Circle of latitude

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 – that is, any two given parallels are everywhere the same distance apart. (Since the earth isn't spherical the distance from the equator to 10 degrees north is less than the distance from 10 to 20 degrees north.) On some map projections, including the Equirectangular projection, they are drawn equidistant.

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, 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 (except 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 (see also great-circle distance), but are rhumb lines. For this reason 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.

Major circles of latitude

Diagram showing the locations of the five major circles of latitude on an equirectangular projection of the Earth.

Diagram showing the derivation of the major circles of latitude on the Earth.

There are five major circles of latitude, listed below from north to south. The position of the Equator is fixed (90 degrees from the Earth's axis of rotation) but the other circles depends on the tilt of this axis relative to the plane of the Earth's orbit, and therefore are not perfectly fixed. The values below are for Epoch 2011.:

• Arctic Circle (66°33′50.2″ N)
• Tropic of Cancer (23°26′09.8″ N)
• Equator (0° latitude)
• Tropic of Capricorn (23°26′09.8″ S)
• Antarctic Circle (66°33′50.2″ S)

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' (in that it is a circle on the surface of the earth, centered on the center of the earth). All the other parallels are smaller and centered only on the earth's axis.

Equator

Polar 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

Tropical Circles

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 tropical circles is equal to the Earth's axial tilt.

Tropic of Cancer

Tropic of Capricorn

Movement of the Tropical and Polar 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". If the Earth were completely 'upright' (its axis at right angles to the orbital plane) there would be no Arctic, Antarctic, or Tropics : at the poles the sun would permanently circle 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. As of 2000, 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 (approximately, and on average) the Tropical Circles are drifting towards the equator (and the Polar Circles towards the poles) by 15 metres per year, and the area of the Tropics is decreasing by 1100 square km per year.

The Earth's axial tilt has additional shorter-term variations due to nutation, of which the main term, with a period of 18.7 years, has an amplitude of 9" 21''' (corresponding to almost 300 metres north and south). There are many still 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 small fluctuations (on the order of 15 meters) called polar motion, which have a small theoretical effect not only on the Tropics and Polar circles, but 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 notable parallels

A number of sub-national and international borders are defined by, or are approximated by, parallels. A major reason why latitude makes for convenient borders in the northern hemisphere is because it is readily measured by sighting the North Star.

Parallel	Description
70°N	On Victoria Island, Canada, two sections of the border between Northwest Territories and Nunavut.
60°N	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."
52°N	In Canada, part of the border between Newfoundland and Labrador and Quebec.
51°N	The southern limit of Russian America from 1799 to 1821.
49°N	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.
45°N	Part of the border between Canada (Quebec) and the United States (New York and Vermont). Also, in the United States, most of the border between Montana and Wyoming.
43°30'N	In the United States, the border between Minnesota and Iowa.
43°N	In the United States, much of the border between South Dakota and Nebraska.
42°30'N	In the United States, the border between Wisconsin and Illinois.
42°N	Originally the northward limit of New Spain. In the United States, 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 United States, 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 United States, 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 from 1945 until Korean War (1950–1953).
37°N	In the United States, 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 United States, defines part of the border between Oklahoma and Texas, most of the border between Missouri and Arkansas. Geographically it is a Westward extension of the border between Virginia and North Carolina and part of the border between Kentucky and Tennessee.
36°N	In the United States, a short section of the border between the Missouri Bootheel and Arkansas.
35°N	In the United States, 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 United States, 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 United States, part of the border between New Mexico and Texas.
31°20'N	Part of the border between the United States and Mexico (Sonora and Chihuahua); the southern border of Arizona and the New Mexico Bootheel.
31°N	Part of the border between Iran and Iraq. In the United States, part of the border between Mississippi and Louisiana, and part of the border between Alabama and Florida.
28°N	In Mexico, the border between Baja California and Baja California Sur.
26°N	Part of the border between Western Sahara (claimed by Morocco) and Mauritania.
25°N	Part of the border between Mauritania and Mali.
22°N	Much of the border between Egypt and Sudan, partly disputed (see also Hala'ib Triangle).
20°N	A short section of the border between Libya and Sudan, 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.
15°N	de facto maritime border between Honduras and Nicaragua.
13°05'N	Part of the border between Chad and Cameroon, over a stretch of 41.6 km, partly in Lake Chad
10°N	Part of the border between Guinea and Sierra Leone.
8°N	Part of the border between Somalia and Ethiopia.
1°N	Part of the border between Equatorial Guinea and Gabon.
1°S	Most of the border between Uganda and Tanzania, and a very short section of the border between Kenya and Tanzania in Lake Victoria.
7°S	A short section of the border between Democratic Republic of the Congo and Angola.
8°S	Two short sections of the border between Democratic Republic of the Congo and Angola.
10°S	A short section of the border between Brazil and Peru.
13°S	Part of the border between Angola and Zambia.
16°S	Part of the border between Mozambique and Zimbabwe.
22°S	A short section of the border between Namibia and Botswana, and parts of the border between Bolivia and Argentina.
26°S	In Australia, the border between South Australia and the Northern Territory, and part of the border between South Australia and Queensland.
28°S	In Argentina, the border between Chaco Province and Santa Fe Province.
29°S	In Australia, much of the border between Queensland and New South Wales.
35°S	In Argentina, part of the border between Córdoba Province and La Pampa Province.
36°S	In Argentina, part of the border between Mendoza Province and La Pampa Province.
42°S	In Argentina, the border between Río Negro Province and Chubut Province.
46°S	In Argentina, the border between Chubut Province and Santa Cruz Province.
52°S	Part of the border between Argentina and Chile.
60°S	The northern boundary of Antarctica for the purposes of the Antarctic Treaty System (see map). The northern boundary of the Southern Ocean.

Many section line boundaries and section line roads run due east and due west along a circle of latitude, rather than a straight line.

Altitude

Note that the features of the spheroid cross-section (orange) in this image are exaggerated with respect to the Earth.

Altitude has an effect on a location's position relative to the plane formed by a circle of latitude. Since altitude 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.

See also

• U.S. Naval Observatory - mean obliquity of the ecliptic