The Antarctic Circle

The Antarctic Circle is the southern mirror of the /learn/the-arctic-circle. Geometrically it is the same construction — the parallel at 90° minus Earth's axial obliquity, but on the southern hemisphere — and the polar phenomena (24-hour day at the solstice, 24-hour night at the opposite solstice) are the same. What is dramatically different is the geography: where the Arctic Circle slices through eight inhabited countries, the Antarctic Circle crosses an uninhabited continent and a few remote islands. This article covers the geometric definition, the geography, the climate of the Antarctic continent, the contrast with the Arctic basin, and the history of exploration from Cook through the modern Antarctic Treaty era.

Definition

The Antarctic Circle lies at latitude −(90° − ε), where ε is Earth's axial obliquity. With ε currently 23.4366° per the NASA Earth Fact Sheet, the Antarctic Circle sits at −66.5634°, or −66°33′38″ S. As on the northern side, the IAU obliquity formula gives the secular variation: obliquity is decreasing at about 0.47″/year, so the Antarctic Circle is drifting south by about 14.5 m/year as 90° − ε grows. Like the Arctic Circle, it is one of the five named parallels in the /learn/parallels-of-latitude family.

The polar phenomena are time-shifted by six months from the northern counterparts:

• On the December solstice, the Sun stays above the horizon for 24 hours at the Antarctic Circle (and longer further south); below the horizon for 24 hours at the Arctic Circle.
• On the June solstice, the Sun stays below the horizon for 24 hours at the Antarctic Circle (polar night); above the horizon for 24 hours at the Arctic Circle.

Atmospheric refraction shifts the visible sun by about 1° south of the geometric circle, the same way it shifts the Arctic Sun north — making the visible midnight sun and the polar night each extend slightly beyond the geometric boundary.

Geography

The Antarctic Circle's geography is dominated by one fact: only Antarctica has substantial land south of the line, and Antarctica has no permanent human population. Going eastward from 0° E:

| Region | Notes | |---|---| | Queen Maud Land (Norway claim) | The line cuts across the coast at about 21° E | | Enderby Land (Australia claim) | Continues across East Antarctic ice sheet | | Indian Ocean coast (Australian Antarctic Territory) | Across the Amery Ice Shelf region | | Wilkes Land / Adélie Land (France) | Across the centre of the East Antarctic plateau | | George V Land / Oates Land (Australia) | Across to the Ross Sea margin | | Ross Sea | The line crosses the open sea south of New Zealand | | Marie Byrd Land (no claim) | West Antarctica | | Ellsworth Land (no claim) | West Antarctic ice sheet | | Antarctic Peninsula (multiple overlapping claims) | The peninsula is north of the line, ending around −63° S | | Bellingshausen Sea | Open ocean back to 0° E |

The Antarctic Peninsula — the only part of Antarctica that extends relatively far north and where most tourist visits happen — sits entirely north of the Antarctic Circle. The tip of the peninsula is at about −63°S, with the South Shetland Islands at about −62°S. The sub-Antarctic island groups (South Orkneys at −61°S, South Sandwich at −59°S, South Georgia at −54°S) are all north of the Circle. The Balleny Islands just south of New Zealand are one of the few non-mainland landmasses the Circle crosses, along with the small Peter I Island in the Bellingshausen Sea.

The mainland geography south of the Circle is dominated by the East Antarctic Ice Sheet — the largest single mass of ice on Earth, reaching up to about 4,000 m thick in places — and by the West Antarctic Ice Sheet, which is marine-based and a current focus of climate research because of its potential instability.

The Antarctic Treaty and the 60°S threshold

The Antarctic Treaty of 1959, which entered into force in 1961, governs all activity south of 60°S latitude. The Antarctic Treaty Secretariat website hosts the full text. The Treaty:

• Reserves Antarctica for peaceful purposes and scientific research
• Freezes — but does not invalidate — the territorial claims of seven states (Argentina, Australia, Chile, France, New Zealand, Norway, the United Kingdom)
• Prohibits military activity, nuclear testing, and the disposal of radioactive waste south of 60°S
• Provides for free inspection of any station by any party

The Treaty's 60°S threshold is not the Antarctic Circle. It sits about 720 km north of it, deliberately chosen to include the sub-Antarctic island groups (the South Shetlands, South Orkneys, and others) within the Treaty area without anchoring the boundary on an astronomical definition that shifts with obliquity. The Treaty is the operative legal framework; the Antarctic Circle is the operative geographic and astronomical one. Both matter, but they govern different questions.

The Antarctic Treaty System (the Treaty plus subsequent agreements like the 1991 Madrid Protocol on Environmental Protection) currently has about 56 signatory states. Around 70 research stations operate in Antarctica, of which about 40 are year-round.

Climate

Antarctica is significantly colder than the Arctic for the structural reasons covered by the British Antarctic Survey and already touched on in the FAQ: it is a continent (ice and rock) rather than an ocean basin (sea ice on water), and the continental ice sheet elevates the surface to an average of about 2,500 m above sea level. The lowest reliably measured surface air temperature on Earth, −89.2°C, was recorded at Vostok Station (78°27′S) on 21 July 1983. Satellite remote sensing has detected lower surface temperatures (down to about −98°C) on the East Antarctic Plateau, with the lowest documented station-measured value of −89.9°C at the Russian station Vostok in 2018.

