International Date Line

This article needs additional citations for verification. Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (September 2009)

The International Date Line (IDL) is a generally north-south imaginary line on the surface of the Earth, passing through the middle of the Pacific Ocean, designating the place where each calendar day begins. It is roughly along 180° longitude, (opposite the Prime Meridian), but it is drawn with diversions to pass around some territories and island groups.

Crossing the IDL travelling east results in a day or 24 hours being subtracted, so that the traveller repeats the date to the west of the line. Crossing west results in a day being added, that is, the date is the eastern side date plus one calendar day. The line is necessary in order to have a fixed, albeit arbitrary, boundary on the globe where the calendar date resets. In practice, there are two such boundaries:^{[citation needed][dubious – discuss]} one is the International Date Line, while the other moves around the globe as the midnight time line (2400 hours local time), marked by the moment at which the date changes to the next day at a given point on the globe.^{[citation needed][dubious – discuss]} These two boundaries mark the transition from one day to the next.

Contents 1 Geography 2 De facto and de jure date lines 3 Historical alterations 4 Cultural references 5 Date lines according to religious principles 5.1 Judaism 5.2 Christianity 6 References

[edit] Geography

For part of its length, the International Date Line follows the meridian of 180° longitude, roughly down the middle of the Pacific Ocean. To avoid crossing nations internally, however, the line deviates to pass around the far east of Russia and various island groups in the Pacific.

In the north, the date line swings to the east of Wrangel island and the Chukchi Peninsula and through the Bering Strait passing between the Diomede Islands at a distance of 1.5 km (1 mi) from each island. It then goes southwest, passing west of St. Lawrence Island and St. Matthew Island, until it passes midway between the United States' Aleutian Islands and Russia's Commander Islands before returning southeast to 180°. This keeps Russia which is north and west of the Bering Sea and the United States' Alaska which is east and south of the Bering Sea, on opposite sides of the line in agreement with the date in the rest of those countries.

Two uninhabited atolls, Howland Island and Baker Island, just north of the equator in the central Pacific Ocean, have the oldest time on Earth of UTC−12:00 hours, along with ships at sea between 172.5°W and 180°. The date line then circumscribes the country of Kiribati by swinging far to the east, almost reaching the 150° meridian. Kiribati's easternmost islands, the sourthern Line Islands south of Hawaii, have the most advanced time on Earth of UTC+14 hours. In the South Pacific, the date line also lies east of 180°, so that Wallis and Futuna, Fiji, Tonga, and New Zealand's Kermadec Islands and Chatham Islands have the same date, but Samoa and New Zealand's Cook Islands are one day earlier. Samoa plans to adjust the dateline on December 29, 2011. ^[1]

If one flies round the world from east to west (the same direction as Magellan), one hour is lost for every 15° of longitude crossed, losing 24 hours for one circuit of the globe; one compensates by adding 24 hours when crossing the International Date Line (also from east to west). The International Date Line must therefore be observed in conjunction with the Earth's time zones: on crossing it, in either direction, the calendar date is adjusted by one day.

For two hours every day, between 10:00 and 11:59 (UTC), there are actually three different days observed at the same time in different places. At UTC time Thursday 10:15, for example, it is Wednesday 23:15 in Samoa, which is eleven hours behind UTC, and it is Friday 00:15 in Kiritimati (separated from Samoa by the IDL), which is 14 hours ahead of UTC. For the first hour (UTC 10:00–10:59), this is true for inhabited territories, whereas during the second hour (UTC 11:00–11:59) it is only true if one counts an uninhabited maritime time zone twelve hours behind UTC.

[edit] 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—the precise course of the line in international waters is arbitrary. 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, 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".

From this comes the utility and importance of UTC, or "Zulu" time: it permits a single and universal reference for time that is valid for all points on the globe at the same moment.

The nautical date line is a de jure construction determined by international agreement. 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 U.S. 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 should adopt the standard time of a country if they are within its territorial waters, but should revert to international time zones (15° wide pole-to-pole gores) as soon as they leave its territorial waters. In reality they use these time zones only for purposes such as radio communication. For internal ship's purposes, e.g. for work and meal hours, they use a time zone of their own choosing. The 15° gore that is offset from UTC by twelve hours is bisected by the nautical date line into two 7.5° gores that differ from UTC by ±12 hours.

