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Longitude is the angular distance of a point's meridian from the Prime (Greenwich) Meridian. It is usually expressed in degrees, minutes, and seconds. Lines of longitude are often referred to as meridians (pronounced /ˈlɒndʒɨtjuːd/ or /ˈlɒŋɡɨtjuːd/),[1] identified by the Greek letter lambda (λ), is the geographic coordinate most commonly used in cartography and global navigation for east-west measurement. Constant longitude is represented by lines running from north to south. The line of longitude (meridian) that passes through the Royal Observatory, Greenwich, in England, establishes the meaning of zero degrees of longitude, or the Prime Meridian. Any other longitude is identified by the east-west angle, referenced to the center of the Earth as vertex, between the intersections with the Equator of the meridian through the location in question and the Prime Meridian. A location's position along a meridian is given by its latitude, which is identified by the north-south angle between the local vertical and the plane of the Equator.



The measurement of longitude is important both to cartography and to provide safe ocean navigation. Mariners and explorers for most of history struggled to determine precise longitude. Finding a method of determining exact longitude took centuries, resulting in the history of longitude recording the effort of some of the greatest scientific minds.

Latitude was calculated by observing with quadrant or astrolabe the inclination of the sun or of charted stars, but longitude presented no such manifest means of study. Amerigo Vespucci was perhaps the first to proffer a solution, after devoting a great deal of time and energy studying the problem during his sojourns in the New World:

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