A circle of latitude or line of latitude on Earth is an abstract east–west small circle connecting all locations around Earth (ignoring elevation) at a given latitude coordinate line.

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Circles of latitude are often called parallels because they are parallel to each other; that is, planes that contain any of these circles never intersect each other. 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. For example, the 60th parallel north or south is half as long as the Equator (disregarding Earth's minor flattening by 0.335%), stemming from ${\displaystyle \cos(60^{\circ })=0.5}$. On the Mercator projection or on the Gall-Peters projection, a circle of latitude is perpendicular to all meridians.^[1] On the ellipsoid or on spherical projection, all circles of latitude are rhumb lines, except the Equator.

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. As such, it is perpendicular to all meridians.

There are 89 integral (whole degree) 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 (e.g. 34.637° N) or with minutes and seconds (e.g. 22°14'26" S).

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 (such as in deserts), 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 the border between the United States and Canada follows 49° N.

There are five major circles of latitude, listed below from north to south. The position of the Equator is fixed (90 degrees from Earth's axis of rotation) 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 16 April 2024:^[2]

• Arctic Circle (66°33′49.9″ N)
• Tropic of Cancer (23°26′10.1″ N)
• Equator (0° latitude)
• Tropic of Capricorn (23°26′10.1″ S)
• Antarctic Circle (66°33′49.9″ S)

These circles of latitude, excluding the Equator, mark the divisions between the five principal geographical zones.

Equator

Main article: 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' (a circle on the surface of the Earth, centered on Earth's center). All the other parallels are smaller and centered only on Earth's axis.

Equator

Polar circles

Main article: Polar circle

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 (at the June and December solstices respectively). 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 (at the December and June Solstices respectively).

The latitude of the polar circles is equal to 90° minus the Earth's axial tilt.

Arctic Circle

Antarctic Circle

Tropical circles

Main articles: Tropic of Cancer and Tropic of Capricorn

The Tropic of Cancer and Tropic of Capricorn 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

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 (to within a few tens of metres) by sighting the North Star.

Normally the circles of latitude are defined at zero elevation. Elevation has an effect on a location with respect to the plane formed by a circle of latitude. Since (in the geodetic system) altitude and depth are determined by the normal to the Earth's surface, locations sharing the same latitude—but having different elevations (i.e., lying along this normal)—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.

• Circles of a sphere
• List of circles of latitude