Latitude and Longitude
The concepts of latitude and longitude create a grid system for the unique expression of any location on Earth's surface.
Latitudes—also known as parallels—mark and measure distance north or south from the equator. Earth's equator (the great circle or middle circumference) is designated 0° latitude. The north and south geographic poles, respectively, measure 90° north (N) and 90° south (S) from the equator. The angle of latitude is determined as the angle between a transverse plane cutting through Earth's equator and the right angle (90°) of the polar axis. The distance between lines of latitude remains constant. One degree of latitude equals 60 nautical miles (approximately 69 statute miles, or 111 km).
Longitudes, also known as meridians, are great circles that run north and south, and converge at the north and south geographic poles. As the designation of 0° longitude is arbitrary, international convention, long held since the days of British sea superiority, establishes the 0° line of longitude—also known as the Prime Meridian—as the great circle that passes through the Royal National Observatory in Greenwich, England (United Kingdom). The linear distance between lines of longitude vary and is a function of latitude. The liner distance between lines of longitude is at its maximum at the equator and decreases to zero at the poles. There are 360° of longitude, divided into 180° east and 180° west of the prime meridian. The line of longitude measuring 180° west is, of course, the same line of longitude measuring 180° east of the prime meridian and, except for some geopolitical local variations, serves as the international date line. Because Earth completes one rotation in slightly less than 24 hours, the angular velocity of rotation is approximately 15° of longitude per hour. This rate of rotation forms the basis for time zone differentiation.
The distance between lines of longitude varies in length at different latitudes, the distance lessening as latitude increases. At the equator, 69.171 statute miles separate lines of longitude, but by 30° latitude, there are only 59.956 statute miles between lines of longitude. At 60° latitude, only 34.697 statute miles separate longitudinal great circles at that latitude. At the poles, all lines of longitude converge.
Every point on Earth can be expressed with a unique set of latitude and longitude coordinates (i.e., lat/lon coordinates). Latitude—specified as degrees north (N) or south (S)—and longitude—specified as degrees east (E) or west (W)—are expressed in degrees, arcminutes, and arcseconds (e.g., a lat/lon of 39:46:05N, 104:52:22W specifies a point in Denver, Colorado).
Lines of latitude and longitude are usually displayed on maps. Although a variety of maps exist, because maps of Earth are two-dimensional representations of a curved three-dimensional oblate spherical surface, all maps distort lines of latitude and longitude. For example, with equatorial cylindrical projections (e.g., a Mercator projection), low latitude regions carry little distortion. Higher latitudes suffer extreme distortion of distance because of erroneously converging lines of latitude (on the surface of the earth they are parallel). Despite this disadvantage, Mercator projections remain useful in navigation because there is no distortion of direction and vertical lines drawn upon such a map indicate true north or south.
Many maps include inserts showing polar conic projections to minimize the distortion of latitude near the poles.
Although it is relatively easy to ascertain latitude—especially in the northern hemisphere where the altitude of the North Star (Polaris) above the horizon gives a fairly accurate estimate of latitude—the accurate determination of longitude proved to be one of great post-enlightenment scientific challenges. The inability to accurately estimate longitude often proved fatal or costly in sea navigation. It was not until the eighteenth century, when British clockmaker John Harrison developed a chronometer that could accurately keep time onboard ship, that the problem of longitude was solved. An accurate clock allows navigators to compare, for example, the time of observed high noon and compare the local time to the time at Royal National Observatory in Greenwich, England (Greenwich Mean Time or GMT). Knowing that Earth rotates at approximately 15° per hour, the time difference between local noon and GMT local noon is directly related to the degrees of longitude between the prime meridian and the observer's location.