Polar circle
A polar circle is a geographic term for a conditional circular line referring either to the Arctic Circle or the Antarctic Circle. It is one of major circles of latitude. On Earth, the Arctic Circle is located at a latitude of N, and the Antarctic Circle is located at a latitude of S. Polar circles are associated with polar regions of Earth which are usually sparsely settled due to their climate environment such as Antarctica.
Areas inside each polar circle and its associated pole, known geographically as the frigid zones, would theoretically experience at least one 24-hour period when the center of the sun is continuously above the horizon and at least one 24-hour period when the center of the sun is continuously below the horizon annually. However, due to atmospheric refraction and the Sun being an extended object rather than a point source, the continuous daylight area is somewhat extended while the continuous darkness area is somewhat reduced.
The latitude of the polar circles is 90 degrees minus the axial tilt of the Earth's axis of daily rotation relative to the ecliptic, the plane of the Earth's orbit. This tilt 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.- The Polar night does not reach as far equatorially as the polar circle, but some 80-100 km toward the pole of this circle.
- Likewise, the Polar day reaches more polarly than the polar circle, by some 80-100 km.
The main reason for this is that the sun rays bend in the atmosphere of planet Earth; this phenomenon is easier to see at high latitudes.