Reported station-measured air temperatures at Vostok on subsequent expeditions have come close to that record but the 1983 value remains the WMO-recognised lowest air temperature on Earth recorded at a fixed station. Surface temperatures detected by satellite radiometry on the East Antarctic Plateau are lower, but they are surface-skin readings, not air temperatures at the standard 2-metre height.

Katabatic winds are a hallmark of the Antarctic interior. Dense, cold air pools on the high inland plateau and drains outward toward the coast under gravity, accelerating as it descends. The result is the strongest sustained winds on the planet outside hurricanes: routine gusts above 200 km/h are recorded at coastal stations like Cape Denison, which holds the record for highest average annual wind speed at any weather station on Earth.

Antarctic sea ice, unlike Arctic sea ice, expands and contracts dramatically each year — from about 3 million km² at the February minimum to about 18 million km² at the September maximum, per NSIDC. The trend in Antarctic sea ice has been more variable than the steady Arctic decline, with recent years showing pronounced low extents.

Subglacial lakes and the ozone hole

Two other phenomena anchor Antarctic science south of the line.

Subglacial lakes. Beneath the East Antarctic Ice Sheet, liquid water is trapped in basins formed by geothermal heat from below and the insulating effect of kilometres of ice above. The largest is Lake Vostok, beneath the Russian Vostok Station, about 250 km long and over 500 m deep. Several hundred subglacial lakes have been identified by radar surveys; some are interconnected by under-ice rivers. They are of interest to astrobiology because they may host ecosystems isolated from the surface for millions of years, an analogue for ice-covered moons like Europa and Enceladus.

The Antarctic ozone hole. Every Southern-Hemisphere spring (roughly August–November), stratospheric ozone over Antarctica thins dramatically — at peak depletion, more than 50% of the column ozone is gone. The phenomenon was first identified by the British Antarctic Survey in 1985 from Halley Station data, and is caused by chlorofluorocarbon-driven catalytic destruction of ozone on polar stratospheric cloud surfaces. The 1987 Montreal Protocol on Substances that Deplete the Ozone Layer is the international response; per NSIDC and atmospheric chemistry monitoring, the ozone layer is on a slow recovery trajectory expected to return to pre-1980 levels by roughly 2065.

Exploration history

The Antarctic Circle was first crossed by James Cook on HMS Resolution on 17 January 1773 at about 39°35′E. Cook went on to cross the Circle two more times on the same voyage. On 30 January 1774 he reached 71°10′S — the highest southern latitude anyone had attained at the time — before turning back. He never sighted the Antarctic mainland, which lay just out of view beyond the pack ice.

First sightings of the continent itself came almost half a century later. Fabian von Bellingshausen on the Russian sloop Vostok sighted ice cliffs of the mainland on 27 January 1820 at about 69°21′S, 2°14′W. Three days later, on 30 January 1820, Edward Bransfield on HMS Williams sighted Trinity Peninsula at the northern tip of the Antarctic Peninsula. Which of the two counts as the “discovery” depends on which document one privileges.

The 19th and early 20th centuries brought a succession of expeditions that progressively unwound the geography of the continent: James Clark Ross's Royal Navy expedition (1839–1843) charting the Ross Sea; Adrien de Gerlache's overwintering on the Belgica (1898); Roald Amundsen's first reaching of the South Pole (14 December 1911); Robert Falcon Scott's arrival there a month later, 17 January 1912; the loss of Scott's party on the return journey. The /learn/the-south-pole support covers the pole itself and the modern Amundsen-Scott Station.

After the Second World War, the International Geophysical Year of 1957–1958 brought 12 countries together for systematic Antarctic science. That coordination led directly to the 1959 Antarctic Treaty.

Two related boundaries

The Antarctic Circle is the astronomical line. Two other boundaries appear frequently in Antarctic science and policy and are easy to confuse with it:

• The Antarctic Convergence (also called the Polar Front) is the oceanographic boundary at roughly 50°S to 60°S where cold northward-flowing Antarctic surface water meets warmer subantarctic water. It is a sharp temperature gradient that has biological significance — it broadly bounds the range of many subantarctic species — and shifts slightly with the seasons.
• The Antarctic Treaty boundary at 60°S, covered above.

The Antarctic Circle at −66.5634°S is south of both of them; the Convergence and the Treaty boundary are intermediate northern boundaries of the broader Antarctic region. Tourism, mostly to the Antarctic Peninsula, takes place primarily north of the Antarctic Circle, between the Convergence and the Circle itself.

Sources

• NASA, Earth Fact Sheet — obliquity.
• IAU WGCCRE — obliquity secular variation.
• USNO Earth's Seasons — solstice dates.
• British Antarctic Survey — Antarctic geography and climate.
• Antarctic Treaty Secretariat — the 1959 Treaty.
• NSIDC — Antarctic ice.

For the northern analogue, see /learn/the-arctic-circle; for the southern limit point of polar phenomena, see /learn/the-south-pole; for the broader family of named parallels, see /learn/parallels-of-latitude.