Polar night: Difference between revisions
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A common misunderstanding is that at each point inside the polar circle, or that at each place where midnight sun occurs, the shortest day is totally dark. Because of [[twilight]], this is not the case. In fact, polar regions get more light throughout the year than regions located closer to the [[equator]]. For example, during seven [[month]]s, [[Kiruna]] at [[Latitude|67° 49′ N]], (slightly north of the [[Arctic Circle]]) has more light than [[Stockholm]] at 59° 39′ N. |
A common misunderstanding is that at each point inside the polar circle, or that at each place where midnight sun occurs, the shortest day is totally dark. Because of [[twilight]], this is not the case. In fact, polar regions get more light throughout the year than regions located closer to the [[equator]]. For example, during seven [[month]]s, [[Kiruna]] at [[Latitude|67° 49′ N]], (slightly north of the [[Arctic Circle]]) has more light than [[Stockholm]] at 59° 39′ N. |
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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 '''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". |
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 [[Geographical pole|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 '''above''' the horizon at the poles is said to be 186 days. The [[wiktionary:Asymmetry|asymmetry]] in numbers is because the time when the sun is ''partially'' above the horizon is counted towards the "daytime". |
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== Kinds of polar night == |
== Kinds of polar night == |
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=== Civil polar night === |
=== Civil polar night === |
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The ''civil polar night'' is period during which no ''[[civil twilight]]'' occurs. Civil twilight happens when the sun is between zero and six degrees below the horizon. Because of |
The ''civil polar night'' is period during which no ''[[civil twilight]]'' occurs. Civil twilight happens when the sun is between zero and six degrees below the horizon. Because of refraction, there still is enough [[light]] for normal outdoor activities. Civil polar night is limited to latitudes above 72° 33′, which is exactly six degrees inside the polar circle. In mainland [[Europe]], there are no places fulfilling this definition. |
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=== Nautical polar night === |
=== Nautical polar night === |
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The ''nautical polar night'' is the period during which there is only a faint glow of light visible during [[midday]]. It happens when there is no ''[[nautical twilight]]''. Nautical twilight happens when the sun is between six and twelve degrees below the horizon. Because of |
The ''nautical polar night'' is the period during which there is only a faint glow of light visible during [[midday]]. It happens when there is no ''[[nautical twilight]]''. Nautical twilight happens when the sun is between six and twelve degrees below the horizon. Because of refraction, there is still a place at the [[horizon]] with clearly more light than other places. 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. |
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=== Astronomical polar night === |
=== Astronomical polar night === |
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Revision as of 19:11, 28 June 2005
The polar night is the night lasting more than 24 hours, usually 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 misunderstanding is that at each point inside the polar circle, or that at each place where midnight sun occurs, the shortest day is totally dark. Because of twilight, this is not the case. In fact, polar regions get more light throughout the year than regions located closer to the equator. For example, during seven months, Kiruna at 67° 49′ N, (slightly north of the Arctic Circle) has more light than Stockholm at 59° 39′ N.
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 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".
Kinds of polar night
Various kinds of polar night exist. This is because polar night is the period during which no twilight occurs; but there are various kinds of twilight.
Civil polar night
The civil polar night is period during which no civil twilight occurs. Civil twilight happens when the sun is between zero and six degrees below the horizon. Because of refraction, there still is enough light for normal outdoor activities. Civil polar night is limited to latitudes above 72° 33′, which is exactly six degrees inside the polar circle. In mainland Europe, there are no places fulfilling this definition.
Nautical polar night
The nautical polar night is the period during which there is only a faint glow of light visible during midday. It happens when there is no nautical twilight. Nautical twilight happens when the sun is between six and twelve degrees below the horizon. Because of refraction, there is still a place at the horizon with clearly more light than other places. 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.
Astronomical polar night
Finally, the astronomical polar night is the period that no trace of light can be seen anywhere and no astronomical twilight occurs. Astronomical twilight happens when the sun is between twelve and eighteen degrees below the horizon. Thus, astronomical polar night happens at latitudes above 84° 33′, which is exactly 18 degrees within the polar circle, or five and a half degrees from the pole.
Effects on humans
The period of polar night is reportedly very taxing on the human body and can trigger depression in many people. Polar days affect people as well. People who suffer from seasonal affective disorder are particularly susceptible.