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On the Earth, a 'circle of latitude' is an imaginary east-west circle that connects all locations with 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 a fixed distance apart, and on some map projections, including the Mercator projection, they are drawn parallel. They are also the intersections of the surface of the Earth with parallel planes. The length circles of latitude gets shorter the farther they are from the equator and closer to the poles.
Arcs of circles of latitude are often used as boundaries between countries or regions that lack distinctive natural borders (e.g. in a desert).
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 it. In other words, circles of latitude are not great circles, and are not really "lines" in the geometry of the sphere (see also great-circle distance). It is for this reason that an aeroplane travelling between a European and North American city on the same latitude will fly further north, over Greenland for example.
A circle of latitude is perpendicular to all meridians at the intersection points, and is hence a special case of a loxodrome.

Contents

Major circles of latitude

Equator

Arctic and Antarctic Circles

Tropics of Cancer and Capricorn

Movement of the Tropics and the Arctic and Antarctic circles

Other notable parallels

Major circles of latitude

The five major circles of latitude are, from north to south:

★ The Arctic Circle (66° 33′ 38″ N)

★ The Tropic of Cancer (23° 26′ 22″ N)

★ The Equator (0° N)

★ The Tropic of Capricorn (23° 26′ 22″ S)

★ The Antarctic Circle (66° 33′ 38″ S)
These circles of latitude (excluding the equator) mark the divisions between the five main geographical zones.

Equator

The equator is the circle that is equidistant from both the North Pole and South Pole.
It divides the Earth into the Northern Hemisphere and the Southern Hemisphere.

Equator

Arctic and Antarctic Circles

The Arctic Circle marks 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 these circles plus the Earth's axial tilt is equal to 90°.

Arctic Circle

Antarctic Circle

Tropics of Cancer and Capricorn

The Tropic of Cancer and Tropic of Capricorn respectively 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 tropic circles is equal to the Earth's axial tilt.

Tropic of Cancer

Tropic of Capricorn

Movement of the Tropics and the Arctic and Antarctic circles

:''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 tilt of the Earth's axis relative to the plane of its orbit around the sun, known technically as the "obliquity of the ecliptic". As of 2000, the mean value of the tilt was about 23°26′21″. However, this angle is not constant, but has a complex motion determined by the superimposition of many different cycles with short to very long periods. As the axial tilt varies, so do the positions of the Tropics and the Arctic and Antarctic circles.
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.

Other notable parallels

Parallel	Description
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."
49th parallel north	Part of the border between the United States and Canada, from Washington to western Minnesota.
45th parallel north	The east-west line that marks the theoretical halfway point between the Equator and the North Pole.
42nd parallel north	The border between California and Oregon and most of the border between Pennsylvania and New York.
40th parallel north	The line originally chosen for the Mason-Dixon Line, but the line was moved several miles south to avoid bisecting the city of Philadelphia.
38th parallel north	The boundary between the Soviet and American occupation zones in Korea in 1945.
37th parallel north	The north-south border between Utah & Arizona, and Colorado & New Mexico respectively.
28th parallel north	The boundary between Baja California and Baja California Sur in Mexico.
22° 19' 35.6736" N	Boundary Street – the boundary between Kowloon and New Kowloon of New Territories.
22nd parallel north	The border between Egypt and Sudan, partly disputed (see also Hala'ib Triangle).
17th parallel north	The division between Republic of Vietnam (South Vietnam) and Democratic Republic of Vietnam (North Vietnam) during the Vietnam War.
26th parallel south	The border between South Australia and the Northern Territory, and part of the border between South Australia and Queensland.
45th parallel south	The east-west line that marks the theoretical halfway point between the Equator and the South Pole.
60th parallel south	The northern boundary of Antarctica for the purposes of the Antarctic Treaty System (see map). The northern boundary of the Southern Ocean.