Polarkreis - zwischen Narvik & Mo i Rana Norwegen

axial tilt vs tropical and polar circles

A polar circle is a geographic term for a conditional circular line (arc) referring either to 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 w ...

or 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. S ...

. These are two of the keynote circles of latitude (parallels). On

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

, the Arctic Circle is currently drifting northwards at a speed of about 14.5 m per year and is now at a mean

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 pol ...

(i.e. without taking into account the

astronomical nutation Astronomical nutation is a phenomenon which causes the orientation of the axis of rotation of a spinning astronomical object to vary over time. It is caused by the gravitational forces of other nearby bodies acting upon the spinning object. Al ...

) of N; the Antarctic Circle is currently drifting southwards at a speed of about 14.5 m per year and is now at a mean latitude (i.e. without taking into account the astronomical nutation) of S. Polar circles are often equated with

polar regions of Earth 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 float ...

. Due to their inherent climate environment, the bulk of the Arctic Circle, much of which is sea, is sparsely settled whereas this applies to all of

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 contine ...

which is mainly land and sheltered ice shelves. If Earth had no atmosphere then both polar circles (arcs) would see at least a day a year when the center of the sun is continuously above 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 ...

''and'' at least a day a year when it is always below the horizon – a polar day and a polar night as is the case for longer, ''within'' the circles. Up to and including the associated poles (

North North is one of the four compass points or cardinal directions. It is the opposite of south and is perpendicular to east and west. ''North'' is a noun, adjective, or adverb indicating Direction (geometry), direction or geography. Etymology T ...

and

South South is one of the cardinal directions or Points of the compass, compass points. The direction is the opposite of north and is perpendicular to both east and west. Etymology The word ''south'' comes from Old English ''sūþ'', from earlier Pro ...

), known geographically as the frigid zones such duration extends up to half of the year, namely, close to the poles. Instead,

atmospheric refraction Atmospheric refraction is the deviation of light or other electromagnetic wave from a straight line as it passes through the atmosphere due to the variation in air density as a function of height. This refraction is due to the velocity of ligh ...

and the Sun's light reaching the planet as an extended object rather than a

point source A point source is a single identifiable ''localised'' source of something. A point source has negligible extent, distinguishing it from other source geometries. Sources are called point sources because in mathematical modeling, these sources can ...

means that just within each circle the Earth's surface does not experience any proper polar night, 24 hours where the sun does not rise. By these same two factors, just outward of each circle still experiences a polar day (a day in which the sun does not fully set). The latitude of the polar circles is + or −90 degrees (which refers to the North and South Pole, respectively) minus the

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 orbi ...

(that is, of the Earth's axis of daily rotation relative to the

ecliptic The ecliptic or ecliptic plane is the orbital plane of the Earth around the Sun. From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the ecliptic again ...

, the plane of the Earth's orbit). This predominant, average tilt of the Earth varies slightly, a phenomenon described as nutation. Therefore, the latitudes noted above are calculated by averaging values of tilt observed over many years. The axial tilt also exhibits long-term variations as described in the reference article (a difference of 1

second of arc A minute of arc, arcminute (arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of one degree. Since one degree is of a turn (or complete rotation), one minute of arc is of a turn. The ...

(″) in the tilt is equivalent to a change of about 31 metres north or south in the positions of the polar circles on the Earth's surface). File:Arctic circle.svg, The north polar circle on a

polar projection The azimuthal equidistant projection is an Azimuthal projection#Azimuthal .28projections onto a plane.29, azimuthal map projection. It has the useful properties that all points on the map are at proportionally correct distances from the center p ...

. World map with polar circles.svg, The polar circle as lines on a modified cylindrical projection.

Effect of atmospheric refraction and the angular diameter of the Sun

The polar circles would almost precisely match the boundaries for the zones where the

polar night The polar night is a phenomenon where the nighttime 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 midnig ...

and the

polar day 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, th ...

would occur throughout 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 winte ...

and

summer solstice The summer solstice, also called the estival solstice or midsummer, occurs when one of Earth's poles has its maximum tilt toward the Sun. It happens twice yearly, once in each hemisphere ( Northern and Southern). For that hemisphere, the summer ...

day respectively. They do so loosely due to the two effects. The first one is

, in which the Earth's atmosphere bends light rays near the horizon. The second effect is caused by the

angular diameter The angular diameter, angular size, apparent diameter, or apparent size is an angular distance describing how large a sphere or circle appears from a given point of view. In the vision sciences, it is called the visual angle, and in optics, it is ...

of the Sun as seen from the Earth's orbital distance (which varies very slightly during each orbit). These factors mean the ground-observed boundaries are 80–100 km away from the circle. A further global factor for this numerical range is Earth's nutation, which is a very small change in tilt. Observers higher above sea level can see a tiny amount of the Sun's disc (see

) where at lower places it would not rise. For 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 w ...

, being 80–100 km north of the circle in winter, and 80–100 km south of the circle in summer; the inverse directions apply to the other circle.Swedish Astronomic calendar 2003 (or any other year) at the times of the winter and summer solstices, around 22 June and 22 December

Notes

{{DEFAULTSORT:Polar Circle Circles of latitude

Effect of atmospheric refraction and the angular diameter of the Sun

See also

Notes