International Date Line (redirected from dateline)

international date line, imaginary line on the earth's surface, generally following the 180° meridian of longitude, where, by international agreement, travelers change dates. Traveling eastward across the line, one subtracts one calendar day; traveling westward, one adds a day. The date line is necessary to avoid a confusion that would otherwise result. For example, if an airplane were to travel westward with the sun, 24 hr would elapse as it circled the globe, but it would still be the same day for those in the airplane while it would be one day later for those on the ground below them. The same problem would arise if two travelers journeyed in opposite directions to a point on the opposite side of the earth, 180° of longitude distant. The eastward traveler would set his clock ahead 1 hr for each 15° of longitude (see standard time), so that his clock would gain a total of 12 hr; the westward traveler would set his clock back 1 hr for each 15°, resulting in a total loss of 12 hr. The two clocks would therefore differ by 24 hr, or one calendar day. The apparent paradox is resolved by requiring that the traveler crossing the date line change his date, thus bringing the travelers into agreement when they meet. The international date line does not follow the 180° meridian along its entire course but bends eastward around the eastern tip of Siberia, westward around the Aleutian Islands, and eastward again around various island groups in the South Pacific in order to avoid a time change in populated areas.

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International Date Line

Imaginary line from the North Pole to the South Pole that arbitrarily separates each calendar day from the next. It corresponds along most of its length to the 180th meridian of longitude but deviates to the east through the Bering Strait to avoid dividing Siberia and then deviates to the west to include the Aleutian Islands with Alaska. South of the Equator, another eastward deviation allows certain island groups to have the same day as New Zealand. The date line is a consequence of the worldwide use of timekeeping systems arranged so that local noon corresponds approximately to the time at which the Sun crosses the local meridian of longitude. See also standard time.