Editing Circle of latitude

{{Use British English|date = April 2020}}
{{Short description|Geographic notion}}
{{Use mdy dates|date = July 2017}}
{{More citations needed|date=April 2012}}
{{longlat}}{{Geodesy|notshowfig=1}}
[[File:Tissot indicatrix world map Mercator proj.svg|thumb|right|The [[Mercator projection]] of a [[world map]]. The angles are untrue for area, especially at [[polar regions of Earth|high latitudes]]. Also note increasing distances between the latitudes towards the poles and the parallel [[meridian (geography)|lines of longitude]]. The only true world map is the [[globe]]. The Mercator projection comes from a globe inside a [[cylinder]].]]
[[File:Usgs map mercator.svg|thumb|right|The [[Mercator projection]] and its use on a world map. This projection first came into use in the 16th century by the Dutch.]]

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''' because they are [[Parallel (geometry)|parallel]] to each other; that is, planes that contain any of these circles never [[Intersection|intersect]] each other. A location's position along a circle of latitude is given by its [[longitude]]. Circles of latitude are unlike circles of longitude, which are all [[great circle]]s with the centre of Earth in the middle, as the circles of latitude get smaller as the distance from the [[Equator]] increases. Their length can be calculated by a common [[sine]] or [[cosine]] function. For example, the [[60th parallel north]] or [[60th parallel south|south]] is half as long as the Equator (disregarding Earth's minor [[flattening]] by 0.335%), stemming from <math>\cos(60^{\circ}) = 0.5</math>. On the [[Mercator projection]] or on the [[Gall-Peters projection]], a circle of latitude is perpendicular to all [[meridian (geography)|meridian]]s.<ref>{{cite web |last=Kher |first=Aparna |title=What Are Longitudes and Latitudes? |url=https://www.timeanddate.com/geography/longitude-latitude.html |website=timeanddate.com |access-date=15 April 2020}}</ref> On the ellipsoid or on spherical projection, all circles of latitude are [[rhumb line]]s, except the Equator. 

The latitude of the circle is approximately the [[angle]] between the Equator and the circle, with the angle's [[vertex (geometry)|vertex]] at Earth's centre. The Equator is at 0°, and the [[North Pole]] and [[South Pole]] are at 90° north and 90° south, respectively. The Equator is the longest circle of latitude and is the only circle of latitude which also is a great circle. As such, it is perpendicular to all meridians.

There are 89 [[integer|integral]] (whole [[Degree (angle)|degree]]) circles of latitude between the Equator and the [[Geographical pole|poles]] in each [[Hemispheres of Earth|hemisphere]], but these can be divided into more precise measurements of latitude, and are often represented as a decimal degree (e.g. 34.637°&nbsp;N) or with [[Minute of arc|minutes and seconds]] (e.g. 22°14′26″&nbsp;S).

On a map, the circles of latitude may or may not be parallel, and their spacing may vary, depending on which [[Map projection|projection]] is used to map the surface of the Earth onto a plane. On an [[equirectangular projection]], centered on the equator, the circles of latitude are horizontal, parallel, and equally spaced. On other cylindrical and pseudocylindrical projections, the circles of latitude are horizontal and parallel, but may be spaced unevenly to give the map useful characteristics. For instance, on a [[Mercator projection]] the circles of latitude are more widely spaced near the poles to preserve local scales and shapes, while on a [[Gall–Peters projection]] the circles of latitude are spaced more closely near the poles so that comparisons of area will be accurate. On most non-cylindrical and non-pseudocylindrical projections, the circles of latitude are neither straight nor parallel.

Arcs of circles of latitude are sometimes used as boundaries between countries or regions where distinctive natural borders are lacking (such as in deserts), or when an artificial border is drawn as a "line on a map", which was made in massive scale during the 1884 [[Berlin Conference]], regarding huge parts of the African continent. North American nations and states have also mostly been created by straight lines, which are often parts of circles of latitudes. For instance, the northern border of [[Colorado]] is at [[41° N|41°&nbsp;N]] while the southern border is at [[37° N|37°&nbsp;N]]. Roughly half the length of the border between the [[United States]] and [[Canada]] follows [[49° N|49°&nbsp;N]].

