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. | The date line is necessary to avoid a confusion that would otherwise result. | 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 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. | 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.