[edit] Historical alterations

The Philippines, as part of the Viceroyalty of New Spain, long had its most important communication with Acapulco in Mexico, and was accordingly placed on the east side of the date line, despite being at the western edge of the Pacific Ocean. 00:01 Tuesday in London was 17:21 Monday in Acapulco and about 08:05 Monday in Manila. During the 1840s, trade interests turned to China, the Dutch East 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, despite being a leap year) was followed by Wednesday, 1 January 1845.^[2]

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, in the Julian calendar (used by Russia at the time), which would have been 18 October in 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 (not Saturday, 7 October).

Samoa changed in 1892, eight years following the international conference that would result in de facto development of the Date Line. King Malietoa Laupepa 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 on Monday, 4 July 1892, so there were 367 days (1892 being a leap year), including two occurrences of Monday, 4 July.

The central Pacific Republic of Kiribati introduced a change of date for its eastern half on 1 January 1995, from time zones −11 and −10 to +13 and +14. Before this, the country was divided by the date line. After the change, the date line in effect moved eastwards to go around this country. As a British colony, Kiribati was centered in the Gilbert Islands, just west of the old date line. Upon independence in 1979, the new republic acquired the Phoenix and Line Islands from the United States and the country found itself straddling the date line. Government offices on opposite sides of the line could only communicate by radio or telephone on the four days of the week which were weekdays on both sides. As a consequence, Kiribati's easternmost territory, the uninhabited Caroline Atoll at 150°25′ west, started the year 2000 before any other country on earth, a feature the Kiribati government capitalized upon as a potential tourist draw. But Ariel and Berger comment that the international community has not taken this date line adjustment very seriously, noting that most world atlases still ignore the Kiribati dateline shift and continue to represent the International Date as a straight line in the Kiribati area.^[3]

In 2011, Samoan Prime Minister Tuilaepa Sailele announced his country would move the International Date Line again, shifting it to the east of the country, so that Samoa would share the same date as Australia, New Zealand and Asia, facilitating trade with its primary partners.^[4] He suggested that the most likely date would be 29 December 2011^[5] with a New Year’s date transition.

[edit] Cultural references

The need for a date line was first flagged when Ferdinand Magellan's expedition circumnavigated the world in 1519–1522. The surviving crew (a mere 18 men out of 217 who departed) returned to a Spanish stopover sure of the day of the week, as attested by carefully maintained sailing logs. Nevertheless, those on land insisted the day was one day later. This phenomenon, now readily understandable, caused great excitement at the time, to the extent that a special delegation was sent to the Pope to explain this temporal oddity to him. The protagonist in The Twenty-One Balloons also travelled toward the west.

The effect of ignoring the date line is also seen in Jules Verne's book Around the World in Eighty Days, in which Phileas Fogg returns to London after a trip around the world, thinking that he has lost the bet that is the central premise of the story. Having travelled in the direction opposite to Magellan, he believes the date there to be one day later than it is. Lest anyone accuse Fogg of cheating by obtaining one extra day, this is not so. On average, each travel day was 18 minutes short of a full 24 hours, accumulating to one full day, which they failed to correct as we would by setting our calendar back a day in mid-Pacific.

An error in navigation due to the date line has also been suggested as a reason for the disappearance of Amelia Earhart and her navigator Fred Noonan in 1937. Earhart was flying her Lockheed Electra between Lae, New Guinea and Howland Island on 2 July 1937 when they disappeared shortly after crossing the date line. Noonan may have made a mistake in using the proper dates. An error of this magnitude could have resulted in a 60-nautical-mile (110 km; 69 mi) difference in the relative location of their intended target. In addition, due to the oddities of the date line, Earhart and Noonan are shown to have been alive on the day after they disappeared, flying for several hours on 3 July local time and then disappearing on 2 July.^[6]

The date line is also a central factor in Umberto Eco's book The Island of the Day Before, in which the protagonist finds himself on a becalmed ship, with an island close at hand on the other side of the International Date Line. Unable to swim, the protagonist's writings indulge in increasingly confused speculation of the physical, metaphysical and religious import of the date line.

[edit] Date lines according to religious principles

[edit] Judaism

The concept of an international date line is first mentioned in a 12th century Talmudic commentary ^[7][8] which seems to indicate that the day changes in an area where the time is six hours ahead of Jerusalem (90 degrees east of Jerusalem, a line running through the Philippines). This line, which he refers to as the K'tzai Hamizrach (the easternmost line), is used to calculate the day of Rosh Hashanah, the Jewish New Year. According to some sources it is alluded to in the Talmud (Rosh Hashanah and Eruvin) as well as in the Jerusalem Talmud.

