Latitude

Latitude is the measure of the geographical distance from a point on the earth's surface to the reference parallel or equator, symbolized by phi. Latitude and longitude are the two angular distances—using either degrees, minutes, and seconds, or decimal degrees—in the geographic coordinate system to precisely communicate the position of a certain location, and their measurement is fundamental for mapping representation and navigation.

Latitude is an angle formed by the plane that crosses a certain location on the earth's surface, the center of the earth, and the equatorial plane. Latitude values range from 0 degrees and 90 degrees to the north of the equator, expressed as a positive angle, and from 0 degrees and 90 degrees in the Southern Hemisphere, expressed as a negative angle—or respectively accompanied by the letters N or S.

The lines that connect the points with the same latitude are called parallels or lines of latitude. There is a constant distance between any two parallels and they cross meridians at right angles. Parallels are constant true east-west lines that decrease in their length toward the poles, represented as straight lines in the Mercator projection. The equator is the longest parallel, which makes it the natural reference or origin parallel and separates the Northern Hemisphere from the Southern Hemisphere. The poles are points, not lines, of latitude 0 degrees. The length of a degree of latitude along any meridian varies little, from 110.567 meters in the equator to 111.699 meters in the poles. There are four other latitudes of specific interest, which are due to the obliquity or the angle between the equatorial plane and the ecliptic of the earth. The Tropic of Cancer (23 degrees 27'N) is the northernmost latitude where the sun is vertical at noon on summer solstice in the Northern Hemisphere. The Tropic of Capricorn (23 degrees 27'S) is the southernmost latitude where the sun is vertical at noon on summer solstice in the Southern Hemisphere. Both parallels comprise the tropical latitudes.

The Arctic Circle (66 degrees 33'N) is the southernmost latitude in the Northern Hemisphere and the Antarctic Circle the northernmost in the Southern Hemisphere where the sun does not set in summer and does not rise in winter. Polar latitudes are comprised within each circle. Mid-latitudes extend between the tropics and the polar circles.

The determination of latitude was achieved very early using a fairly accurate method, based on celestial navigation. This method involved measuring the angle of the sun above the horizon at noon during the day, or the position of any other astronomical object using common instruments such as the astrolabe, sextant or octant. Celestial navigation during the night consists of measuring the angle of the pole star in the Northern Hemisphere and the Southern Cross in the opposite. Latitude is the result of operating 90 degrees minus the measured altitude angle plus the sun's declination for the date.