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The International Date Line

an imaginary line set at 180 longitude that "adds a day" when crossed going to the west

In 1884, members of the International Prime Meridian Conference agreed that the longitude of Greenwich, England, would become zero degrees longitude and established the 24 time zones relative to the "Prime Meridian." That same conference established 180 longitude as the International Date Line, since it is exactly opposite the Prime Meridian.

The International Date Line is not, however, an absolutely straight line. It zigzags, for political reasons, so that it never crosses the territory of any nation. For example, all of Siberia is in the Asian system, even though its easternmost peninsula should actually lie across the line. In the South Pacific, the line was adjusted so that all islands within any given political association were on one or the other side.

map showing the International Date Line

See Also

Time Zones

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The Robinson Library >> Practical and Spherical Astronomy

This page was last updated on 09/27/2018.