circle of latitude

Map of Earth
Longitude (λ)
Lines of longitude appear curved and vertical in this projection, but are actually halves of great circles.
Latitude (φ)
Lines of latitude appear straight and horizontal in this projection, but are actually circular with different radii. All locations with a given latitude are collectively referred to as a circle of latitude.
The equator divides the planet into a Northern Hemisphere and a Southern Hemisphere, and has a latitude of 0°.

On the Earth, a circle of latitude is an imaginary east-west circle connecting all locations that have 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. Circles of latitude become shorter 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 intersection points, 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 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.

Arcs of circles of latitude are sometimes used as boundaries between countries or regions when 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 of major circles of latitude on the Earth.

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

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