Editing Circle of latitude (section)

==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]].]]
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[[Image:Earth-lighting-summer-solstice EN.png|thumb|300px|Diagram showing the derivation of the major circles of latitude on the [[Earth]].]]
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[[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"
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|[[Equator]]
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===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]].

{|
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|[[Image:World map with arctic circle.svg|300px]]
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|[[Arctic Circle]]
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|[[Image:World map with antarctic circle.svg|300px]]
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|[[Antarctic Circle]]
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===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.

{|
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{| class="wikitable"
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|[[Image:World map with tropic of cancer.svg|300px]]
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|[[Tropic of Cancer]]
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{| class="wikitable"
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|[[Image:World map with tropic of capricorn.svg|300px]]
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|[[Tropic of Capricorn]]
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===Movement of the Tropical and Polar Circles<span class="anchor" id="Movement of the Tropical and Polar circles"></span>===
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{{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.