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One of the two coordinates (the other being latitude) used to locate a position at sea; marked in degrees east or west of the prime meridien (0 degrees longitude) universally accepted to be at Greenwich, England. As there are 360 degrees in a circle, longitude may range up to 180 degrees East or West. 180 degrees East and West, in fact, meet on the other side of the globe from Greenwich, at the International Date Line. Longitude is comparable to the y-axis on a graph.