The
polar night occurs when the night lasts for
more than 24 hours. This only occurs inside the
polar circles. The opposite phenomenon, when
the sun stays above the horizon for a long time is called the
polar day, or
midnight
sun.
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 midnight sun is experienced, this is not true.
Due to
twilight, these regions experience
polar twilight instead of the polar night. In fact,
polar regions typically get more twilight
throughout the year than
equatorial
regions.
In regions inside the polar circles, the length of the time when
the sun is below the horizon varies from 20 hours at the
Arctic Circle and
Antarctic Circle to 179 days at the
Poles. However not all this time
is classified as polar night, since there may be plenty of sunlight
because of
refraction. Also, one might
notice that the time when the sun is
above the
horizon at the poles is said to be 186 days. The
asymmetry in numbers is because the
time when the sun is
partially above the horizon is
counted towards the "daytime".
Types of polar night
Since there are various kinds of twilight, there are also various
kinds of polar night. This is because the polar night is defined as
the period during where no
twilight occurs.
These latitude definitions are based on relatively clear skies, so
the sky will be darker if there are thick clouds about.
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,
although there may still be enough light for normal outdoor
activities during civil twilight because of scattering of light by
the upper atmosphere and refraction. It happens when there is no
true
daylight and only
civil twilight occurs at the
solar culmination. Street-lamps
may remain on and one looking out a window from a brightly lit room
may see their reflection even at noon, as the level of outdoor
illuminance will remain 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 will
probably resort to 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 night" in that sense.
Civil polar night
The
civil polar night is the period during which there is
only a faint glow of light visible at midday. It happens when there
is no
civil twilight 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. In mainland Europe, there are no places fulfilling
this definition.
On the Norwegian Territory of Svalbard, however,
civil polar night lasts from about 12
November until the end of January. Dikson, in Russia, experiences civil polar night for
around a month. If there are thick clouds, it gets darker,
and places like the coast of Finnmark
(about 70°) in Norway will get a
rather dark "day".
Nautical polar night
The
nautical polar night is the period that no trace of
light can be seen anywhere but the sky is not completely dark at
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 still a place at the
horizon with clearly more light than other
places 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, Nunavut, the
northernmost settlement in Canada and the
world, experiences this from late November to mid
January.
Astronomical polar night
Finally, the
astronomical polar night is the period of
continuous
night 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 ever visible to the naked eye, will be visible
throughout the entire
day.
There are no permanent settlements anywhere in this range of
latitude.
This portion of the Arctic Ocean is generally permanently ice capped.
Some
scientific stations in Antarctica, including the Amundsen-Scott
South Pole Station, experience this.
Effects on humans
The period of polar night can trigger
depression in some people. People who
suffer from
seasonal
affective disorder are particularly susceptible to these
conditions. The polar night may also be implicated in some
instances of
solipsism
syndrome.
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