Latitude and longitude

Latitude and longitude are sets of parallel lines that are drawn perpendicular to one another on a map or a globe. They are referenced in combination in order to describe a precise location on the surface of the earth. Lines of latitude run from east to west, parallel to the equator, and lines of longitude run from the North Pole to the South Pole. While lines of longitude converge at each pole, lines of latitude never touch.

Both latitude and longitude have a reference point that has been designated 0 degrees. Latitude is measured by the number of degrees north or south of the equator, with the North and South Poles being 90 degrees north and south, respectively, while longitude is measured in degrees east or west of the prime meridian, which runs through the city of Greenwich, England. For example, the coordinates of Washington, DC, are expressed as 38°53′42.4″N 77°02′12.0″W. This means that the city is 38 degrees, 53 arc minutes (or minutes for short), and 42.4 arc seconds (or seconds) north of the equator and 77 degrees, 2 minutes, and 12 seconds west of the prime meridian. An arc minute is equal to one-sixtieth of a degree, and an arc second is one-sixtieth of an arc minute.

Brief History

Throughout history, longitude and latitude have been of tremendous importance to the fields of exploration, navigation, and commerce. Latitude is fairly easy to determine because the equator acts as a natural starting point, being equidistant from the North and South Poles. Sailors on the ocean could calculate their approximate latitude by using simple instruments to measure the position of the sun or stars above the horizon.

Establishing longitude represented a much greater challenge, since there is no “natural” starting point. The problem was studied by great minds such as Amerigo Vespucci and Galileo Galilei before finally being unraveled by John Harrison (1693–1776), a clockmaker from England. It was already known that a sailor on a ship could determine longitude based on the difference between local time at the ship’s position and the time at a given starting point; fifteen degrees traveled east or west equaled one hour of time difference. However, clocks of the era were too motion sensitive to remain accurate onboard a ship, so sailors had no way to keep track of the time at the starting point. After several decades and three prototypes, Harrison eventually developed a working marine chronometer, which used a coiled spring and a rapidly oscillating balance wheel to keep time. It was essentially the first accurate pocket watch ever constructed.

Until the nineteenth century, countries or regions tended to choose their own capital city as a longitudinal reference point. (Greenwich, now a borough of London, was outside the city at the time, but it was chosen as Great Britain’s reference point due to the presence of the Royal Greenwich Observatory.) Only in 1884 did the International Meridian Conference meet in Washington, DC, and agree to use the Greenwich meridian (line of longitude) as the prime meridian.

Overview

In addition to the primary function of allowing mapmakers and navigators to identify locations, latitude and longitude also allow Earth to be divided into different sections for ease of reference. Thus, lines of longitude—specifically the prime meridian—make it possible to refer to the Western Hemisphere, denoting everything to the west of Greenwich, and the Eastern Hemisphere, meaning everything to the east. These designations quickly took on a life of their own, leading to the common characterization of “the East” as synonymous with either East Asia or Russia and the former Soviet republics. In a similar fashion, “the West” usually refers to either the United States or Europe, despite the fact that most of Europe is in fact in the Eastern Hemisphere, and often carries the connotation of a more modern, economically privileged culture.

On the other side of the globe, the Eastern and Western Hemispheres meet at the 180th meridian, so called because it is 180 degrees from the prime meridian in either direction. This is the line along which most world maps are divided. Most of the 180th meridian runs through the middle of the Pacific Ocean, though parts of it pass through Russia and Fiji, and it forms the basis of the international date line, which delineates the earth’s earliest time zone from its latest. If someone were to continue traveling east and cross the international date line from the Eastern to the Western Hemisphere, the local time at that person’s position would suddenly become twenty-four hours earlier—one calendar day.

Other regions of Earth are defined by boundaries known as circles of latitude. There are five main circles of latitude: the equator, which is defined as 0 degrees latitude and divides the globe into the Northern and Southern Hemispheres; the Tropic of Cancer and the Tropic of Capricorn, which mark the northernmost and southernmost latitudes, respectively, at which the sun can be seen directly overhead on the summer or winter solstice; and the Arctic Circle and the Antarctic Circle, which surround the North and South Poles and mark the southernmost and northernmost latitudes in the Northern and Southern Hemispheres, respectively, at which the sun remains above or below the horizon for twenty-four hours on the summer or winter solstice.

The latter four circles of latitude (aside from the equator) divide the planet into five major geographical zones. Their precise locations vary depending on the angle of Earth’s axis in a given year, but in general, the Tropics of Cancer and Capricorn are approximately 23.5 degrees from the equator, and the Arctic and Antarctic Circles are approximately 66.5 degrees from the equator. The region between the Tropics of Cancer and Capricorn is the torrid zone, often colloquially called the “tropics”; the region between the Tropic of Cancer and the Arctic Circle or the Tropic of Capricorn and the Antarctic Circle is called the north or south temperate zone; and the region between the Arctic Circle and the North Pole or the Antarctic Circle and the South Pole is called the north or south frigid zone.

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