Polar night
Encyclopedia
The polar night occurs when the night lasts for more than 24 hours. This occurs only inside the polar circle
Polar circle
A polar circle is either the Arctic Circle or the Antarctic Circle. On Earth, the Arctic Circle is located at a latitude of  N, and the Antarctic Circle is located at a latitude of  S....

s. The opposite phenomenon, the polar day, or midnight sun
Midnight sun
The midnight sun is a natural phenomenon occurring in summer months at latitudes north and nearby to the south of the Arctic Circle, and south and nearby to the north of the Antarctic Circle where the sun remains visible at the local midnight. Given fair weather, the sun is visible for a continuous...

, occurs when the sun stays above the horizon for more than 24 hours.

Description

A common misconception is that the shortest day is totally dark at each point where the midnight sun occurs inside the polar circle. In places very close to the poles this is true, but in regions that are located at the inner border of the polar circles where the midnight sun is experienced, this is not the case. Because of twilight
Twilight
Twilight is the time between dawn and sunrise or between sunset and dusk, during which sunlight scattering in the upper atmosphere illuminates the lower atmosphere, and the surface of the earth is neither completely lit nor completely dark. The sun itself is not directly visible because it is below...

, these regions experience polar twilight instead of the polar night. In fact, polar region
Polar region
Earth's polar regions are the areas of the globe surrounding the poles also known as frigid zones. The North Pole and South Pole being the centers, these regions are dominated by the polar ice caps, resting respectively on the Arctic Ocean and the continent of Antarctica...

s typically get more twilight throughout the year than equatorial regions
Tropics
The tropics is a region of the Earth surrounding the Equator. It is limited in latitude by the Tropic of Cancer in the northern hemisphere at approximately  N and the Tropic of Capricorn in the southern hemisphere at  S; these latitudes correspond to the axial tilt of the Earth...

.

For regions inside the polar circles, the lengths of the time when the sun is below the horizon varies from 20 hours at the Arctic Circle
Arctic Circle
The Arctic Circle is one of the five major circles of latitude that mark maps of the Earth. For Epoch 2011, it is the parallel of latitude that runs north of the Equator....

 and Antarctic Circle
Antarctic Circle
The Antarctic Circle is one of the five major circles of latitude that mark maps of the Earth. For 2011, it is the parallel of latitude that runs south of the Equator.-Description:...

  to 179 days at the Poles
Geographical pole
A geographical pole is either of the two points—the north pole and the south pole—on the surface of a rotating planet where the axis of rotation meets the surface of the body...

. However not all this time is classified as polar night, since sunlight may be visible because of refraction
Refraction
Refraction is the change in direction of a wave due to a change in its speed. It is essentially a surface phenomenon . The phenomenon is mainly in governance to the law of conservation of energy. The proper explanation would be that due to change of medium, the phase velocity of the wave is changed...

 . Also, the time when the sun is above the horizon at the poles is 186 days. The numerical asymmetry occurs because the time when the sun is partially above the horizon is counted towards the "daytime". Also, the above numbers are average numbers: owing to the ellipticity of the Earth's orbit, where the South pole receives a week more of sun-below-horizon than the North pole (see equinox
Equinox
An equinox occurs twice a year, when the tilt of the Earth's axis is inclined neither away from nor towards the Sun, the center of the Sun being in the same plane as the Earth's equator...

).

Types of polar night

As there are various kinds of twilight, there also exist various kinds of polar night. This is because the polar night is defined as the period during which no twilight
Twilight
Twilight is the time between dawn and sunrise or between sunset and dusk, during which sunlight scattering in the upper atmosphere illuminates the lower atmosphere, and the surface of the earth is neither completely lit nor completely dark. The sun itself is not directly visible because it is below...

 occurs. These latitude definitions are based on relatively clear skies, so the sky will be darker in the presence of dense cloud.

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
Winter solstice
Winter solstice may refer to:* Winter solstice, astronomical event* Winter Solstice , former band* Winter Solstice: North , seasonal songs* Winter Solstice , 2005 American film...

, although there may still be enough light for normal outdoor activities during civil twilight because of light scattering by the upper atmosphere and refraction. It happens when there is no true daylight
Daylight
Daylight or the light of day is the combination of all direct and indirect sunlight outdoors during the daytime. This includes direct sunlight, diffuse sky radiation, and both of these reflected from the Earth and terrestrial objects. Sunlight scattered or reflected from objects in outer space is...

and only civil twilight
Civil twilight
Civil twilight can mean:* The brightest phase of twilight.* Civil Twilight , three-piece rock music band who originated in Cape Town, South Africa before moving to the USA.* "Civil Twilight", a song by The Weakerthans from their 2007 album Reunion Tour....

occurs at the solar culmination. 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
Illuminance
In photometry, illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of the intensity of the incident light, wavelength-weighted by the luminosity function to correlate with human brightness perception. Similarly, luminous emittance is the luminous flux per...

 will be below that of many illuminated indoor spaces. However, it is possible to conduct outdoor activities without any artificial light during the time of civil twilight.

Sufferers of Seasonal Affective Disorder
Seasonal affective disorder
Seasonal affective disorder , also known as winter depression, winter blues, summer depression, summer blues, or seasonal depression, is a mood disorder in which people who have normal mental health throughout most of the year experience depressive symptoms in the winter or summer, spring or autumn...

 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
Lux
The lux is the SI unit of illuminance and luminous emittance, measuring luminous flux per unit area. It is used in photometry as a measure of the intensity, as perceived by the human eye, of light that hits or passes through a surface...

