A circle of latitude on Earth is an abstract east–west circle connecting all locations around Earth at a given latitude. Circles of latitude are often called parallels because they are parallel to each other; that is, any two circles are always the same distance apart. 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 circles 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. The 60th parallel north or south is half as long as the Equator. A circle of latitude is perpendicular to all meridians. The latitude of the circle is approximately the angle between the Equator and the circle, with the angle's 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. There are 89 integral circles of latitude between the equator and the Poles in each hemisphere, but these can be divided into more precise measurements of latitude, and are often represented as a decimal degree or with minutes and seconds. Latitude can be in theory measured down to planck length, and so there are vast number of circles of latitude on Earth. On a map, the circles of latitude may or may not be parallel, and their spacing may vary, depending on which 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, 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 while the southern border is at 37°N. Roughly half the length of border between the United States and Canada follows 49°N.
Major circles of latitude
There are five major circles of latitude, listed below from north to south. The position of the Equator is fixed 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 :
These circles of latitude, excluding the Equator, mark the divisions between the five principal geographical zones.
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'. All the other parallels are smaller and centered only on Earth's axis.
Polar Circles
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. 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. The latitude of the polar circles is equal to 90° less the Earth's axial tilt.
Tropical Circles
The Tropic of Cancer and Tropic of Capricorn mark the northernmost and southernmost latitudes at which the sun may be seen directly overhead. The latitude of the tropical circles is equal to the Earth's axial tilt.
Movement of the Tropical and Polar Circles
By definition, the positions of the Tropic of Cancer, Tropic of Capricorn, Arctic Circle and Antarctic Circle all depend on the tilt of the Earth's axis relative to the plane of its orbit around the sun. If the Earth were "upright" 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 with short to very long periods. In the year 2000 AD the mean value of the tilt was about 23° 26′ 21″. 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 0.47″ per year. As a result, the Tropical Circles are drifting towards the equator by 15 metres per year, and the area of the Tropics is decreasing by per year. The Earth's axial tilt has additional shorter-term variations due to nutation, of which the main term, with a period of 18.6 years, has an amplitude of 9.2". 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 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
A number of sub-national and international borders 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 by sighting the North Star.
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.
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."
Much of 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.
48°N
In Canada, part of the border between Quebec and New Brunswick.
46°N
In the United States, part of the border between Washington and Oregon.
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.
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.
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.
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.
In the US, a short section of the border between the Missouri Bootheel and Arkansas.
35°N
In the US, the southern border of Tennessee, which meets Mississippi, Alabama and Georgia. Also, part of the border between North Carolina and Georgia.
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.
32°N
In the US, part of the border between New Mexico and Texas.
31°20'N
Part of the border between the US and Mexico ; the southern border of Arizona and the New Mexico Bootheel.
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.
Elevation has an effect on a location with respect to the plane formed by a circle of latitude. Since altitude and depth are determined by the normal to the Earth's surface, locations sharing the same latitude—but having different elevations —no longer lie within this plane. Rather, all points sharing the same latitude—but of varying elevation and longitude—occupy the surface of a truncated cone formed by the rotation of this normal around the Earth's axis of rotation.
