International Date Line
From Freepedia
Image:International Date Line.png 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 connecting flights might choose the wrong date for the reservation.
If one crosses the date line at precisely midnight, going westward, one skips an entire day, while going eastward, the day repeats.
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, losing 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, led by Phileas Fogg, 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.
Lest anyone accuse Fogg of cheating by obtaining one extra day, this is not so. Assuming a constant eastward speed, each day was 18 minutes short of a full 24 hours, accumulating one full day, which they regain by setting their calendars back a day in mid-Pacific.
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.
De facto and de jure date lines
The IDL drawn on the map on this page and all other maps is now and always has been an artificial construct of cartographers—it is de facto (of fact). No international organization nor any treaty between nations has fixed the 'straight line' segments and their junctions. All nations unilaterally determine their standard time zones, which are applicable only on land and adjacent territorial waters. These national zones do not extend into international waters. Indeed, the 1884 International Meridian Conference explicitly refused to propose or agree to any time zones, despite 'common knowledge' that they did, stating that they were outside its purview. The conference resolved that the Universal Day (midnight-to-midnight Greenwich Mean Time), which it did agree to, "shall not interfere with the use of local or standard time where desirable."
However, the nautical date line which governs ships is determined by international agreement—it is de jure (of law). It is the result of the 1917 Anglo-French Conference on Time-keeping at Sea, which recommended that all ships, both military and civilian, adopt hourly standard time zones on the high seas. The United States, for example, adopted its recommendation for US military and merchant marine ships in 1920. This date line is implied but not explicitly drawn on time zone maps. It follows the 180° meridian except where it is interrupted by territorial waters adjacent to land, forming gaps—it is a pole-to-pole dashed line. Ships must adopt the standard time of a country if they are within its territorial waters, but must revert to international time zones (15° wide pole-to-pole gores) as soon as they leave its territorial waters. The 15° gore that is offset from GMT or UT1 (not UTC) by twelve hours is bisected by the nautical date line into two 7.5° gores that differ from GMT by ±12 hours.
Historical alterations
The Philippines, a Spanish colony until 1898, decidedly on the west side of the date line today, historically was on the east side, being in communication by ocean transport with Acapulco, Mexico. 17:20 in Acapulco was about 08:04 in Manila. A 9 hour, 16 minute difference, Manila ran nearly a half-day behind Acapulco.
During the 1840s, trade interests turned to China, the Dutch Indies and adjacent areas, and the Philippines was changed to the west side of the date line. Monday, 30 December 1844 (ending up as a 365-day year) was followed by Wednesday, 1 January 1845.
Until 1867, Alaska began Russia's day, with the date line following the partially-defined border between Russian Alaska and British North America, including the colony of British Columbia. The day before the purchase by the United States took effect, it was Friday, 6 October 1867, on the Julian calendar (still used by Russia), which would have been 18 October by the Gregorian calendar. The time in New Archangel would have been 12:00 when it was 12:02, Thursday, 17 October, at the future site of Whitehorse, Yukon and 12:49, 17 October, at the future site of Vancouver, British Columbia.
With the transfer of governance, the date line was shifted (moving Alaska back a day), and the calendar was changed (moving Alaska ahead 12 days), and being effective at midnight, the calendar moved ahead one day as well, for a net change of 12 days. Friday, 6 October was followed by Friday, 18 October.
Western Samoa changed in 1892, eight years following the official sanction of the Date Line. The king was persuaded by American traders to adopt the American date, being three hours behind California, to replace the former Asian date, being four hours ahead of Japan. The change was made at the end of the day, Monday, 4 July 1892, so that year, there were 367 days, including two occurrences of Monday, July 4.
