Latitude and Longitude, system of geometrical coordinates used in designating the location of places on the surface of the earth. (For the use of these terms in astronomy, see Coordinate System; Ecliptic.) Latitude, which gives the location of a place north or south of the equator, is expressed by angular measurements ranging from 0° at the equator to 90° at the poles. Longitude, the location of a place east or west of a north-south line called the prime meridian, is measured in angles ranging from 0° at the prime meridian to 180° at the International Date Line.
Midway between the poles, the equator, a great circle, divides the earth into northern and southern hemispheres. Parallel to the equator and north and south of it are a succession of imaginary circles that become smaller and smaller the closer they are to the poles. This series of east-west-running circles, known as the parallels of latitude, is crossed at right angles by a series of half-circles extending north and south from one pole to the other, called the meridians of longitude.
Although the equator was an obvious choice as the prime parallel, being the largest, no one meridian was uniquely qualified as prime. Until a single prime meridian could be agreed upon, each nation was free to choose its own, with the result that many 19th-century maps of the world lacked a standardized grid. The problem was resolved in 1884, when an international prime meridian, passing through London's Greenwich Observatory, was officially designated. A metallic marker there indicates its exact location.
Degrees of latitude are equally spaced, but the slight flattening at the poles causes the length of a degree of latitude to vary from 110.57 km (68.70 mi) at the equator to 111.70 km (69.41 mi) at the poles. At the equator, meridians of longitude 1 degree apart are separated by a distance of 111.32 km (69.17 mi); at the poles, meridians converge. Each degree of latitude and longitude is divided into 60 minutes, and each minute divided into 60 seconds, thereby allowing the assignment of a precise numerical location to any place on earth.