A circle of latitude on the Earth is an imaginary east-west circle connecting all locations (not taking into account elevation) having 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 parallel to each other – that is, any two parallels are everywhere the same distance apart. (Since the Earth isn't spherical the distance from the equator to 10 degrees north is slightly less than the distance from 10 to 20 degrees north. On some map projections, like the Equirectangular projection, they are drawn equidistant.)
Circles of latitude become smaller the farther they are from the equator and the closer they are to the poles. A circle of latitude is perpendicular to all meridians, and is hence a special case of a loxodrome.
The latitude of the circle is (roughly) the angle between the Equator and the circle, with the angle's vertex at the Earth's centre. The Equator is at 0°, and the North and South Pole are at 90° north and 90° south respectively. 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). There is no limit to how precisely latitude can be measured, and so there are an infinite number of circles of latitude on Earth.
Many maps show circles of latitude as straight lines, but a circle of latitude is not (except the Equator) the shortest distance between two points on the Earth. In other words, circles of latitude (except for the Equator) are not great circles (see also great-circle distance), but are rhumb lines. So an aircraft flying between a European and North American city that share the same latitude will fly farther north, over Greenland for example.
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", as happened in Korea.
Read more about Circle Of Latitude: Major Circles of Latitude, Other Notable Parallels, Altitude
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