==Major circles of latitude==
[[Image:World map with major latitude circles.svg|thumb|300px|The five major circles of latitude shown on an [[equirectangular projection]] of [[Earth]].]]
<!--
[[Image:Earth-lighting-summer-solstice EN.png|thumb|300px|Diagram showing the derivation of the major circles of latitude on the [[Earth]].]]
-->
[[File:axial_tilt_vs_tropical_and_polar_circles.svg|thumb|400px|Relationship of Earth's axial tilt (ε) to the tropical and polar circles]]

There are five major circles of latitude, listed below from north to south. The position of the Equator is fixed (90 degrees from Earth's axis of rotation) but the latitudes of the other circles depend on the tilt of this axis relative to the plane of Earth's orbit, and so are not perfectly fixed. The values below are for {{TODAY}}:<ref>{{cite web|url=https://groups.google.com/a/googleproductforums.com/forum/?fromgroups#!msg/gec-geology-moderated/CMXesn-SROc/mEqdXT1LaLoJ |title=''Trópico en movimiento'' (in Spanish) |access-date=2014-05-13}}</ref>

* [[Arctic Circle]] ({{Circle of latitude|Polar}}&nbsp;N)
* [[Tropic of Cancer]] ({{Circle of latitude|Tropical}}&nbsp;N)
* [[Equator]] (0°&nbsp;latitude)
* [[Tropic of Capricorn]] ({{Circle of latitude|Tropical}}&nbsp;S)
* [[Antarctic Circle]] ({{Circle of latitude|Polar}}&nbsp;S)

These circles of latitude, excluding the Equator, mark the divisions between the five principal [[geographical zone]]s.

===Equator===
{{main|Equator}}
The equator is the circle that is equidistant from the [[North Pole]] and [[South Pole]]. It divides the Earth into the [[Northern Hemisphere]] and the [[Southern Hemisphere]]. Of the parallels or circles of latitude, it is the longest, and the only '[[great circle]]' (a circle on the surface of the Earth, centered on Earth's center). All the other parallels are smaller and centered only on Earth's axis.

{| class="wikitable"
|-
|[[Image:World map with equator.svg|300px]]
|-
|[[Equator]]
|}

===Polar circles===
{{main|Polar circle}}
The Arctic Circle is the southernmost latitude in the Northern Hemisphere at which the Sun can remain continuously above or below the horizon for 24 hours (at the [[June solstice|June]] and [[December solstice]]s respectively). Similarly, the Antarctic Circle marks the northernmost latitude in the Southern Hemisphere at which the Sun can remain continuously above or below the horizon for 24 hours (at the [[December solstice|December]] and [[June Solstice]]s respectively).

The latitude of the polar circles is equal to 90° minus the Earth's [[axial tilt]].

{|
|-
|
{| class="wikitable"
|-
|[[Image:World map with arctic circle.svg|300px]]
|-
|[[Arctic Circle]]
|}
| style="width:20px;"|&nbsp;
|
{| class="wikitable"
|-
|[[Image:World map with antarctic circle.svg|300px]]
|-
|[[Antarctic Circle]]
|}
|}

===Tropical circles===
{{main|Tropic of Cancer|Tropic of Capricorn}}
The Tropic of Cancer and Tropic of Capricorn mark the northernmost and southernmost latitudes at which the Sun may be seen directly overhead at the [[June solstice]] and [[December solstice]] respectively.

The latitude of the tropical circles is equal to the Earth's axial tilt.

