latitude, angular distance of any point on the surface of the earth north or south of the equator. The equator is latitude 0°, and the North Pole and South Pole are latitudes 90°N and 90°S, respectively. The length of one degree of latitude averages about 69 mi (110 km); it increases slightly from the equator to the poles as a result of the earth's polar flattening. Latitude is commonly determined by means of a sextant or other instrument that measures the angle between the horizon and the sun or another celestial body, such as the North Star (see Polaris). The latitude is then found by means of tables that give the position of the sun and other bodies for that date and hour. An imaginary line on the earth's surface connecting all points equidistant from the equator (and thus at the same latitude) is called a parallel of latitude. On most globes and maps parallels are usually shown in multiples of 5°. Because of their special meanings, four fractional parallels are also shown. These are the Tropic of Cancer (231/2°N) and the Tropic of Capricorn (231/2°S), marking the farthest points north and south of the equator where the sun's rays fall vertically (see tropics), and the Arctic Circle (661/2°N) and the Antarctic Circle (661/2°S), marking the farthest points north and south of the equator where the sun appears above the horizon each day of the year (see also midnight sun). Parallels of latitude and meridians of longitude together form a grid by which any point on the earth's surface can be specified. The term latitude is also used in various celestial coordinate systems (see ecliptic coordinate system).