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Map of United States Time Zones

crossing multiple time zones
You're on a plane heading somewhere, you cross several time zones, and you will possibly suffer from jet lag later because your inner clock rebels.
Image: kk

What is a time zone?

24 spherical lunes
Earth is divided into 24 segments known as lunes.
Standard-US Time Zones Standard US Time-Zones (click map to enlarge)

A time zone is defined as an area where a uniform standard time applies for legal, economic and social purposes. Standard time zones are 15 degrees of longitude wide.

A time zone is ideally one of 24 spherical lunes (a section on the globe in a north/south direction) with equal-width (15 degrees), each assigned with one of the 24 hours (makes together one day and one rotation of the Earth).

Normally, a uniform standard time is maintained in each section to track the day and night cycle. This means that people in each geographic region within a time zone use the same time.

All these zones are defined by an offset from the Coordinated Universal Time (UTC; see below) by a number of hours (UTC−12 to UTC+14) centered on the prime meridian (0°; since 1884 the Greenwich meridian [1]).

Today's use of time zones tends to follow more often the boundaries of countries and their subdivisions instead of the standard parallels because it is convenient in close economic or political areas to keep the same time. For example, most of Alaska uses Alaska Time, which spans three standard time zones. The state used two time zones until 1983, when it placed the capital, Juneau, in the same time zone as the more populous Anchorage and Fairbanks.
China spans four time zones but uses only one. The time difference between Bangkok, Thailand, and Kuala Lumpur, Malaysia, is one hour, although the two capitals are on almost the same longitude.

U.S. Time Zones
The time zone map below shows the six time zones of the United States, and a series of clocks show the actual time in the different time zones.

U.S. Daylight Saving Time
In the United States, Daylight Saving Time begins at 2 a.m. on the second Sunday in March and ends at 2 a.m. on the first Sunday in November. Two states don't observe daylight saving time, Arizona and Hawaii.

Time Zones Map of United States including Hawaii and Alaska
U.S. Time Zone Map
The map shows the six time zones into which the United States is divided. The territory of the 50 US states extends over eight standard time zones, from UTC (GMT) −4 to −11 hours. There are six designated US time zones; and there are four time zones in the contiguous USA.

US Time Zones

The US time zones are (from west to east):

  • Hawaii–Aleutian Time Zone
    The Aleutian Islands, a spur of Alaska, and the Hawaiian Islands in the Pacific Ocean are 3,750 km (2,330 mi) apart but lie at similar longitudes.
    Hawaii–Aleutian Standard Time (HST) UTC − 10 h; Hawaii does not observe daylight saving time.
    The Alaskan Aleutian Islands observe Hawaii–Aleutian Daylight Time (HDT) UTC − 9 h
  • Alaska Time Zone
    Alaska is located in the far northwest of North America and has no border with any other U.S. state.
    Alaska Standard Time (AKST) UTC − 9 h; Alaska Daylight (AKDT) UTC − 8 h.
  • Contiguous USA Time Zones
    48 states
  • Pacific Time Zone (PT)
    Pacific Standard Time (PST) UTC − 8 h; Pacific Daylight Time (PDT) UTC − 7 h
  • Mountain Time Zone (MT)
    Mountain Standard Time (MST) UTC − 7 h; Mountain Daylight Time (MDT) UTC − 6 h (Note: most of Arizona does not observe daylight saving time)
  • Central Time Zone (CT) Central Standard Time (CST) UTC − 6 h; Central Daylight Time (CDT) UTC − 5 h
  • Eastern Time Zone (ET)
    Eastern Standard Time (EST) UTC − 5 h; Eastern Daylight Time (EDT) UTC − 4 h

UTC: Coordinated Universal Time
Time zones around the world are expressed as positive or negative offsets from UTC or GMT (arguably the same as Coordinated Universal Time). Local time is UTC and the time zone offset for that location, e.g., Eastern Standard Time (EST) = UTC −5 hours.
UTC replaced Greenwich Mean Time on 1 January 1972 as the basis for the main reference time scale or civil time in various regions.

