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 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:
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.
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 7Spain 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.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,
- 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.