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

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
Civil twilight
Civil twilight can mean:* The brightest phase of twilight.* Civil Twilight , three-piece rock music band who originated in Cape Town, South Africa before moving to the USA.* "Civil Twilight", a song by The Weakerthans from their 2007 album Reunion Tour....

and only nautical twilight occurs at the solar culmination. Civil twilight happens when the sun is between zero and six degrees below the horizon. The civil polar night is limited to latitudes above 72° 33', which is exactly 6 degrees inside the polar circle. Nowhere on mainland Europe is this definition met. On the Norwegian territory of Svalbard
Svalbard
Svalbard is an archipelago in the Arctic, constituting the northernmost part of Norway. It is located north of mainland Europe, midway between mainland Norway and the North Pole. The group of islands range from 74° to 81° north latitude , and from 10° to 35° east longitude. Spitsbergen is the...

, however, civil polar night lasts from about 11 November until 30 January. Dikson
Dikson (urban-type settlement)
Dikson is an urban locality in Taymyrsky Dolgano-Nenetsky District of Krasnoyarsk Krai, Russia. Population: Dikson is the northernmost port in Russia and one of the northernmost settlements in the world. It is located so far north that one may experience complete darkness with no civil twilight...

, in Russia, experiences civil polar night for approximately a month. If there is thick cloud cover places like the coast of Finnmark
Finnmark
or Finnmárku is a county in the extreme northeast of Norway. By land it borders Troms county to the west, Finland to the south and Russia to the east, and by water, the Norwegian Sea to the northwest, and the Barents Sea to the north and northeast.The county was formerly known as Finmarkens...

 (about 70°) in Norway
Norway
Norway , officially the Kingdom of Norway, is a Nordic unitary constitutional monarchy whose territory comprises the western portion of the Scandinavian Peninsula, Jan Mayen, and the Arctic archipelago of Svalbard and Bouvet Island. Norway has a total area of and a population of about 4.9 million...

 will get a darker "day".

Nautical polar night

During the nautical polar night period there is no trace of natural light, except for around midday. It happens when there is no nautical twilight and only astronomical twilight occurs at the solar culmination. Nautical twilight happens when the sun is between six and twelve degrees below the horizon, although there is a location at the horizon
Horizon
The horizon is the apparent line that separates earth from sky, the line that divides all visible directions into two categories: those that intersect the Earth's surface, and those that do not. At many locations, the true horizon is obscured by trees, buildings, mountains, etc., and the resulting...

 with more light than others because of refraction. The nautical polar night is limited to latitudes above 78° 33', which is exactly 12 degrees within the polar circle, or eleven and a half degrees from the pole. Alert
Alert, Nunavut
Alert, in the Qikiqtaaluk Region, Nunavut, Canada, is the northernmost permanently inhabited place in the world, from the North Pole. It takes its name from HMS Alert, which wintered east of the present station, off what is now Cape Sheridan, in 1875–1876.Alert was reported to have five permanent...

, Nunavut
Nunavut
Nunavut is the largest and newest federal 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, though the actual boundaries had been established in 1993...

, the northernmost settlement in Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...

 and the world, experiences this from late November to mid January.

Astronomical polar night

The astronomical polar night period is continuous night
Night
Night or nighttime is the period of time when the sun is below the horizon. This occurs after dusk. The opposite of night is day...

where no astronomical twilight occurs. Astronomical twilight happens when the sun is between twelve and eighteen degrees below the horizon. Thus, the astronomical polar night is limited to latitudes above 84° 33', which is exactly 18 degrees within the polar circle, or five and a half degrees from the pole. 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
Day
A day is a unit of time, commonly defined as an interval equal to 24 hours. It also can mean that portion of the full day during which a location is illuminated by the light of the sun...

.

There are no permanent settlements anywhere in this range of latitude. This portion of the Arctic Ocean
Arctic Ocean
The Arctic Ocean, located in the Northern Hemisphere and mostly in the Arctic north polar region, is the smallest and shallowest of the world's five major oceanic divisions...

 is generally permanently ice-capped. Some scientific stations in Antarctica, including the Amundsen-Scott South Pole Station
Amundsen-Scott South Pole Station
The Amundsen–Scott South Pole Station is the American scientific research station on the high plateau of Antarctica. This station is located at the southernmost place on the Earth, the Geographic South Pole, at an elevation of 2,835 meters above sea level.The original Amundsen-Scott Station was...

, experience this.

Effects on humans

The period of polar night can trigger depression
Clinical depression
Major depressive disorder is a mental disorder characterized by an all-encompassing low mood accompanied by low self-esteem, and by loss of interest or pleasure in normally enjoyable activities...

 in some people. People who suffer from seasonal affective disorder
Seasonal affective disorder
Seasonal affective disorder , also known as winter depression, winter blues, summer depression, summer blues, or seasonal depression, is a mood disorder in which people who have normal mental health throughout most of the year experience depressive symptoms in the winter or summer, spring or autumn...

 are particularly susceptible to these conditions. The polar night may also be implicated in some instances of solipsism syndrome
Solipsism syndrome
Solipsism syndrome is a postulated condition which may be faced by those living in space or on another planet for extended periods of time. It may be best described as 'severe Big Brother anomie'.- Overview :...

.

Popular culture

The concept of a night of almost one month long has been the subject of the vampire movies Frostbiten
Frostbiten
Frostbite is a Swedish comedy horror film from 2006 directed by Anders Banke.The film was produced outside the bigger Swedish films studios, which would be considered an independent film in America...

and 30 Days of Night
30 Days of Night (film)
30 Days of Night is a 2007 American horror film based on the comic book miniseries of the same name. The film is directed by David Slade and stars Josh Hartnett, Melissa George, and Danny Huston...

. In these films, the vampires are drawn to the long duration of darkness, allowing them to do as they please.

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
x
OK