Circle of latitude

A circle of latitude, also known as a parallel, is an imaginary east-west line on Earth’s surface that connects all points sharing the same latitude, ignoring elevation. These circles are parallel to one another, meaning the planes containing them never intersect. A location’s position along a given parallel is specified by its longitude. Unlike circles of longitude, which are all great circles centered on Earth’s core, circles of latitude shrink in circumference as they move away from the Equator.

Equator

Their length can be calculated using a sine or cosine function; for instance, the 60th parallel north or south is half the length of the Equator, disregarding Earth’s minor flattening. The latitude of a circle is approximately the angle between the Equator and that circle, with the vertex at Earth’s center. The Equator, at 0°, is the longest parallel and the only one that is a great circle, perpendicular to all meridians. The North and South Poles lie at 90° north and south, respectively.

Movement of the Tropical and Polar Circles

Between the Equator and each pole, there are 89 integral-degree parallels, though latitude can be expressed in decimal degrees or minutes and seconds. On maps, parallels may appear horizontal and parallel, as on an equirectangular projection, or spaced unevenly on projections like Mercator (wider near poles for shape preservation) or Gall-Peters (closer near poles for area accuracy). On non-cylindrical projections, they are neither straight nor parallel. Arcs of parallels often serve as political boundaries where natural borders are absent; for example, the northern and southern borders of Colorado lie at 41° N and 37° N, and roughly half the U.S.-Canada border follows 49° N. Five major circles exist: the Equator, the Arctic and Antarctic Circles, and the Tropics of Cancer and Capricorn.

Polar circles

The Equator divides the Northern and Southern Hemispheres. The polar circles mark the southernmost and northernmost latitudes where the Sun can remain above or below the horizon for 24 hours at solstices; their latitude equals 90° minus Earth’s axial tilt. The tropical circles mark the northernmost and southernmost latitudes where the Sun can be directly overhead at solstices; their latitude equals Earth’s axial tilt. These four circles’ positions shift slowly due to changes in axial tilt, which is a complex motion from multiple cycles.