Circle of latitude
Abstract east–west parallels defining Earth's latitudinal grid.
A circle of latitude, or line of latitude, on Earth is an abstract east–west small circle connecting all locations around Earth at a given latitude coordinate line, ignoring elevation. Often called parallels because they are parallel to each other, these circles are fundamental to geographic coordinate systems, with the Equator being the longest and the only great circle among them. Their length decreases as distance from the Equator increases, calculable by a cosine function, and they serve as key references for navigation, mapping, and defining borders.
- type
- Geographic concept
- key_circles
- Equator, Tropic of Cancer, Tropic of Capricorn, Arctic Circle, Antarctic Circle
- number_of_integral_circles
- 89 per hemisphere between Equator and poles
- basis
- Angle between Equator and circle, vertex at Earth's centre
Lore & Background
Circles of latitude are often called parallels because planes containing any of these circles never intersect each other. A location's position along a circle of latitude is given by its longitude. Unlike circles of longitude, which are all great circles with Earth's centre in the middle, circles of latitude get smaller as distance from the Equator increases; for example, the 60th parallel north or south is half as long as the Equator, stemming from cos(60°) = 0.5. On the Mercator or Gall-Peters projection, a circle of latitude is perpendicular to all meridians, and on the ellipsoid or spherical projection, all circles of latitude are rhumb lines except the Equator.
Reader's Guide
Circles of latitude are essential for geographic orientation, mapping, and defining climatic zones. The five major circles—Arctic Circle, Tropic of Cancer, Equator, Tropic of Capricorn, and Antarctic Circle—mark divisions between principal geographical zones. Arcs of circles of latitude are sometimes used as boundaries between countries or regions where natural borders are lacking, such as the northern border of Colorado at 41° N and the southern border at 37° N, and roughly half the length of the border between the United States and Canada follows 49° N. On maps, their appearance varies by projection: on equirectangular projection they are horizontal, parallel, and equally spaced; on Mercator they are more widely spaced near poles; on Gall-Peters they are spaced more closely near poles; on most non-cylindrical projections they are neither straight nor parallel.
Did You Know?
- The Equator is the only circle of latitude that is also a great circle.
- The 60th parallel north or south is half as long as the Equator, due to cos(60°) = 0.5.
- The positions of the Tropical and Polar Circles are not fixed; they drift by about 15 m per year due to changes in Earth's axial tilt.
- Arcs of circles of latitude are sometimes used as boundaries, such as the 49° N border between the United States and Canada.
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