# Antarctic circles of latitude
The [Antarctic Circle](https://www.edgepedia.org/antarctic-circle) is a parallel of latitude at about 66°33′ south of the Equator, marking the northernmost latitude in the [Southern Hemisphere](https://www.edgepedia.org/southern-hemisphere) at which the Sun can remain above the horizon for a full 24 hours and the northernmost at which it can remain below the horizon for a full 24 hours[1](https://www.antarctica.gov.au/about-antarctica/weather-and-climate/weather/sunlight-hours/). Its mirror in the north is the [Arctic Circle](https://www.edgepedia.org/arctic-circle) at 66°33′ N; together the two are called the polar circles[1](https://www.antarctica.gov.au/about-antarctica/weather-and-climate/weather/sunlight-hours/). The precise value is 66°33′48.8″ S[2](https://www.astro4edu.org/resources/glossary/term/424/r491/), and it drifts slightly from year to year as Earth's axial tilt changes[3](https://www.antarcticatourism.org/antarctic-circle-cruise/).
| Key fact | Detail |
|---|---|
| Latitude | 66°33′48.8″ S (about 66.5° S), the southern polar circle[2](https://www.astro4edu.org/resources/glossary/term/424/r491/) |
| Geometric origin | 90° minus Earth's axial tilt of about 23.5° gives 66.5°[4](https://astronomy.swinburne.edu.au/cosmos/A/Antarctic%2BCircle) |
| Daylight meaning | South of the circle the Sun is completely below the horizon on Midwinter's Day (20 or 21 June) and completely above it on Midsummer's Day (21 or 22 December)[1](https://www.antarctica.gov.au/about-antarctica/weather-and-climate/weather/sunlight-hours/) |
| First crossing | James Cook in HMS Resolution, 17 January 1773[5](https://teara.govt.nz/en/antarctica-and-new-zealand/page-1) |
| Long-term movement | Earth's obliquity has varied between 22.1° and 24.5° over the last million years, so the circle's latitude shifts with it[6](https://science.nasa.gov/science-research/earth-science/climate-science/milankovitch-orbital-cycles-and-their-role-in-earths-climate/) |
| Political contrast | The Antarctic Treaty defines Antarctica as all territory below 60° S, about 6.5 degrees of latitude north of the circle[7](https://www.nationalgeographic.com/travel/article/bus2antarctica_antarctic_circl) |
## Definition and geometry
A circle of latitude is an imaginary ring linking all points sharing a parallel[8](https://education.nationalgeographic.org/resource/latitude/). The Antarctic Circle is one of the five named parallels commonly drawn on world maps, alongside the Equator (0°), the [Tropic of Cancer](https://www.edgepedia.org/tropic-of-cancer) (23°27′ N), the [Tropic of Capricorn](https://www.edgepedia.org/tropic-of-capricorn) (23°27′ S), and the Arctic Circle (66°33′ N)[9](https://astro.unl.edu/naap/motion1/tc_units.html).
Its position follows directly from the tilt of [Earth's rotation](https://www.edgepedia.org/earths-rotation) axis. Because Earth's rotational axis is inclined about 23.5° from the perpendicular to the orbital plane, the Sun's noon position at the solstices reaches 23.5° from the Equator, and the polar circles sit at the complement: 90° − 23.5° = 66.5°[4](https://astronomy.swinburne.edu.au/cosmos/A/Antarctic%2BCircle) • [19](https://www.ametsoc.org/ams/education-careers/education-program/k-12-teachers/project-atmosphere/training-opportunities/project-atmosphere-peer-led-training/project-atmosphere-peer-training-resources/sunlight-seasons-module/). The US Naval Observatory gives Earth's obliquity as approximately 23°.44[10](https://aa.usno.navy.mil/faq/asa_glossary), which places the circle at 66.56° S[11](https://www.anglesey.bham.sch.uk/_site/data/files/migrated/geography/4-itk-latitude-longitude-night-and-day.pdf).
## Daylight, midnight sun, and polar night
The circle's practical meaning is astronomical rather than physical; there is nothing marked on the water or ground at the line[3](https://www.antarcticatourism.org/antarctic-circle-cruise/). On the day of the southern summer solstice, around 22 December, an observer standing on the Antarctic Circle sees the Sun above the horizon for a full 24 hours[4](https://astronomy.swinburne.edu.au/cosmos/A/Antarctic%2BCircle). On Midwinter's Day, 20 or 21 June, when Earth's south pole reaches its maximum tilt away from the Sun, the Sun stays completely below the horizon[1](https://www.antarctica.gov.au/about-antarctica/weather-and-climate/weather/sunlight-hours/).