The date line poses a problem for religious travellers relative to the day on which to observe the Shabbat and Holidays. The Shabbat is on the seventh day of the week, which is constant if one stays on the same side of the date line. The problem occurs when a Jewish traveller crosses the line and for whom it is Friday but for the city the traveller is visiting, it is Saturday. There are several different opinions regarding where exactly the date line is according to Jewish law.^[8][9]

The halachic ruling of Rabbi Moshe Heinemann Rabbinic Administrator of the Star-K, is as follows:^[8] In New Zealand and Japan, the local Saturday is according to the majority opinion Shabbat, and it should therefore be fully observed as Shabbat, with Shabbat Prayers, etc. However, since according to the Chazon Ish, Shabbat is on the local Sunday, one should not perform any Shabbat Torah prohibitions on Sunday. Nevertheless, on Sunday, one should pray the regular weekday prayers, donning tefillin during morning prayers.

In Hawaii, Saturday is Shabbat according to the majority opinion. Therefore, the local Saturday is fully observed as Shabbat. The day known locally as Friday is Shabbat according to the minority opinion, and one should not perform Shabbat Torah prohibitions on that day. Cooking for Shabbat should therefore be done on Thursday.

Determining the majority opinion on the Aleutian Islands or South Pacific Islands, including Fiji and American Samoa, is complicated and beyond the scope of this article. However, in the following locations, Shabbat is observed on the local Saturday, and a second day is not necessary: Australia, China, Mainland Russia, Taiwan, Hong Kong, Mainland Alaska and Manila, and other areas of the Philippines west of 125.2°E.

[edit] Christianity

In Christianity it is also important to hold services on the correct day. Generally, the Christian calendar follows the legal calendar, and the Christian date line is on the same place as the international date line. One important issue exists in the Orthodox countries, where the Julian calendar is followed for religious purposes but the Gregorian calendar for civilian purposes. The two calendars have the same weekdays, so the main issue is when to celebrate Easter, Christmas and other main holidays. In Tonga, the Seventh Day Adventists (who usually observe the sabbath on a Saturday) observe the sabbath on a Sunday, officially due to an anomaly in the International Date Line which makes Sunday really Saturday. In reality, this practice is most likely a practical one. In Tonga, Sunday is culturally (and partially legally) reserved for Christian worship and little else.

[edit] References

v · d · eTime measurement and standards Major subjects Time · Chronometry · Orders of magnitude · Metrology International standards UTC · UTC offset · UT · DUT1 · IERS · ISO 31-1 · ISO 8601 · TAI · 12-hour clock · 24-hour clock · Barycentric Coordinate Time · Civil time · Daylight saving time · Geocentric Coordinate Time · International Date Line · Leap second · Solar time · Terrestrial Time · Time zone Obsolete standards Barycentric Dynamical Time · Ephemeris time · Greenwich Mean Time · Prime Meridian Time in physics Absolute time and space · Spacetime · Chronon · Continuous time · Coordinate time · Cosmological decade · Discrete time · Planck epoch · Planck time · Proper time · Theory of relativity · Time dilation · Gravitational time dilation · Time domain · T-symmetry Horology Clock · Astrarium · Atomic clock · Complication · Equation of time · History of timekeeping devices · Hourglass · Marine chronometer · Marine sandglass · Radio clock · Sundial · Watch · Water clock Calendar Astronomical · Calculating the day of the week · Dominical letter · Epact · Equinox · Gregorian · Intercalation · Islamic · Julian · Leap year · Lunar · Lunisolar · Seven-day week · Solar · Solstice · Tropical year · Week-day names Archaeology & geology Dating methodologies · Geologic time scale · International Commission on Stratigraphy Astronomical chronology Galactic year · Nuclear time scale · Precession · Sidereal time Units of time Century · Day · Decade · Fortnight · Hour · Jiffy · Lustrum · Millennium · Minute · Month · Saeculum · Second · Shake · Tide · Week · Year Related topics Chronology · Duration · Mental chronometry · Metric time · System time · Time value of money · Timekeeper

Coordinates: 0°N 180°W / 0°N 180°W / 0; -180