{|
|-
|
{| class="wikitable"
|-
|[[Image:World map with tropic of cancer.svg|300px]]
|-
|[[Tropic of Cancer]]
|}
| style="width:20px;"|&nbsp;
|
{| class="wikitable"
|-
|[[Image:World map with tropic of capricorn.svg|300px]]
|-
|[[Tropic of Capricorn]]
|}
|}

===Movement of the Tropical and Polar Circles<span class="anchor" id="Movement of the Tropical and Polar circles"></span>===
<!--This section is linked from 'Arctic Circle' and 'Antarctic Circle'-->
{{See also|Axial tilt}}
By definition, the positions of the [[Tropic of Cancer]], [[Tropic of Capricorn]], [[Arctic Circle]] and [[Antarctic Circle]] all depend on the [[Axial tilt|tilt of the Earth's axis]] relative to the plane of its orbit around the Sun (the "obliquity of the ecliptic"). If the Earth were "upright" (its axis at right angles to the orbital plane) there would be no Arctic, Antarctic, or Tropical circles: at the poles the Sun would always circle along the horizon, and at the equator the Sun would always rise due east, pass directly overhead, and set due west.

The positions of the Tropical and Polar Circles are not fixed because the axial tilt changes slowly – a complex motion determined by the superimposition of many different cycles (some of which are described below) with short to very long periods. At noon of January 1st 2000 AD, the mean value of the tilt was 23°&nbsp;26′&nbsp;21.406″ (according to IAU 2006, theory P03), the corresponding value being 23°&nbsp;26′&nbsp;10.633″ at noon of January 1st 2023 AD. 

The main long-term cycle causes the axial tilt to fluctuate between about 22.1° and 24.5° with a period of 41,000 years. Currently, the ''average'' value of the tilt is decreasing by about 0.468″ per year. As a result (approximately, and on average), the Tropical Circles are drifting towards the equator (and the Polar Circles towards the poles) by 15 m per year, and the area of the [[Tropics]], defined astronomically, is decreasing by {{convert|1100|km2|abbr=on}} per year. (However, the tropical belt as defined based on atmospheric conditions is expanding due to [[global warming]].<ref>{{cite journal |last1=Quan |first1=Xiao-Wei |last2=Hoerling |first2=Martin P. |last3=Perlwitz |first3=Judith |last4=Diaz |first4=Henry F. |last5=Xu |first5=Taiyi |title=How Fast Are the Tropics Expanding? |journal=Journal of Climate |date=1 March 2014 |volume=27 |issue=5 |pages=1999–2013 |doi=10.1175/JCLI-D-13-00287.1 |bibcode=2014JCli...27.1999Q }}</ref>)

The Earth's axial tilt has additional shorter-term variations due to [[astronomical nutation|nutation]], of which the main term, with a period of 18.6 years, has an [[amplitude]] of 9.2″ (corresponding to almost 300 m north and south).<ref>{{Cite web|url = http://www2.jpl.nasa.gov/basics/bsf2-1.php#nutation|title = Basics of Space Flight, Chapter 2|date = 2013-10-29|access-date = 2015-03-26|website = Jet Propulsion Laboratory|publisher = Jet Propulsion Laboratory/NASA}}</ref> There are many smaller terms, resulting in varying daily shifts of some metres in any direction.

Finally, the Earth's rotational axis is not exactly fixed in the Earth, but undergoes small fluctuations (on the order of 15 m) called [[polar motion]], which have a small effect on the Tropics and Polar Circles and also on the Equator.

Short-term fluctuations over a matter of days do not directly affect the location of the extreme latitudes at which the Sun may appear directly overhead, or at which 24-hour day or night is possible, except when they actually occur at the time of the solstices. Rather, they cause a theoretical shifting of the parallels, that would occur if the given axis tilt were maintained throughout the year.

==== Other planets ====

These circles of latitude can be defined on other planets with axial inclinations relative to their orbital planes. Objects such as [[Pluto]] with tilt angles greater than 45 degrees will have the tropic circles closer to the poles and the polar circles closer to the equator.

==Other notable parallels==
{{see also|Baseline (surveying)}}

A number of sub-national and international [[border]]s were intended to be defined by, or are approximated by, parallels. Parallels make convenient borders in the northern hemisphere because astronomic latitude can be roughly measured (to within a few tens of metres) by sighting the [[Polaris|North Star]].

