international date line

international date line

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 (mostly extremely around Kiribati) to avoid a date change within island nations and territories or between important trade partners.

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international date line

[¦in·tər¦nash·ən·əl ′dāt ‚līn]

(astronomy)

A jagged arbitrary line, roughly equal to the 180° meridian, where a date change occurs: if the line is crossed from east to west a day is skipped, if from west to east the same day is repeated.

McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

international date line

The Greenwich antimeridian, or 180° longitude. The international date line, however, does not follow the Greenwich antimeridian exactly but is distorted to place inhabited areas definitely on one side or the other of the line to avoid a difference in date in an inhabited area. Also called a calendar line or a date line.

An Illustrated Dictionary of Aviation Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved

International Date Line

the line approximately following the 180? meridian from Greenwich on the east side of which the date is one day earlier than on the west

Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005