South of the circle these periods lengthen: viewers watch the Sun stay above the horizon for days on end, while the [South Pole](https://www.edgepedia.org/south-pole) gets a six-month stretch of daylight starting at the autumnal equinox, followed by a six-month period of darkness beginning at the vernal equinox.[4](https://astronomy.swinburne.edu.au/cosmos/A/Antarctic%2BCircle) The reverse holds in the Arctic: when the [Northern Hemisphere](https://www.edgepedia.org/northern-hemisphere) has polar nights around the December solstice, areas within the Antarctic Circle experience polar days[12](https://www.timeanddate.com/astronomy/polar-night.html).
Even within the polar night the darkness is graded. Civil twilight, when the Sun's center is up to 6° below the horizon, still allows terrestrial objects to be visible; nautical twilight runs from 6° to 12° below; astronomical twilight, from 12° to 18° below, leaves a sky with no color and no visible horizon[1](https://www.antarctica.gov.au/about-antarctica/weather-and-climate/weather/sunlight-hours/). [Atmospheric refraction](https://www.edgepedia.org/atmospheric-refraction) also lifts the apparent Sun near the horizon, so the observed day is slightly longer than the geometric one[10](https://aa.usno.navy.mil/faq/asa_glossary).
## Movement of the circle over time
Because the circle's latitude equals 90° minus the obliquity, any change in Earth's tilt moves it. NASA reports that obliquity has varied between 22.1° and 24.5° over the last million years, with greater tilt producing more extreme seasons as each hemisphere receives more solar radiation in summer and less in winter[6](https://science.nasa.gov/science-research/earth-science/climate-science/milankovitch-orbital-cycles-and-their-role-in-earths-climate/). These [Milankovitch cycles](https://www.edgepedia.org/milankovitch-cycles), named for the Serbian astronomer Milutin Milankovitch, are confirmed by deep-sea sediment cores and ice cores from [Greenland](https://www.edgepedia.org/greenland) and [Antarctica](https://www.edgepedia.org/antarctica) as drivers of glacial-interglacial cycles over hundreds of thousands of years[6](https://science.nasa.gov/science-research/earth-science/climate-science/milankovitch-orbital-cycles-and-their-role-in-earths-climate/). On human timescales the drift is tiny, but it is enough that reference works quote slightly different values, from 66°30′ S in the 1911 Encyclopædia Britannica[13](https://en.wikisource.org/wiki/1911_Encyclop%C3%A6dia_Britannica/Antarctic) to 66°33′48.8″ S today[2](https://www.astro4edu.org/resources/glossary/term/424/r491/).
## Exploration history
The circle served as a milestone in the European exploration of Antarctica. On 17 January 1773, [James Cook](https://www.edgepedia.org/james-cook) and the crew of HMS Resolution became the first people known to have crossed the Antarctic Circle[5](https://teara.govt.nz/en/antarctica-and-new-zealand/page-1). Cook crossed it again in December 1773, and on 30 January 1774 he reached 71°10′ S, his Farthest South, coming within about 75 miles of the [Antarctic](https://www.edgepedia.org/antarctic) mainland without seeing it[14](https://www.britishantarcticterritory.org.uk/heritage/uk-antarctic-timeline/). His second voyage circumnavigated the continent through the [Southern Ocean](https://www.edgepedia.org/southern-ocean), reaching south of 60° S in four areas and crossing the circle three times, without sighting land[15](https://mawsonshuts.antarctica.gov.au/preparation/introduction/).
Later expeditions used the circle as a reference point in their reports. During 1839–40, the American expedition under Charles Wilkes made repeated reports of land near the Antarctic Circle, spanning the longitudes of 157°46′ E to 106°19′ E[16](https://www.antarctica.gov.au/about-antarctica/history/exploration-and-expeditions/australasian-antarctic-expedition/home-of-the-blizzard-book/appendix3/). In 1874 the Challenger scientific expedition under George Strong Nares penetrated within the Antarctic Circle in longitude 78°22′ E during its circumnavigation[16](https://www.antarctica.gov.au/about-antarctica/history/exploration-and-expeditions/australasian-antarctic-expedition/home-of-the-blizzard-book/appendix3/).
The first people to winter south of the circle did so involuntarily. Adrien de Gerlache's Belgian expedition of 1897–98 was frozen into the pack ice and drifted for a whole year south of the Antarctic Circle; its members were the first to experience an Antarctic winter[16](https://www.antarctica.gov.au/about-antarctica/history/exploration-and-expeditions/australasian-antarctic-expedition/home-of-the-blizzard-book/appendix3/).