{| class="wikitable"
|-
! Parallel !! Description
|- valign="center"
| [[81st parallel north|81°N]]
| rowspan="2" | In [[Svalbard]], [[Norway]], the northern and southern limits of the area comprised by the [[Svalbard Treaty]] of 9 February 1920.
|- valign="top"
| [[74th parallel north|74°N]]
|- valign="top"
| [[70th parallel north|70°N]]
| On [[Victoria Island (Canada)|Victoria Island]], Canada, two sections of the border between [[Northwest Territories]] and [[Nunavut]].
|- valign="top"
| [[60th parallel north|60°N]]
| In Canada, the southern border of [[Yukon]] with the northern border of [[British Columbia]]; the southern border of Northwest Territories with the northern borders of British Columbia, [[Alberta]] and [[Saskatchewan]]; and the southern border of mainland Nunavut with the northern border of [[Manitoba]], leading to the expression "north of sixty" for the territories.
|- valign="top"
| [[Parallel 54°40′ north|54°40′N]]
| The border between 19th century Russian territories to the north and conflicting American and British land claims in western North America. The conflicting claims led to the [[Oregon boundary dispute]] between Britain and the United States, giving rise to the slogan "Fifty-four forty or fight."
|- valign="top"
| [[52nd parallel north|52°N]]
| In Canada, part of the border between [[Newfoundland and Labrador]] and [[Quebec]].
|- valign="top"
| [[51st parallel north|51°N]]
| The southern limit of [[Russian America]] from 1799 to 1821.
|- valign="top"
|[[49th parallel north|49°N]]
| Much of [[Canada–United States border|the border]] between Canada and the United States, from British Columbia to Manitoba; "49th parallel" is a common expression for the border, though the majority of Canada's population actually lives south of the parallel.
|- valign="top"
|[[48th parallel north|48°N]]
| In Canada, part of the border between Quebec and [[New Brunswick]].
|- valign="top"
|[[46th parallel north|46°N]]
| In the United States, part of the border between [[Washington (state)|Washington]] and [[Oregon]].
|- valign="top"
| [[45th parallel north|45°N]]
| Approximates the portion of the [[Canada–United States border]] between [[Quebec]] (Canada) and [[New York (state)|New York]] and [[Vermont]] (US). Also approximates most of the border between [[Montana]] and [[Wyoming]].
|- valign="top"
| 43°30′N
| In the US, the border between [[Minnesota]] and [[Iowa]].
|- valign="top"
| [[43rd parallel north|43°N]]
| In the US, much of the border between [[South Dakota]] and [[Nebraska]].
|- valign="top"
| 42°30′N
| In the US, the border between [[Wisconsin]] and [[Illinois]].
|- valign="top"
| [[42nd parallel north|42°N]]
| Originally the northward limit of [[New Spain]].  In the US, the southern borders of [[Oregon]] and [[Idaho]] where they meet the northern borders of [[California]], [[Nevada]] and [[Utah]]. The parallel also defines much of the border between [[Pennsylvania]] and [[New York (state)|New York]].
|- valign="top"
| [[41st parallel north|41°N]]
| In the US, part of the border between Wyoming and Utah, the border between Wyoming and [[Colorado]], and part of the border between Nebraska and Colorado.
|- valign="top"
| [[40th parallel north|40°N]]
| In the US, the border between Nebraska and [[Kansas]]. The parallel was originally chosen for the [[Mason–Dixon line]], but the line was moved several miles south to avoid bisecting the city of [[Philadelphia]].
|- valign="top"
| [[38th parallel north|38°N]]
| The boundary between the [[Soviet Union|Soviet]] and [[United States of America|American]] occupation zones in Korea, and later between [[North Korea]] and [[South Korea]], from 1945 until the [[Korean War]] (1950–1953).
|- valign="top"
| [[37th parallel north|37°N]]
| In the US, the southern border of Utah with the northern border of [[Arizona]]. The southern border of Colorado with the northern borders of [[New Mexico]] and [[Oklahoma]]. The southern border of Kansas with the northern border of Oklahoma.
|- valign="top"
| [[Parallel 36°30′ north|36°30′N]]
| [[Image:Missouri Compromise Line.svg|thumb|100px]]The historic [[Missouri Compromise]] line (the historic division between slave and free states). In the US, defines part of the border between Oklahoma and [[Texas]], most of the border between [[Missouri]] and [[Arkansas]]. Geographically it is a Westward extension of the border between [[Virginia]] and [[North Carolina]] and part of the border between [[Kentucky]] and [[Tennessee]].
|- valign="top"
| [[36th parallel north|36°N]]
| In the US, a short section of the border between the [[Missouri Bootheel]] and Arkansas.
|- valign="top"
| [[35th parallel north|35°N]]
| In the US, the southern border of Tennessee, which meets [[Mississippi]], [[Alabama]] and [[Georgia (U.S. state)|Georgia]]. Also, part of the border between North Carolina and Georgia.
|- valign="top"
| [[33rd parallel north|33°N]]
