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 or the Antarctic Circle. 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 because ...

(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 surf ...

, 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 po ...

(i.e. without taking into account the astronomical nutation) 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 co ...

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 ''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 ''no ...

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 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 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 (″) 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 map projection. It has the useful properties that all points on the map are at proportionally correct distances from the center point, and that all points on the map are at the correct azimut ...

. 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 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 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 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 Nutation () is a rocking, swaying, or nodding motion in the axis of rotation of a largely axially symmetric object, such as a gyroscope, planet, or bullet in flight, or as an intended behaviour of a mechanism. In an appropriate reference frame ...

, which is a very small change in tilt. Observers higher above sea level can see a tiny amount of the Sun's disc (see horizon) where at lower places it would not rise. For the Arctic circle, 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