Circle of latitude
Imaginary east–west lines parallel to the Equator.
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A circle of latitude, or line of latitude, is an abstract east–west small circle on Earth connecting all locations at a given latitude coordinate, ignoring elevation. Often called parallels because they are parallel to each other, these circles are fundamental to geographic coordinate systems and mapmaking. Their length decreases with distance from the Equator, which is the longest circle of latitude and the only one that is also a great circle. A location’s position along a given circle is specified by its longitude.
Equator
Unlike circles of longitude, which are all great circles centered on Earth’s center, circles of latitude become smaller as one moves away from the Equator; for instance, the 60th parallel north or south is half as long as the Equator, a relationship derived from a sine or cosine function. On the Mercator or Gall-Peters projections, a circle of latitude is perpendicular to all meridians, while on an ellipsoid or spherical projection, all such circles except the Equator are rhumb lines. The latitude of a circle approximates the angle between the Equator and that circle, with the vertex at Earth’s center; the Equator is at 0°, and the poles at 90° north and south.
There are 89 integral-degree circles between the Equator and each pole, though finer measurements use decimal degrees or minutes and seconds. On maps, the appearance of these circles varies by projection: on an equirectangular projection centered on the Equator, they are horizontal, parallel, and equally spaced; on a Mercator projection, they are spaced more widely near the poles to preserve local shapes, while on a Gall-Peters projection, they are spaced more closely near the poles for accurate area comparisons. On most non-cylindrical projections, they are neither straight nor parallel.
Movement of the Tropical and Polar Circles
Arcs of these circles often serve as political boundaries where natural borders are absent, such as the 49° N border between the United States and Canada or the 37° N and 41° N borders of Colorado. Five major circles exist: the Equator (fixed at 0°), the Arctic and Antarctic Circles, and the Tropics of Cancer and Capricorn. Their positions depend on Earth’s axial tilt, which slowly changes over time, so these circles are not perfectly fixed. The Equator divides the Northern and Southern Hemispheres.
Quick Facts
- Longest circle
- Equator (0° latitude)
- Major circles
- Arctic Circle
- Tropic of Cancer
- Equator
- Tropic of Capricorn
- Antarctic Circle
- Number of integral circles
- 89 between Equator and each pole per hemisphere
Facts from the source article.
Lore & Background
Circles of latitude are defined as small circles on Earth's surface, with the Equator being the sole great circle among them. Their lengths follow a cosine function; for instance, the 60th parallel is half as long as the Equator. On maps, their appearance varies by projection: on an equirectangular projection they are horizontal, parallel, and equally spaced, while on a Mercator projection they are more widely spaced near the poles to preserve local shapes, and on a Gall–Peters projection they are spaced more closely near the poles for accurate area comparisons.
Reader's Guide
Circles of latitude serve as essential reference lines for navigation, mapping, and defining political boundaries. They are often used as borders where natural features are absent, such as the northern border of Colorado at 41° N and the southern border at 37° N, or roughly half the U.S.–Canada border along 49° N. The five major circles—Arctic Circle, Tropic of Cancer, Equator, Tropic of Capricorn, and Antarctic Circle—mark divisions between principal geographical zones. Shorter-term variations from nutation and polar motion also cause small daily shifts. On other planets, analogous circles can be defined based on axial tilt.
Frequently Asked Questions
What is a circle of latitude?
A circle of latitude is an imaginary east–west line that wraps around Earth at a fixed angular distance from the Equator. Every point on that line shares the same north–south coordinate, and all such lines run parallel to one another.
Why is the Equator unique among all circles of latitude?
The Equator at 0° is the longest parallel because it is the only one that also counts as a great circle. Every other parallel shrinks in circumference the farther it stretches toward the poles.
What are the five most well-known circles of latitude?
The five major named parallels are the Arctic Circle, Tropic of Cancer, Equator, Tropic of Capricorn, and Antarctic Circle. Together they mark key boundaries for seasonal daylight extremes and broad climate zones.
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Sources
Compiled from Wikipedia and the sources listed below. Text from Wikipedia is available under CC BY-SA 4.0; this entry is adapted from it.
- Wikipedia: Circle of latitude (CC BY-SA 4.0).
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