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Longitude is the measure of the geographical distance from a point on the earth’s surface to the reference or prime meridian, the Greenwich meridian, symbolized by lambda. Latitude and longitude are the two angular distances-using either degrees, minutes, and seconds, or decimal degrees-in the geographic coordinate system that precisely communicate the position of a certain location, and their measurement is fundamental for mapping representation and navigation.
Longitude is an angle formed by the meridian plane that crosses a location on the earth’s surface, the center of the earth, and the Greenwich meridian plane. Longitude values range from 0 degrees and 180 degrees to the east of Greenwich, expressed as a positive angle, and from 0 degrees and 180 degrees to the west of Greenwich, expressed as a negative angle-or respectively accompanied by the letters E or W.
The lines that connect the points with the same longitude are termed meridians. They converge at both poles, are true north-south lines, and cross parallels at right angles. The distance between any two meridians increases toward the equator and decreases toward the poles, except for opposite meridians, which form a great circle.
Since the earth makes one 360 degree revolution in approximately 24 hours, and one hour represents a displacement of 15 degrees of longitude, the earth has been divided into 24 time zones-plus other fractions-allowing for the same standard time in a time zone. The antipodal meridian of Greenwich forms the International Date Line in the middle of the Pacific Ocean.
The Greenwich Meridian, which separates the Eastern and Western Hemispheres, was agreed upon as the prime meridian in October 1884 at the International Meridian Conference held in the United States. This reference meridian is arbitrary but commonly accepted; historically multiple prime meridians were used for mapping.
Finding longitude accurately at sea was a problem for centuries, making navigation unsafe; many vessels were lost in shipwrecks and voyages impeded until a solution was found. The usual method used to calculate the vessel’s position was dead reckoning, measuring the distance traveled in a fixed direction from a previous position. This repeatedly proved unsatisfactory since error was high. While some complex calculations had existed since the 17th century (including measurement of the moons of Jupiter), a reliable solution remained elusive.
Seeking a resolution to the problem, the British Parliament passed the Longitude Act in 1714, and a Board of Longitude was commissioned to examine and judge proposals submitted by the public for methods of calculating longitude at sea, with a prize attached as an incentive.
The problem was solved not through mathematics and celestial observation, but rather through the application of mechanical invention. Since longitude is a function of time, knowing both local time and time at a known position and simply calculating the difference allows a precise measurement of east/ west position. With this in mind, John Harrison, a clockmaker, designed an ingeniously accurate chronometer that could keep time without a pendulum (and therefore at sea). Despite the elitist hesitation of the Board of Longitude-who would have preferred both an elegantly mathematical solution and a winner from a higher social class-Harrison was acknowledged the prize winner in 1773.
- William J.H. Andrewes, ed., The Quest for Longitude (Harvard University Press, 1996);
- Lev Bugayevskiy and John P. Snyder, Map Protections. A Reference Manual (Taylor and Francis, 1995);
- Derek Howse, Greenwich Time and the Discovery of the Longitude (Oxford University Press, 1980);
- Arthur Robinson et al., Elements of Cartography (John Wiley & Sons, 1995);
- Dava Sobel, Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time (Penguin, 1995).