| In the US, the southern border of Arkansas, which meets the northern border of [[Louisiana]], is approximated by the parallel. Historically, it defined the southern border of the [[Louisiana Territory]].
|- valign="top"
| [[32nd parallel north|32°N]]
| In the US, part of the border between New Mexico and Texas.
|- valign="top"
| 31°20′N
| Part of the border between the US and [[Mexico]] ([[Sonora]] and [[Chihuahua (state)|Chihuahua]]); the southern border of Arizona and the [[New Mexico Bootheel]].
|- valign="top"
| [[31st parallel north|31°N]]
| Part of the border between [[Iran]] and [[Iraq]]. In the US, part of the border between [[Mississippi]] and [[Louisiana]], and part of the border between [[Alabama]] and [[Florida]].
|- valign="top"
| [[28th parallel north|28°N]]
| In Mexico, the border between [[Baja California]] and [[Baja California Sur]].
|- valign="top"
| [[26th parallel north|26°N]]
| Part of the border between [[Western Sahara]] (claimed by [[Morocco]]) and [[Mauritania]].
|- valign="top"
| [[25th parallel north|25°N]]
| Part of the border between Mauritania and [[Mali]].
|- valign="top"
| [[22nd parallel north|22°N]]
| Much of the border between [[Egypt]] and [[Sudan]], partly disputed (see also [[Hala'ib Triangle]]).
|- valign="top"
| [[20th parallel north|20°N]]
| A short section of the border between [[Libya]] and Sudan, and within Sudan, the northern border of the [[Darfur]] region.
|- valign="top"
| [[17th parallel north|17°N]]
| The division between Republic of Vietnam ([[South Vietnam]]) and Democratic Republic of Vietnam ([[North Vietnam]]) during the [[Vietnam War]].
|- valign="top"
| [[15th parallel north|15°N]]
| ''de facto'' maritime border between [[Honduras]] and [[Nicaragua]].<ref>{{cite web|url=http://www.icj-cij.org/docket/files/120/13746.pdf|title=Maritime Delimitation between Nicaragua and Honduras in the Caribbean Sea (Nicaragua v. Honduras)|access-date=2014-05-13|archive-url=https://web.archive.org/web/20150924042116/http://www.icj-cij.org/docket/files/120/13746.pdf|archive-date=2015-09-24|url-status=dead}}</ref>
|- valign="top"
| 13°05′N
| Part of the border between [[Chad]] and [[Cameroon]], over a stretch of 41.6&nbsp;km, partly in [[Lake Chad]]
|- valign="top"
| [[10th parallel north|10°N]]
| Part of the border between [[Guinea]] and [[Sierra Leone]].
|- valign="top"
| [[8th parallel north|8°N]]
| Part of the border between [[Somalia]] and [[Ethiopia]].
|- valign="top"
| [[1st parallel north|1°N]]
| Part of the border between [[Equatorial Guinea]] and [[Gabon]].
|- valign="top"
| [[1st parallel south|1°S]]
| Most of the border between [[Uganda]] and [[Tanzania]], and a very short section of the border between [[Kenya]] and Tanzania in [[Lake Victoria]].
|- valign="top"
| [[7th parallel south|7°S]]
| A short section of the border between [[Democratic Republic of the Congo]] and [[Angola]].
|- valign="top"
| [[8th parallel south|8°S]]
| Two short sections of the border between Democratic Republic of the Congo and Angola.
|- valign="top"
| [[10th parallel south|10°S]]
| A short section of the border between [[Brazil]] and [[Peru]].
|- valign="top"
| [[13th parallel south|13°S]]
| Part of the border between Angola and [[Zambia]].
|- valign="top"
| [[16th parallel south|16°S]]
| Part of the border between [[Mozambique]] and [[Zimbabwe]].
|- valign="top"
| [[22nd parallel south|22°S]]
| A short section of the border between [[Namibia]] and [[Botswana]], and parts of the border between [[Bolivia]] and [[Argentina]].
|- valign="top"
| [[26th parallel south|26°S]]
| In [[Australia]], the border between [[South Australia]] and the [[Northern Territory]], and part of the border between South Australia and [[Queensland]].
|- valign="top"
| [[28th parallel south|28°S]]
| In Argentina, the border between [[Chaco Province]] and [[Santa Fe Province]].
|- valign="top"
| [[29th parallel south|29°S]]
| In Australia, much of the border between Queensland and [[New South Wales]].
|- valign="top"
| [[35th parallel south|35°S]]
| In Argentina, part of the border between [[Córdoba Province, Argentina|Córdoba Province]] and [[La Pampa Province]].
|- valign="top"
| [[36th parallel south|36°S]]
| In Argentina, part of the border between [[Mendoza Province]] and La Pampa Province, and part of the border between [[San Luis Province]] and La Pampa Province.
|- valign="top"
| [[42nd parallel south|42°S]]
| In Argentina, the border between [[Río Negro Province]] and [[Chubut Province]].
|- valign="top"
| [[46th parallel south|46°S]]
| In Argentina, the border between Chubut Province and [[Santa Cruz Province (Argentina)|Santa Cruz Province]].
|- valign="top"
| [[52nd parallel south|52°S]]
| Part of the border between Argentina and [[Chile]].
|- valign="top"
| [[60th parallel south|60°S]]
| The northern boundary of [[Antarctica]] for the purposes of the [[Antarctic Treaty System]] (see [http://sd-www.jhuapl.edu/FlareGenesis/Antarctica/1999/pictures/antarctica_pol_map.jpg map]). The northern boundary of the [[Southern Ocean]].
|}

