A circle of latitude, on the Earth, is an imaginary east-west circle connecting all locations (not taking into account elevation) that share a given latitude. A location's position along a circle of latitude is given by its longitude.
Circles of latitude are often called parallels because they are parallel to each other. On some map projections, including the Mercator projection, they are drawn at equidistant intervals.
Circles of latitude become smaller the farther they are from the equator and the closer they are to the poles. A circle of latitude is perpendicular to all meridians at the points of intersection, and is hence a special case of a loxodrome.
Contrary to what might be assumed from their straight-line representation on some map projections, a circle of latitude is not, with the sole exception of the Equator, the shortest distance between two points lying on the Earth. In other words, circles of latitude (except for the Equator) are not great circles, and are not really "lines" in the geometric sense (see also great-circle distance). It is for this reason that an airplane traveling between a European and North American city that share the same latitude will fly farther north, over Greenland for example.
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", as happened in Korea.
The five major circles of latitude are, from north to south:
These circles of latitude (excluding the equator) mark the divisions between the five principal geographical zones.
The latitude of these circles plus the Earth's axial tilt is equal to 90°.
The latitude of the tropic circles is equal to the Earth's axial tilt.
The main long-term cycle causes the axial tilt to fluctuate between about 22.5° and 24.5° with a 41,000 year periodicity. As a consequence of this cycle the average value of the tilt is currently decreasing by about 0.47″ per year. This causes the Tropics of Cancer and Capricorn to drift towards the equator by about 15 metres per year, and the Arctic and Antarctic Circles to drift towards the Poles by the same amount. As a result of the movement of the Tropics of Cancer and Capricorn, the area of the Tropics decreases worldwide by about 1100 square kilometres per year on average.
The Earth's axial tilt is subject to additional shorter-term variations due to nutation, of which the main term, with a period of 18.7 years, has an amplitude of 9"21 (corresponding to almost 300 metres north and south). There are then still many smaller terms, resulting in varying daily shifts of some metres in any direction.
Finally, the Earth's rotational axis is not exactly fixed with respect to the Earth, but undergoes very small fluctuations, called polar motion, which have a small theoretical effect on the positions of the abovementioned parallels.
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.
|70°N||On Victoria Island, , two sections of the border between Northwest Territories and Nunavut.|
|60°N||In , 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).|
|54°40'N||The border between 19th century Russian and British land claims in western North America which played a role in the Oregon boundary dispute between Britain and the United States, giving rise to the slogan "Fifty-four forty or fight."|
|52°N||In , part of the border between Newfoundland and Labrador and Quebec.|
|49°N||Much of the border between and the , from Washington to western Minnesota.|
|45°N||The theoretical halfway point between the Equator and the North Pole. Part of the border between (Quebec) and the (New York and Vermont). Also, in the , most of the border between Montana and Wyoming.|
|43°N||In the , much of the border between South Dakota and Nebraska.|
|42°N||In the , 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 , 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 , 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.|
|38°N||The boundary between the Soviet and American occupation zones in Korea in 1945.|
|37°N||In the , 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.|
|36°30'N||The historic Missouri Compromise line. In the , defines part of the border between Oklahoma and Texas, most of the border between Missouri and Arkansas, and part of the border between Kentucky and Tennessee.|
|36°N||In the , a short section of the border between the Missouri Bootheel and Arkansas.|
|35°N||In the , 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 , the southern border of Arkansas, which meets the northern border of Louisiana. Historically, it defined the southern border of the Louisiana Territory.|
|32°N||In the , part of the border between New Mexico and Texas.|
|31°N||Part of the border between and . In the , part of the border between Mississippi and Louisiana, and part of the border between Alabama and Florida.|
|28°N||In , the border between Baja California and Baja California Sur.|
|26°N||Part of the border between Western Sahara (claimed by ) and .|
|25°N||Part of the border between and .|
|22°N||Much of the border between and , partly disputed (see also Hala'ib Triangle).|
|20°N||A short section of the border between and , and within Sudan, the northern border of the Darfur region.|
|17°N||The division between Republic of Vietnam (South Vietnam) and Democratic Republic of Vietnam (North Vietnam) during the Vietnam War.|
|10°N||Part of the border between and .|
|1°N||Part of the border between and .|
|1°S||Most of the border between and , and a very short section of the border between and in Lake Victoria.|
|7°S||A short section of the border between and .|
|8°S||Two short sections of the border between and .|
|10°S||A short section of the border between and .|
|13°S||Part of the border between and .|
|22°S||A short section of the border between and , and parts of the border between and .|
|26°S||In , the border between South Australia and the Northern Territory, and part of the border between South Australia and Queensland.|
|29°S||In , much of the border between Queensland and New South Wales.|
|35°S||In , part of the border between Córdoba Province and La Pampa Province.|
|36°S||In , part of the border between Mendoza Province and La Pampa Province.|
|42°S||In , the border between Río Negro Province and Chubut Province.|
|45°S||The theoretical halfway point between the Equator and the South Pole.|
|46°S||In , the border between Chubut Province and Santa Cruz Province.|
|52°S||Part of the border between and .|
|60°S||The northern boundary of Antarctica for the purposes of the Antarctic Treaty System (see map). The northern boundary of the Southern Ocean.|
Altitude has an effect on a location's position relative to the plane formed by a circle of latitude. Since latitude is determined by the normal to the Earth's surface, locations sharing the same latitude—but having different elevations (e.g., lying along this normal)—no longer lie within this plane. Rather, all points sharing the same latitude and of varying elevation occupy a cone formed by the rotation of this normal around the Earth's axis.
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