International Date Line

The International Date Line is an imaginary line on the surface of the Earth, which offsets the hours that are added or subtracted as one travels east or west through successive time zones. It is for the most part at ±180° of longitude, on the side of the Earth that lies opposite the Prime Meridian, but has an odd shape to pass around Russia and islands in the Pacific.

The first date-line problem occurred in association with Magellan's circumnavigation of the globe. The crew returned to a Spanish stopover sure of the day of the week, as attested by various carefully maintained sailing logs. Nevertheless, those on land insisted the day was different. Although now readily understandable, this phenomenon caused great excitement at the time, to the extent that a special delegation was sent to the Pope to explain this oddity to him.

For the most part, the International Date Line follows the line of longitude of 180 degrees. The two largest deviations from this meridian both occur to keep the date line from crossing nations internally. In the north, the date line swings to the east through Bering Strait and then west past the Aleutian Islands in order to keep Alaska (part of the United States) and Russia on opposite sides of the line. In the central Pacific, the date line was moved in 1995 to extend around, rather than through, the territory of Kiribati. Prior to this time zone change, Kiribati straddled the date line; as a consequence, government offices on opposite sides of the line could only communicate on the four days of the week when both sides experienced weekdays simultaneously. A secondary consequence of this time zone revision was the new status of the Caroline Atoll as the easternmost inhabited land to enter the year 2000, a feature which the Kiribati government capitalized upon as a tourist draw.

However, even into the 21st century, many mapmakers do not take account of this Kiribati line shift, preferring to continue to represent the International Date as a straight line in the Kiribati area.

The International Date Line can cause confusion among airline travelers. The most troublesome situation usually occurs with short journeys from west to east. For example, to travel from Tonga to Samoa by air takes approximately two hours. Thus, if a person leaves at noon on Tuesday, they will arrive at 2 pm on Monday. Meanwhile, someone in Samoa inquiring about the departing flight may be told there is no flight until the next day. There could also be problems with the traveler having to repeat Monday. Journal entries and photographs may end up out of sequence, and there could be errors in someone's medication schedule. In addition, those making ownward airline connections might chose the wrong date for the reservation.

It should be noted that the International Date Line must be observed in conjunction with earth's time zones. If someone were to circumnavigate the globe in an airplane from east to west (the same direction as Magellan), they should subtract one hour for every 15° of longitude crossed, lossing 24 hours for one circuit of the globe, but 24 hours are added when crossing the International Date Line (from east to west), so the net result is zero.

The effect of ignoring the date line is also seen in Jules Verne's novel Around the World in Eighty Days, in which the travellers return to London after a trip around the world, thinking that they have lost the bet that is the central premise of the story. Having circumnavigated in the direction opposite Magellan's, they believe the date there to be one day later than it truly is.

Anyone travelling west and passing the line must add a day to what they would otherwise expect the date and time to be. Correspondingly, those going east must subtract a day. Magellan's crew and Verne's travellers neglected those respective adjustments.

External link

• A History of the International Date Line (http://www.phys.uu.nl/~vgent/idl/idl.htm)cs:Datová hranice

da:Datolinjen de:Datumsgrenze es:Línea internacional de cambio de fecha fr:Ligne de changement de date id:Batas penanggalan internasional it:Linea internazionale del cambio di data he:קו התאריך nl:Internationale datumgrens ja:国際日付変更線 pl:Międzynarodowa linia zmiany daty ru:Линия перемены дат sv:Datumgränsen