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Time zone

2007 Schools Wikipedia Selection. Related subjects: General Geography;
General Physics

   A time zone is a region of the Earth that has adopted the same standard
   time, usually referred to as the local time. Most adjacent time zones
   are exactly one hour apart, and by convention compute their local time
   as an offset from Greenwich Mean Time (see also UTC).
   Standard Time Zones of the World
   Enlarge
   Standard Time Zones of the World

   Standard time zones can be defined by geometrically subdividing the
   Earth's spheroid into 24 lunes (wedge-shaped sections), bordered by
   meridians each 15° of longitude apart. The local time in neighbouring
   zones is then exactly one hour different. However, political and
   geographical practicalities can result in irregularly-shaped zones that
   follow political boundaries or that change their time seasonally (as
   with daylight saving time), as well as being subject to occasional
   redefinition as political conditions change.

   There are variations of the definitions of time zone which generally
   fall into two meanings: a time zone can represent a region where the
   local time is some fixed offset from a global reference (usually UTC),
   or a time zone can represent a region throughout which the local time
   is always consistent even though the offset may fluctuate seasonally.

   Before the adoption of time zones, people used local solar time
   (originally apparent solar time, as with a sundial; and, later, mean
   solar time). Mean solar time is the average over a year of apparent
   solar time. Its difference from apparent solar time is the equation of
   time.

   This became increasingly awkward as railways and telecommunications
   improved, because clocks differed between places by an amount
   corresponding to the difference in their geographical longitude, which
   was usually not a convenient number. This problem could be solved by
   synchronizing the clocks in all localities, but then in many places the
   local time would differ markedly from the solar time to which people
   are accustomed. Time zones are thus a compromise, relaxing the complex
   geographic dependence while still allowing local time to approximate
   the mean solar time. There has been a general trend to push the
   boundaries of time zones farther west of their designated meridians in
   order to create a permanent daylight saving time effect. The increase
   in worldwide communication has further increased the need for
   interacting parties to communicate mutually comprehensible time
   references to one another.

Standard time zones

   Originally, time zones based their time on Greenwich Mean Time (GMT,
   also called UT1), the mean solar time at longitude 0° (the Prime
   Meridian). But as a mean solar time, GMT is defined by the rotation of
   the Earth, which is not constant in rate. So, the rate of atomic clocks
   was annually changed or steered to closely match GMT. But on January 1,
   1972 it became fixed, using predefined leap seconds instead of rate
   changes. This new time system is Coordinated Universal Time (UTC). Leap
   seconds are inserted to keep UTC within 0.9 seconds of UT1. In this
   way, local times continue to correspond approximately to mean solar
   time, while the effects of variations in Earth's rotation rate are
   confined to simple step changes that can be easily subtracted if a
   uniform time scale ( International Atomic Time or TAI) is desired. With
   the implementation of UTC, nations began to use it in the definition of
   their time zones instead of GMT. As of 2005, most but not all nations
   have altered the definition of local time in this way (though many
   media outlets fail to make a distinction between GMT and UTC). Further
   change to the basis of time zones may occur if proposals to abandon
   leap seconds succeed.

   Due to daylight saving time, UTC is local time at the Royal
   Observatory, Greenwich only between 01:00 UTC on the last Sunday in
   October and 01:00 UTC on the last Sunday in March. For the rest of the
   year, local time there is UTC+1, known in the United Kingdom as British
   Summer Time (BST). Similar circumstances apply in many places.

   The definition for time zones can be written in short form as UTC±n (or
   GMT±n), where n is the offset in hours. These examples give the local
   time at various locations at 12:00 UTC when daylight saving time (or
   summer time, etc.) is not in effect:
     * San Francisco, California, United States: UTC-8; 04:00
     * Toronto, Ontario, Canada: UTC-5; 07:00
     * Stockholm, Sweden: UTC+1; 13:00
     * Cape Town, South Africa: UTC+2; 14:00
     * Mumbai, India: UTC+5:30; 17:30
     * Tokyo, Japan: UTC+9; 21:00

   Where the adjustment for time zones results in a time at the other side
   of midnight from UTC, then the date at the location is one day later or
   earlier. Some examples when UTC is 23:00 on Monday when daylight saving
   time is not in effect:
     * Cairo, Egypt: UTC+2; 01:00 on Tuesday
     * Wellington, New Zealand: UTC+12; 11:00 on Tuesday

   Some examples when UTC is 02:00 on Tuesday when daylight saving time is
   not in effect:
     * New York City, New York, United States: UTC-5; 21:00 on Monday
     * Honolulu, Hawaii, United States: UTC-10; 16:00 on Monday

   The time-zone adjustment for a specific location may vary because of
   Daylight Saving Time. For example New Zealand, which is usually UTC+12,
   observes a one-hour daylight saving time adjustment during the southern
   hemisphere summer, resulting in a local time of UTC+13.

   See also Sidereal time, Calculating local time.

