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Longitude - Tropical

Longitude 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.

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