Time Zone, any of 24 geographic areas into which the earth is divided for the purpose of maintaining a standard time system (see Time). Clocks within a given time zone are set to the same time, which is generally one hour later than the zone immediately to the west. Each time zone is defined by its distance east or west of Greenwich, England.
Until the late 1800s most towns and cities set clocks based upon the rising and setting of the sun. Because of the earth’s rotation, dawn and dusk occur at different times at different places, but time differences between distant locations were barely noticeable because of long travel times and the lack of long-distance communications.
With the growth of rapid locomotive travel and long-distance telegraph communications during the 1830s, schedule and message conflicts began to arise. For example, because each train station set its own clock, it was difficult to coordinate train schedules. In the 1870s American railroads maintained 50 different time zones. Technology had created a need for a unified time-keeping system.
Cleveland Abbe, an American meteorologist, developed a system of weather reporting and forecasting using the telegraph to share information between weather stations. In order to compile his information, Abbe required a time-keeping system that was consistent between the stations. To accomplish this he divided the United States into four standard time zones. In 1883 Abbe convinced North American railroad companies to adopt his time zone system. In 1884 Britain, which had already adopted its own standard time system for England, Scotland, and Wales, helped gather international consensus for global time zones.
Since the earth rotates 15 degrees of longitude per hour, the earth’s 360 degrees were divided into 24 zones, each measuring about 15 degrees in width. The 0° longitude line, or meridian, was defined as a line running through the old Greenwich Observatory in Greenwich, England. Time in each of the 12 zones east of Greenwich increases one hour for each zone. Time in each of the 12 zones to the west of Greenwich decreases by one hour. The International Date Line lies at the 180° meridian on the opposite side of the earth from Greenwich and divides the eastern and western time zones. The time difference between each side of the International Date Line is 24 hours. Thus, a traveler heading west across the date line loses one day while a traveler headed east gains a day.
Major variations in time zone boundaries exist to follow physical, geographical, and political boundaries and to avoid bisecting highly populated areas. The state of Georgia, for example, was originally divided into two time zones; the boundary was subsequently moved west to run along the Alabama-Georgia border. China, which spans about 50 degrees of longitude, observes a single time zone based upon the time in the eastern part of the country. Guyana, Liberia, Mongolia, and Saudi Arabia still use their own local time systems. The International Date Line bends around the Bering Strait so that all of eastern Russia lies within the same day’s time. Under the 1966 Uniform Time Act, the Department of Transportation administers and adjusts time zones within the United States.
The time in any given time zone or country may shift by one hour for certain periods of the year to gain maximum daylight hours and balance these hours from morning to evening. One such system is Daylight Savings Time in the United States.
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