Lines of longitude appearvertical with varying curvature in this projection, but are actually halves of great ellipses, with identical radii at a given latitude.
Lines of latitude appearhorizontal with varying curvature in this projection; but are actually circular with different radii. All locations with a given latitude are collectively referred to as acircle of latitude.
TheMercator projection of aworld map. The angles are untrue for area, especially athigh latitudes. Also note increasing distances between the latitudes towards the poles and the parallellines of longitude. The only true world map is theglobe. The Mercator projection comes from a globe inside acylinder.TheMercator projection and its use on a world map. This projection first came into use in the 16th century by the Dutch.
Circles of latitude are often calledparallels because they areparallel to each other; that is, planes that contain any of these circles neverintersect each other. A location's position along a circle of latitude is given by itslongitude. Circles of latitude are unlike circles of longitude, which are allgreat circles with the centre of Earth in the middle, as the circles of latitude get smaller as the distance from theEquator increases. Their length can be calculated by a commonsine orcosine function. For example, the60th parallel north orsouth is half as long as the Equator (disregarding Earth's minorflattening by 0.335%), stemming from. On theMercator projection or on theGall-Peters projection, a circle of latitude is perpendicular to allmeridians.[1] On the ellipsoid or on spherical projection, all circles of latitude arerhumb lines, except the Equator.
The latitude of the circle is approximately theangle between the Equator and the circle, with the angle'svertex at Earth's centre. The Equator is at 0°, and theNorth Pole andSouth 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 89integral (wholedegree) circles of latitude between the Equator and thepoles in eachhemisphere, but these can be divided into more precise measurements of latitude, and are often represented as a decimal degree (e.g. 34.637° N) or withminutes and seconds (e.g. 22°14′26″ S).
On a map, the circles of latitude may or may not be parallel, and their spacing may vary, depending on whichprojection is used to map the surface of the Earth onto a plane. On anequirectangular 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 aMercator projection the circles of latitude are more widely spaced near the poles to preserve local scales and shapes, while on aGall–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 1884Berlin 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 ofColorado is at41° N while the southern border is at37° N. Roughly half the length of the border between theUnited States andCanada follows49° N.
The five major circles of latitude shown on anequirectangular projection ofEarth.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 15 February 2026:[2]
The equator is the circle that is equidistant from theNorth Pole andSouth Pole. It divides the Earth into theNorthern Hemisphere and theSouthern 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.
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 theJune andDecember solstices 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 theDecember andJune Solstices respectively).
The latitude of the polar circles is equal to 90° minus the Earth'saxial tilt.
The Tropic of Cancer and Tropic of Capricorn mark the northernmost and southernmost latitudes at which the Sun may be seen directly overhead at theJune solstice andDecember solstice respectively.
The latitude of the tropical circles is equal to the Earth's axial tilt.
By definition, the positions of theTropic of Cancer,Tropic of Capricorn,Arctic Circle andAntarctic Circle all depend on thetilt 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° 26′ 21.406″ (according to IAU 2006, theory P03), the corresponding value being 23° 26′ 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, theaverage 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 theTropics, defined astronomically, is decreasing by 1,100 km2 (420 sq mi) per year. (However, the tropical belt as defined based on atmospheric conditions is expanding due toglobal warming.[3])
The Earth's axial tilt has additional shorter-term variations due tonutation, of which the main term, with a period of 18.6 years, has anamplitude of 9.2″ (corresponding to almost 300 m north and south).[4] 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) calledpolar 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.
These circles of latitude can be defined on other planets with axial inclinations relative to their orbital planes. Objects such asPluto with tilt angles greater than 45 degrees will have the tropic circles closer to the poles and the polar circles closer to the equator.
A number of sub-national and internationalborders 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 theNorth Star.
In Canada, the southern border ofYukon with the northern border ofBritish Columbia; the southern border of Northwest Territories with the northern borders of British Columbia,Alberta andSaskatchewan; and the southern border of mainland Nunavut with the northern border ofManitoba, leading to the expression "north of sixty" for the territories.
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 theOregon boundary dispute between Britain and the United States, giving rise to the slogan "Fifty-four forty or fight."
Much ofthe 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.
In the US, the border between Nebraska andKansas. The parallel was originally chosen for theMason–Dixon line, but the line was moved several miles south to avoid bisecting the city ofPhiladelphia.
In the US, the southern border of Utah with the northern border ofArizona. The southern border of Colorado with the northern borders ofNew Mexico andOklahoma. The southern border of Kansas with the northern border of Oklahoma.
The historicMissouri Compromise line (the historic division between slave and free states). In the US, defines part of the border between Oklahoma andTexas, most of the border betweenMissouri andArkansas. Geographically it is a Westward extension of the border betweenVirginia andNorth Carolina and part of the border betweenKentucky andTennessee.
In the US, the southern border of Arkansas, which meets the northern border ofLouisiana, is approximated by the parallel. Historically, it defined the southern border of theLouisiana Territory.
The features of theellipsoid cross-section (orange) in this image are exaggerated with respect to those of the Earth.
Normally the circles of latitude are defined at zeroelevation. Elevation has an effect on a location with respect to theplane formed by a circle of latitude. Since (in thegeodetic system) altitude and depth are determined by thenormal to the Earth's surface, locations sharing the same latitude—but having different elevations (i.e., lying along this normal)—no longer lie within this plane. Rather, all points sharing the same latitude—but of varying elevation and longitude—occupy the surface of a truncatedcone formed by the rotation of this normal around the Earth's axis of rotation.
^Quan, Xiao-Wei; Hoerling, Martin P.; Perlwitz, Judith; Diaz, Henry F.; Xu, Taiyi (March 1, 2014). "How Fast Are the Tropics Expanding?".Journal of Climate.27 (5):1999–2013.Bibcode:2014JCli...27.1999Q.doi:10.1175/JCLI-D-13-00287.1.