US states with more than one time zone.
As you can see on the time zone map, there are 15 US states with more than one time zone in effect.
Below is a list of US states with more than one time zone: list

More about Time Zones

American railroad scene: lightning express trains leaving the junction
One of the driving forces behind the development of time zones was the increasing use of faster modes of transportation in the late 19th and early 20th centuries. In the US, much of the national rail network was completed by 1900. In March 1918, Congress officially established the standard timekeeping system that is associated with this arrangement of time zones in the United States.
Image: American railroad scene: lightning express trains leaving the junction (Currier & Ives, 1874; New York)

As is well known, Earth is a sphere that is supplied with light and heat by the sun. Due to Earth's spherical shape, the sun's rays are most intense at the location nearest to the sun.

According to an ancient tradition, when on earth the sun is at its highest it is 12 o'clock, noon -- time for lunch.
Since the earth rotates on its axis, it is not also noon in other parts of the globe, but morning or evening or night.

Before the Industrial Revolution, when life was dominated by farming and artisan crafts, the times to sow, fertilize and harvest were important.

With the progress of technology and the sciences and as the world moved closer together through ever faster means of transport such as steamships and the railroad, people started to think about how to get the time problem under control.

Time zones are an invention of modern time. Before the creation of standard time zones, every place had its own time. With the spread of the telegraph and the construction of railroad networks in the 19th and early 20th centuries, communication and travel gained new dimensions - and new problems. Due to the lack of time standardization, it was difficult to coordinate train schedules on the mostly single-track lines; a signal at the wrong time could lead to fatal collisions.

However, the impetus for the introduction of standardized time did not come from the railroads alone. Astronomers and geophysicists, seeking simultaneous observations from scattered geographic locations, had long advocated a uniform time. [LoC]

An Air France Lockheed Constellation ("Connie") airliner. American Aircraft Used by European Airlines Air France Constellation
Time zones became important in commercial air travel, especially for scheduling and coordinating flights.

An Air France Lockheed Constellation ("Connie") airliner built by Lockheed Corporation starting in 1943. Passenger plane travel in tourism between Europe and the US began to gain popularity in the post-World War II era, particularly in the 1950s and 1960s. The introduction of larger and more comfortable passenger aircraft such as the Boeing 707 and Douglas DC-8 made long-distance air travel more practical and affordable for a growing number of people.

In the early days of air travel, airlines used a variety of different time zones, which made scheduling and coordination difficult. However, by the 1950s, most airlines had adopted a system based on Coordinated Universal Time (UTC), which provided a standard reference point for time across the world.

Image: The U.S. National Archives

Before the introduction of time zones, there were between 140 and 300 different local time zones in North America. As the story goes, a missed train in Ireland by Sandford Fleming led eventually to the creation of a coordinated Universal Time for the whole planet. Fleming, a Scottish-born Canadian engineer, was one of the leaders in the effort to develop a worldwide system of keeping time. He proposed a system of time zones in 1879 and presented it at several international conferences.

The British already had a standardized time. In 1847, the national Railway Clearing Union ordered that Greenwich Mean Time (GMT) should apply at all railroad stations. The British Post Office transmitted time signals from Greenwich by telegraph to most parts of the country to set the clocks.
Since 1875, Cleveland Abbe, an astronomer, meteorologist, and the first head of the U.S. Weather Bureau, had lobbied the American Meteorological Society (AMS) for a uniform standard time.
In October 1884, the International Meridian Conference was held in Washington, D.C., "for the purpose of fixing upon a meridian proper to be employed as a common zero of longitude and standard of time-reckoning throughout the globe."
The standard timekeeping system associated with this arrangement of time zones was officially established in the United States by Congress in March 1918.
On 1 January 1960, Coordinated Universal Time (UTC), the international basis of civil and scientific time, was introduced. The unit of UTC is the atomic second. UTC replaced Greenwich Mean Time (GMT), based on the motion of the Earth, as the World Standard for time in 1972.

Time zones have become an indispensable part of our interconnected world of global communication and transportation systems, allowing people to coordinate activities across long distances and reducing confusion and errors caused by differences in local times.

Weather Washington, D.C.





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