## The circle in travel and law
The circle is not a political boundary. The Antarctic Treaty defines Antarctica as all territories below 60° S, well north of the circle[7](https://www.nationalgeographic.com/travel/article/bus2antarctica_antarctic_circl). In tourism, however, crossing it is a distinct milestone: the great majority of Antarctic voyages turn around well before 66°33′ S, and circle itineraries typically run 13 to 16 nights against 10 to 12 for a standard peninsula voyage, with the extra days spent steaming south along the western Antarctic Peninsula, often crossing near Crystal Sound[3](https://www.antarcticatourism.org/antarctic-circle-cruise/) • [17](https://www.swoop-antarctica.com/cruises/circle). About 98 percent of all tourism voyages occur in the Antarctic Peninsula region during the austral summer from October to April[18](https://www.ats.aq/e/tourism.html), and a crossing is always dependent on sea ice and weather[3](https://www.antarcticatourism.org/antarctic-circle-cruise/).
## References
1. [Sunlight hours in Antarctica, Australian Antarctic Program](https://www.antarctica.gov.au/about-antarctica/weather-and-climate/weather/sunlight-hours/)
2. [Glossary term: Polar Circle, astro4edu](https://www.astro4edu.org/resources/glossary/term/424/r491/)
3. [Antarctic Circle Cruise: Crossing the Polar Line, Antarctica Tourism](https://www.antarcticatourism.org/antarctic-circle-cruise/)
4. [Antarctic Circle, COSMOS, Swinburne University](https://astronomy.swinburne.edu.au/cosmos/A/Antarctic%2BCircle)
5. [Voyages of discovery, Te Ara Encyclopedia of New Zealand](https://teara.govt.nz/en/antarctica-and-new-zealand/page-1)
6. [Milankovitch (Orbital) Cycles and Their Role in Earth's Climate, NASA Science](https://science.nasa.gov/science-research/earth-science/climate-science/milankovitch-orbital-cycles-and-their-role-in-earths-climate/)
7. [Bus2Antarctica: Crossing the Antarctic Circle, National Geographic](https://www.nationalgeographic.com/travel/article/bus2antarctica_antarctic_circl)
8. [Latitude, National Geographic Education](https://education.nationalgeographic.org/resource/latitude/)
9. [Units of Longitude and Latitude, NAAP, University of Nebraska-Lincoln](https://astro.unl.edu/naap/motion1/tc_units.html)
10. [Glossary, Astronomical Almanac, US Naval Observatory](https://aa.usno.navy.mil/faq/asa_glossary)
11. [Longitude, latitude, night and day teaching materials, Anglesey](https://www.anglesey.bham.sch.uk/_site/data/files/migrated/geography/4-itk-latitude-longitude-night-and-day.pdf)
12. [Polar Night — The Opposite of Midnight Sun, timeanddate.com](https://www.timeanddate.com/astronomy/polar-night.html)
13. [Antarctic, 1911 Encyclopædia Britannica, Wikisource](https://en.wikisource.org/wiki/1911_Encyclop%C3%A6dia_Britannica/Antarctic)
14. [UK Antarctic Timeline, British Antarctic Territory](https://www.britishantarcticterritory.org.uk/heritage/uk-antarctic-timeline/)
15. [Preparation: Introduction, Home of the Blizzard, Mawson's Huts](https://mawsonshuts.antarctica.gov.au/preparation/introduction/)
16. [The Home of the Blizzard (1915), Appendix III, Australian Antarctic Program](https://www.antarctica.gov.au/about-antarctica/history/exploration-and-expeditions/australasian-antarctic-expedition/home-of-the-blizzard-book/appendix3/)
17. [Antarctic Circle Cruises, Swoop Antarctica](https://www.swoop-antarctica.com/cruises/circle)
18. [Tourism and Non-Governmental Activities, Antarctic Treaty Secretariat](https://www.ats.aq/e/tourism.html)
19. [Sunlight Seasons Module, ametsoc.org](https://www.ametsoc.org/ams/education-careers/education-program/k-12-teachers/project-atmosphere/training-opportunities/project-atmosphere-peer-led-training/project-atmosphere-peer-training-resources/sunlight-seasons-module/)
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*Topic: Encyclopedia › Physical world and mathematics › Earth sciences › Earth systems and geophysics*
*Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 15, 2026; Oct 25, 2026 · Last review: Oct 15, 2026*
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License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license