The Antarctic Circle is the most southerly of the five major

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 ...

that mark maps of Earth. The region south of this circle is known as the

Antarctic The Antarctic ( or , American English also or ; commonly ) is a polar region around Earth's South Pole, opposite the Arctic region around the North Pole. The Antarctic comprises the continent of Antarctica, the Kerguelen Plateau and othe ...

, and the zone immediately to the north is called the Southern Temperate Zone. South of the Antarctic Circle, the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

is 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 ...

for 24 continuous hours at least once per year (and therefore visible at midnight) and the centre of the Sun (ignoring refraction) is below the horizon for 24 continuous hours at least once per year (and therefore not visible at noon); this is also true within the equivalent

polar circle 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 (parallels). On Earth, the Arctic Circle is currently d ...

in the

Northern Hemisphere The Northern Hemisphere is the half of Earth that is north of the Equator. For other planets in the Solar System, north is defined as being in the same celestial hemisphere relative to the invariable plane of the solar system as Earth's Nort ...

, 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 ...

. The position of the Antarctic Circle is not fixed and currently runs south of the

Equator The equator is a circle of latitude, about in circumference, that divides Earth into the Northern and Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the North and South poles. The term can also ...

. This figure may be slightly inaccurate because it does not allow for the effects of

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 ...

, which can be up to 10″. Its latitude depends on the Earth's

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 ...

, which fluctuates within a margin of more than 2° over a 41,000-year period, due to

tidal force The tidal force is a gravitational effect that stretches a body along the line towards the center of mass of another body due to a gradient (difference in strength) in gravitational field from the other body; it is responsible for diverse phenomen ...

s resulting from the

orbit of the Moon The Moon orbits Earth in the prograde direction and completes one revolution relative to the Vernal Equinox and the stars in about 27.32 days (a tropical month and sidereal month) and one revolution relative to the Sun in about 29.53 days (a ...

. Consequently, the Antarctic Circle is currently drifting southwards at a speed of about per year.

Midnight sun and polar night

axial_tilt_vs_tropical_and_polar_circles

The Antarctic Circle is the northernmost latitude in the Southern Hemisphere at which the centre of the sun can remain continuously above the horizon for twenty-four hours; as a result, at least once each year at any location within the Antarctic Circle the centre of the sun is visible at local midnight, and at least once the centre of the sun is below the horizon at local noon. Directly on the Antarctic Circle these events occur, in principle, exactly once per year: at the

December December is the twelfth and final month of the year in the Julian and Gregorian calendars and is also the last of seven months to have a length of 31 days. December got its name from the Latin word ''decem'' (meaning ten) because it was ori ...

and

June June is the sixth month of the year in the Julian and Gregorian calendars and is the second of four months to have a length of 30 days, and the third of five months to have a length of less than 31 days. June contains the summer solstice in ...

solstice A solstice is an event that occurs when the Sun appears to reach its most northerly or southerly excursion relative to the celestial equator on the celestial sphere. Two solstices occur annually, around June 21 and December 21. In many count ...

s, respectively. However, because of

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

mirage A mirage is a naturally-occurring optical phenomenon in which light rays bend via refraction to produce a displaced image of distant objects or the sky. The word comes to English via the French ''(se) mirer'', from the Latin ''mirari'', meanin ...

s, and because the sun appears as a disk and not a point, part of the midnight sun may be seen on the night of the southern

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 ...

up to about 50 minutes (′) () north of the Antarctic Circle; similarly, on the day of the southern

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 ...

, part of the sun may be seen up to about 50′ south of the Antarctic Circle. That is true at sea level; those limits increase with elevation above sea level, although in mountainous regions there is often no direct view of the true horizon. Mirages on the Antarctic continent tend to be even more spectacular than in Arctic regions, creating, for example, a series of apparent sunsets and sunrises while in reality the sun remains below the horizon.

Human habitation

There is no permanent human population south of the Antarctic Circle, but there are several

research stations in Antarctica Multiple governments have set up permanent research stations in Antarctica and these bases are widely distributed. Unlike the drifting ice stations set up in the Arctic, the research stations of the Antarctic are constructed either on rock ...

operated by various nations that are inhabited by teams of scientists who rotate on a seasonal basis. In previous centuries some semi-permanent

whaling Whaling is the process of hunting of whales for their usable products such as meat and blubber, which can be turned into a type of oil that became increasingly important in the Industrial Revolution. It was practiced as an organized industr ...

stations were established on the continent, and some whalers would live there for a year or more. At least three children have been born in Antarctica, albeit in stations north of the Antarctic Circle.

Geography

The circumference of the Antarctic Circle is roughly . The area south of the Circle is about and covers roughly 4% of Earth's surface. Most of the

continent A continent is any of several large landmasses. Generally identified by convention rather than any strict criteria, up to seven geographical regions are commonly regarded as continents. Ordered from largest in area to smallest, these seven ...

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 ...

is within the Antarctic Circle.

Sites along the Circle

Starting at the

prime meridian A prime meridian is an arbitrary meridian (a line of longitude) in a geographic coordinate system at which longitude is defined to be 0°. Together, a prime meridian and its anti-meridian (the 180th meridian in a 360°-system) form a great ...

and heading eastwards, the Antarctic Circle passes through: :

References

External links