==Elevation==
[[Image:Circle of latitude elevation.svg|thumb|right|256px|The features of the [[ellipsoid]] cross-section (orange) in this image are exaggerated with respect to those of the Earth.]]
Normally the circles of latitude are defined at zero [[elevation]]. Elevation has an effect on a location with respect to the [[Plane (mathematics)|plane]] formed by a circle of latitude. Since (in the [[Geodetic datum|geodetic system]]) altitude and depth are determined by the [[surface normal|normal]] to the Earth's surface, locations sharing the same latitude&mdash;but having different elevations (i.e., lying along this normal)&mdash;no longer lie within this plane. Rather, all points sharing the same latitude&mdash;but of varying elevation and longitude&mdash;occupy the surface of a truncated [[Conical surface|cone]] formed by the rotation of this normal around the Earth's axis of rotation.

==See also==
* [[Circles of a sphere]]
* [[List of circles of latitude]]

==References==
{{Reflist}}

==External links==
*[https://web.archive.org/web/20071012021011/http://aa.usno.navy.mil/faq/docs/SunApprox.php U.S. Naval Observatory – mean obliquity of the ecliptic]. {{Webarchive|url=https://web.archive.org/web/20071012021011/http://aa.usno.navy.mil/faq/docs/SunApprox.php |date=October 12, 2007 }}.
{{Geographical coordinates}}
{{Greek astronomy}}
{{Authority control}}

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[[Category:Circles of latitude| ]]