International Date Line
English, Van H. Department of Geography, Dartmouth College, Hanover, New Hampshire.
Last reviewed:February 2016
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The 180° meridian, where each day officially begins and ends. As a person travels eastward, against the apparent movement of the Sun, 1 h is gained for every 15° of longitude; traveling westward, time is lost at the same rate. Two people starting from any meridian and traveling around the world in opposite directions at the same speed would have the same time when they meet, but would be 1 day apart in date. If there were no international agreement as to where each day should begin and end, there could be any number of places so designated. To eliminate such confusion, the International Meridian Conference, in 1884, designated the 180° meridian as the location for the beginning of each day. Thus, when a traveler goes west across the line, a day is lost; if it is Monday to the east, it will be Tuesday immediately as the traveler crosses the International Date Line. In traveling eastward a day is gained; if it is Monday to the west of the line, it will be Sunday immediately after the line is crossed.
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