History

   Greenwich Mean Time (GMT) was established in 1675 as an aid to
   determine longitude at sea by mariners. The first time zone in the
   world was established by British railways on December 1, 1847 — with
   GMT hand-carried on chronometers. About August 23, 1852, time signals
   were first transmitted by telegraph from the Royal Observatory,
   Greenwich. Even though 98% of Great Britain's public clocks were using
   GMT by 1855, it was not made Britain's legal time until August 2, 1880.
   Some old clocks from this period have two minute hands — one for the
   local time, one for GMT . This only applied to the island of Great
   Britain, and not to the island of Ireland.

   On November 2, 1868, New Zealand (then a British colony) officially
   adopted a standard time to be observed throughout the colony, and was
   perhaps the first country to do so. It was based on the longitude 172°
   30' East of Greenwich, that is 11 hours 30 minutes ahead of GMT. This
   standard was known as New Zealand Mean Time.

   Timekeeping on the American railroads in the mid nineteenth century was
   somewhat confused. Each railroad used its own standard time, usually
   based on the local time of its headquarters or most important terminus,
   and the railroad's train schedules were published using its own time.
   Some major railroad junctions served by several different railroads had
   a separate clock for each railroad, each showing a different time; the
   main station in Pittsburgh, Pennsylvania, for example, kept six
   different times. The confusion for travellers making a long journey
   involving several changes of train can be imagined.

   Charles F. Dowd proposed a system of one-hour standard time zones for
   American railroads about 1863, although he published nothing on the
   matter at that time and did not consult railroad officials until 1869.
   In 1870, he proposed four ideal time zones (having north–south
   borders), the first centered on Washington, D.C., but by 1872 the first
   was centered 75°W of Greenwich, with geographic borders (for example,
   sections of the Appalachian Mountains). Dowd's system was never
   accepted by American railroads. Instead, U.S. and Canadian railroads
   implemented their own version on Sunday, November 18, 1883, also called
   "The Day of Two Noons", when each railroad station clock was reset as
   standard-time noon was reached within each time zone. The zones were
   named Intercolonial, Eastern, Central, Mountain, and Pacific. Within
   one year, 85% of all cities with populations over 10,000, about 200
   cities, were using standard time. A notable exception was Detroit,
   Michigan (which is nearly half-way between the meridians of eastern
   time and central time, though actually a little closer to central),
   which kept local time until 1900, then tried Central Standard Time,
   local mean time, and Eastern Standard Time before a May 1915 ordinance
   settled on EST and was ratified by popular vote in August 1916. This
   hodgepodge ended when Standard zone time was formally adopted by the
   U.S. Congress on 19 March 1918.

   Time zones were first proposed for the entire world by Canada's Sir
   Sandford Fleming in 1876 as an appendage to the single 24-hour clock he
   proposed for the entire world (located at the centre of the Earth and
   not linked to any surface meridian!). In 1879 he specified that his
   universal day would begin at the anti-meridian of Greenwich (now called
   180°), while conceding that hourly time zones might have some limited
   local use. He continued to advocate his system at subsequent
   international conferences. In October 1884, the International Meridian
   Conference did not adopt his time zones because they were not within
   its purview. The conference did adopt a universal day of 24 hours
   beginning at Greenwich midnight, but specified that it "shall not
   interfere with the use of local or standard time where desirable".

   Nevertheless, most major countries had adopted hourly time zones by
   1929. Today, all nations use standard time zones for secular purposes,
   but they do not all apply the concept as originally conceived. Israel,
   for example, legally starts the day at 6:00 PM instead of midnight—so,
   the international date 1 January begins at what most other countries
   call 6:00 PM on 31 December. Newfoundland, India, and parts of
   Australia use half-hour deviations from standard time, and some nations
   use quarter-hour deviations.

Nautical time zones

   Before 1920, all ships kept local apparent time on the high seas by
   setting their clocks at night or at the morning sight so that, given
   the ship's speed and direction, it would be 12 o'clock when the Sun
   crossed the ship's meridian (12 o'clock = local apparent noon). During
   1917, at the Anglo-French Conference on Time-keeping at Sea, it was
   recommended that all ships, both military and civilian, should adopt
   hourly standard time zones on the high seas. A ship within the
   territorial waters of any nation would use that nation's standard time.
   The captain was permitted to change his ship's clocks at a time of his
   choice following his ship's entry into another time zone—he often chose
   midnight. These zones were adopted by all major fleets between 1920 and
   1925 but not by many independent merchant ships until World War II.

