A brief history of time zones
Prior to the invention of clocks, people around the world kept time using some type of instrument to observe the Sun’s zenith at noon. The earliest time measuring devices used either the Sun’s shadow or the rate in which water runs out of a vessel.
The pendulum clock was developed during the 17th century – these clocks were sufficiently accurate to be used at sea to determine longitude and for scientific time measurement in the 18th century.
In 1764, English horologist John Harrison, discovered that a clock could be used to locate a ship's position at sea with extraordinary accuracy. A new Longitude Act, known as the Act 5 George III, followed in 1765. Chronometers, which measure time accurately in spite of motion or varying conditions, became popular instruments among many merchant mariners during the 19th century.
Still, even after developments regarding longitude, many towns and cities set clocks based on sunsets and sunrises. Dawn and dusk occur at different times but time differences between distant locations were barely noticeable prior to the 19th century because of long travel times and the (lack of) long-distance communications. The use of local solar time became increasingly awkward as railways and telecommunications improved. Time zones were therefore a compromise, relaxing the complex geographic dependence while still allowing local time to be approximate with mean solar time.
19th century challenges
American railroads maintained many different time zones during the late 1800s. Each train station set its own clock making it difficult to coordinate train schedules and confusing passengers. Time calculation became a serious problem for people travelling by train (sometimes hundreds of miles in a day), according to the Library of Congress. Every city in the United States used a different time standard so there were more than 300 local sun times to choose from. Railroad managers tried to address the problem by establishing 100 railroad time zones, but this was only a partial solution to the problem.
Operators of the new railroad lines needed a new time plan that would offer a uniform train schedule for departures and arrivals. Four standard time zones for the continental United States were introduced on November 18, 1883. Britain, which already adopted its own standard time system for England, Scotland, and Wales, helped gather international consensus for global time zones in 1884.
The call for one prime meridian
Various meridians were used for longitudinal references among different countries prior to the late 1800s. However, the Greenwich Meridian was the most popular of these. The Greenwich Observatory's reputation for the reliability and accuracy in publications of its navigational data was one factor that contributed to the Greenwich Meridian’s popularity. Moreover, the shipping industry would benefit from having just one prime meridian. Many people informally recognized the Greenwich Meridian as the prime meridian prior to the International Meridian Conference in 1884.
Sir Sandford Fleming was one of the key players in developing a satisfactory worldwide system of keeping time, according to sources such as the Canadian Encyclopedia. He advocated the adoption of a standard or mean time and hourly variations from that in accordance with established time zones. He also helped convene the International Meridian Conference in 1884, where the international standard time system was adopted.
The International Meridian Conference
The International Meridian Conference at Washington DC, USA, adopted a proposal in October 1884. The proposal stated that the prime meridian for longitude and timekeeping should be one that passes through the centre of the transit instrument at the Greenwich Observatory in the United Kingdom (UK). The conference therefore established the Greenwich Meridian as the prime meridian and Greenwich Mean Time (GMT) as the world’s time standard. The international 24-hour time-zone system grew from this, in which all zones referred back to GMT on the prime meridian.
The main factors that favored Greenwich as the site of the prime meridian were:
- Britain had more shipping and ships using the Greenwich Meridian than the rest of the world put together (at the time). The British Nautical Almanac started these charts in 1767.
- The Greenwich Observatory produced data of the highest quality for a long time.
(Sheen, cited in Brannel Astronomy, n.d.).
Time zones in the 20th century
Interestingly, many French maps showed zero degrees at Paris for many years despite the International Meridian Conference’s outcomes in 1884. GMT was the universal reference standard – all other times being stated as so many hours ahead or behind it – but the French continued to treat Paris as the prime meridian until 1911. Even so, the French defined legal time as Paris Mean Time minus nine minutes and 21 seconds. In other words, this was the same time as GMT. France did not formally use to Coordinated Universal Time (UTC) as a reference to its standard time zone (UTC+1) until August in 1978 (Sheen, cited in Brannel Astronomy, n.d.).
Standard time, in terms of time zones, was not established in United States law until the Act of March 19, 1918, sometimes called the Standard Time Act. The act also established daylight saving time in the nation. Daylight saving time was repealed in 1919, but standard time in time zones remained in law, with the Interstate Commerce Commission (ICC) having the authority over time zone boundaries.
Many countries adopted hourly time zones by the late 1920s. Many nations today use standard time zones but some places adopt half-hour deviations from standard time or use quarter hour deviations. Moreover, countries such as China use a single time zone even though their territory extends beyond the 15 degrees of longitude.
How Many Time Zones?
Given a 24-hour day and 360 degrees of longitude around the Earth, it is obvious that the world's standard time zones have to be 15 degrees wide, on average. Some sources claim there are 24 standard time zones, while others say there are 25 time zones. The perspective of the number of time zones depends on the definition of a time zone versus the International Date Line. The world also has non-standard time zones.
Note: timeanddate.com wishes to acknowledge sources such as Brannel Astronomy, the Library of Congress (USA), the National Institute of Standards and Technology (NIST), the UK’s National Maritime Museum, and The Canadian Encyclopedia, for historical information made available about time zones. It is also important to note that any mention of the United Kingdom (U.K.) or United States (U.S.A.) in this article refers to what is now considered to be the UK or USA.
In this Article
- 19th century challenges
- The call for one prime meridian
- The International Meridian Conference
- Time zones in the 20th century
- How Many Time Zones?
Time Zone Library
- Time Zone History
- The UK’s Time Zones
- Half Hour Time Zones
- International Date Line
- Australia’s Time Zones
- Time Zones in Antarctica
- Only one Time Zone in Arizona
- Indiana’s Time Zones
- AM and PM: What do they mean?