night, stars of the sixth magnitude, which are the dimmest stars visible to the naked eye, will be visible throughout the entire

day A day is the time period of a full rotation of the Earth with respect to the Sun. On average, this is 24 hours, 1440 minutes, or 86,400 seconds. In everyday life, the word "day" often refers to a solar day, which is the length between two ...

. This happens when the sun is between 18° and 23° 26’ below the horizon. These conditions last about 11 weeks at the poles. The

Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest cont ...

experiences this from May 11 to August 1 or August 2 depending on the year. The

North Pole The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is the point in the Northern Hemisphere where the Earth's axis of rotation meets its surface. It is called the True North Pole to distinguish from the Ma ...

experiences this from November 13 to January 29.

Polar Sun cycle

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.NASA: The Sun and Seasons
NASA. (See last paragraph, section 164.) By David Stern. Last updated Sept. 17, 2004. Downloaded Feb. 17, 2017.

References

External links

The polar night and polar darkness

These clearly show the progression into and away from winter at a latitude within the Arctic Circle. {{DEFAULTSORT:Polar Night Earth phenomena Polar regions of the Earth Night