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 ...

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. So ...

. These are two of the keynote

circles of latitude A circle of latitude or line of latitude on Earth is an abstract east– west small circle connecting all locations around Earth (ignoring elevation) at a given latitude coordinate line. Circles of latitude are often called parallels bec ...

(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 sur ...

, 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 ...

(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. Alt ...

) 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. 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 cont ...

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 or geography. Etymology The word ''north ...

and South), known geographically as the frigid zones such duration extends up to half of the year, namely, close to the poles. Instead, atmospheric refraction and the Sun's light reaching the planet as an extended object rather than a point source 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 orb ...

(that is, of the Earth's axis of daily rotation relative to the ecliptic, 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. 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 and the polar day 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 winter ...

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 atmospheric refraction, 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 ...

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 ...

, 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