   Time on a ship's clocks and in a ship's log had to be stated along with
   a "zone description", which was the number of hours to be added to zone
   time to obtain GMT, hence zero in the Greenwich time zone, with
   negative numbers from −1 to −12 for time zones to the east and positive
   numbers from +1 to +12 to the west (hours, minutes, and seconds for
   nations without an hourly offset). These signs are opposite to those
   given below because ships must obtain GMT from zone time, not zone time
   from GMT. All zones were pole-to-pole staves 15° wide, except −12 and
   +12 which were each 7.5° wide, with the 180° meridian separating them.
   Unlike the zig-zagging land-based International Date Line, the nautical
   International Date Line follows 180° except where it is interrupted by
   territorial waters and the lands they border, including islands. About
   1950, a letter suffix was added to the zone description, assigning Z to
   the zero zone, and A–M (except J) to the east and N–Y to the west (J
   may be assigned to local time in non-nautical applications; zones M and
   Y have the same clock time but differ by 24 hours: a full day). These
   were to be vocalized using a phonetic alphabet which included Zulu for
   GMT, leading sometimes to the use of the term "Zulu Time". The
   Greenwich time zone runs from 7.5°W to 7.5°E Longitude, while zone A
   runs from 7.5°E to 22.5°E Longitude, etc.

   These nautical letters have been added to some time zone maps, like the
   map of Standard Time Zones by Her Majesty's Nautical Almanac Office
   (NAO), which extended the letters by adding an asterisk (*) or dagger
   (†) for areas that do not use a nautical time zone, and a double dagger
   (‡) for areas that do not have a legal standard time (Greenland's ice
   sheet and all of Antarctica. The United Kingdom specifies UTC-3 for the
   Antarctic Peninsula, but no other country recognizes that). They
   conveniently ignore any zone that does not have an hour or half-hour
   offset, so a double dagger (‡) has been co-opted for these zones in the
   list of time zones.

   In maritime usage, GMT retains its historical meaning of UT1, the mean
   solar time at Greenwich. UTC, atomic time at Greenwich, is too
   inaccurate, differing by as much as 0.9 s from UT1, creating an error
   of 0.4 km in longitude at the equator. However, DUT can be added to UTC
   to correct it to within 50 ms of UT1, reducing the error to only 20 m.

Skewing of zones

   Ideal time zones, such as nautical time zones, are based on the mean
   solar time of a particular meridian located in the middle of that zone
   with boundaries located 7[DEL: + :DEL] ^1⁄[2] degrees east and west of
   the meridian. In practice, zone boundaries are often drawn much farther
   to the west with often irregular boundaries, and some locations base
   their time on meridians located far to the east. For example, Spain
   uses the mean solar time of 15 degrees east even though the Prime
   Meridian (0°) passes through it. The tendency to draw time zone
   boundaries far to the west of their meridians allows greater
   utilization of more daylight in the afternoon hours. Many of these
   locations also use daylight saving time which means that solar noon
   could occur as late as 2:00 P.M. by the clock.

Trivia

     * In terms of the largest number of time zones, Russia is first, with
       eleven time zones, including Kaliningrad on the Baltic Sea. The
       United States is tied with Canada for second with six time zones.
       If the possessions of the United Kingdom, the United States and
       France are included it increases the number of time zones in each.
       Taking into account overseas territories and possessions, France
       has the most, with over twelve time zones, followed by the United
       States with nine and the United Kingdom with over eight.
     * In terms of area, China is the largest country with only one time
       zone ( UTC+8), although before the Chinese Civil War in 1949 China
       was separated into five time zones. China also has the widest
       spanning time zone.
     * Stations in Antarctica generally keep the time of their supply
       bases, thus both the Amundsen-Scott South Pole Station (U.S.) and
       McMurdo Station (U.S.) use New Zealand time ( UTC+12 southern
       winter, UTC+13 southern summer).
     * The 27° latitude passes back and forth across time zones in South
       Asia. Pakistan: +5, India +5:30, Nepal +5:45, India (Sikkim) +5:30,
       China +8:00, Bhutan +6:00, India ( Arunachal Pradesh) +5:30,
       Myanmar +6:30. This switching was more odd in 2002, when Pakistan
       enabled Daylight Saving Time. Thus from west to east, time zones
       were: +6:00, +5:30, +5:45, +5:30, +8:00, +6:00, +5:30 and +6:30.
     * Because the earliest and latest time zones are 26 hours apart, any
       given calendar date exists at some point on the globe for 50 hours.
       For example, April 11 begins in time zone UTC+14 at 10:00 UTC April
       10, and ends in time zone UTC-12 at 12:00 UTC April 12.
     * There are numerous places where several time zones meet, for
       instance at the tri-country border of Finland, Norway and Russia.
     * There are about 39 time zones instead of 24 (as popularly
       believed). This is due to fractional hour offsets and zones with
       offsets larger than 12 hours near the International Date Line. Some
       micronations may use offsets that are not recognized by all
       authorities.
     * The largest time gap along a political border is the 3.5 hour gap
       along the border of China (UTC +8) and Afghanistan ( UTC+4:30).
     * One of the most unusual time zones is the Australian Central
       Western Time zone (CWST), which is in effect in a small strip of
       Western Australia from the border of South Australia west to just
       before Caiguna. It is 8¾ hours ahead of UTC (UTC+8:45) and covers
       an area of about 35,000 km², larger than Belgium, but has a
       population of about 200. Although unofficial, it is universally
       respected in the area. See Time in Australia.

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