The Arctic Circle and Antarctic Circle are two imaginary circles of latitude that mark the approximate boundaries of the regions where at least one day each year can experience twenty four hours of continuous daylight and at least one day can experience twenty four hours of continuous darkness. The Arctic Circle lies at approximately 66.56 degrees north latitude, while the Antarctic Circle lies at approximately 66.56 degrees south latitude.
Their positions are related directly to Earths axial tilt, which is approximately 23.44 degrees relative to the perpendicular to the plane of its orbit. The circles can be expressed as approximately 90 degrees minus 23.44 degrees from either pole. Unlike the tropics, which identify the furthest latitudes of the overhead Sun, the polar circles identify the approximate limits of extreme seasonal daylight conditions.
The Arctic Circle is the northern polar circle and surrounds the Arctic region, while the Antarctic Circle is the southern polar circle and surrounds the Antarctic region. The Arctic Circle crosses parts of countries including Norway, Sweden, Finland, Russia, the United States through Alaska, Canada, Denmark through Greenland and Icelandic territory depending on the exact line and geographic treatment. The Antarctic Circle lies primarily across the Antarctic continent and surrounding Antarctic waters.
Unlike a political border, neither circle has a legal status simply because it is a latitude. Their locations are geographical reference lines. They may be marked by monuments and signs, particularly in places where tourism or transportation routes cross them, but most of each circle has no visible physical boundary.
The most important astronomical effect of the polar circles is the occurrence of the midnight Sun and polar night. At locations north of the Arctic Circle, there are periods around the Northern Hemisphere summer when the Sun remains above the horizon for at least twenty four hours. Around the Northern Hemisphere winter, there are corresponding periods when the Sun remains below the horizon for at least twenty four hours.
The Antarctic Circle experiences the opposite seasonal pattern because the hemispheres have opposite seasons. At the circles themselves, the extreme event occurs approximately around the relevant solstice, when geometric conditions can allow the Sun to remain on or very near the horizon for an extended period. Farther toward either pole, the duration of continuous daylight or darkness becomes progressively longer.
The exact latitude of the Arctic and Antarctic Circles is not permanently fixed. Earths axial tilt changes slowly over long periods, meaning the latitude associated with the polar circles changes correspondingly. Modern maps generally use approximately 66.5 degrees north and south. Precise astronomical calculations can use the current value of Earths obliquity. This is an important distinction because educational maps usually simplify the value, while scientific calculations require greater precision. The circles therefore represent dynamic astronomical reference lines rather than permanently fixed marks embedded in Earths surface. Over geological and astronomical timescales, their positions move gradually as Earths orientation changes.
The Arctic and Antarctic Circles are not the same as the geographical boundaries of the Arctic and Antarctic regions. The Arctic is commonly understood as a broad northern region with definitions based on geography, climate, vegetation, oceanography or political arrangements, and different organisations use different boundaries. Antarctica is a continent surrounded by the Southern Ocean, whereas the Antarctic region can refer to a larger area around it. The Antarctic Circle is specifically a latitude line. It should therefore not be treated as the exact southern boundary of all definitions of the Antarctic or as a political border surrounding Antarctica.
Polar circles are closely connected with seasonal variation in solar radiation. During the summer of each hemisphere, the corresponding polar region receives sunlight for exceptionally long periods, although the Sun remains relatively low above the horizon. During winter, the region experiences exceptionally long periods without direct sunlight. These patterns affect temperatures, sea ice, ecosystems, atmospheric processes and human activity.
Continuous daylight does not mean continuous strong heating because solar elevation remains low and the angle of incoming radiation spreads energy over a large surface area. Similarly, continuous darkness does not mean that all polar regions remain equally cold because ocean currents, atmospheric circulation, elevation, sea ice and local geography strongly influence conditions.
A common misconception is that the Arctic Circle and Antarctic Circle mark places where it is permanently dark or permanently light. They instead mark the approximate starting latitudes for at least one full day of continuous daylight or darkness each year. The number of such days increases toward the poles. Another misconception is that the circles are related to the distance between Earth and the Sun.
Their positions are primarily determined by axial tilt, not by seasonal changes in Earths orbital distance. Earths orbit is slightly elliptical, but that fact is not what establishes the latitude of the polar circles or causes the basic pattern of polar day and polar night.
The Arctic and Antarctic Circles are therefore important geographical and astronomical reference lines used to understand Earths seasons, solar illumination and high latitude environments. Together with the Equator and the two tropics, they form a basic framework of major lines of latitude. The Equator is at zero degrees, the tropics are approximately 23.44 degrees north and south, and the polar circles are approximately 66.56 degrees north and south. The poles lie at 90 degrees north and 90 degrees south. This arrangement reflects the geometry of Earths rotation and revolution and provides a practical framework for interpreting global maps, seasonal sunlight, polar climate and astronomical observations.
