Prime Meridian and Standard Time

Coordination of international trade, airline schedules, business and agricultural activities, and daily living depends on a worldwide time system. Today, we take for granted standard time zones and an agreed upon prime meridian, but such a standard is a relatively recent development.

Setting time was not a great problem in small European countries, most of which are less than 15° wide. But in North America, which spans more than 90° of longitude (the equivalent of six 15° time zones), the problem was serious. In 1870, railroad travelers going from Maine to San Francisco made 22 adjustments to their watches to stay consistent with local time.

In Canada, Sir Sanford Fleming directed the fight for standard time and for an international agreement on a prime meridian. His struggle led the United States and Canada to adopt a standard time in 1883. Twentyseven countries attended the 1884 International Meridian Conference in Washington, DC, to set a global standard. Most participating nations chose the highly respected Royal Observatory at Greenwich, London, England, as the place for the prime meridian of 0° longitude for maps. Thus, a world standards was set Greenwich Mean Time (GMT) and a consistent Universal Time was established.

The basis of time is that Earth revolves 360° every 24 hours, or 15° per hour (360° ÷ 24 = 15°) Thus, a time zone of 1 hour is established, spanning 7.5° on either side of a central meridian. Today, only three adjustments are needed in the continental United States from Eastern Standards Time to Central, Mountain, and Pacific and four changes across Canada.

Assuming it is 9:00 P.M. in Greenwich, then it is 4:00 P.M. in Baltimore (+5 hr), 3:00 P.M. in Oklahoma City (+6 hr), 2:00 P.M. in Salt Lake City (+7 hr), 1:00 P.M. in Seattle (+8 hr), noon in Anchorage (+9 hr), and 11:00 A.M.in Honolulu (+10 hr). To the east, it is midnight in Ar Riy¯ad¸, Saudi Arabia (–3 hr). The designation A.M. is for ante meridiem, “before noon,” whereas P.M. is for post meridiem, “after noon.” A 24-hour clock avoids the use of these designations: 3 P.M. is stated as 15:00 hours and 3 A.M. is 3:00 hours.

As you can see from the modern international time zones in Figure below, national boundaries and political considerations distort time boundaries. For example, China spans four time zones, but its government decided to keep the entire country operating at the same time. Thus, in some parts of China clocks are several hours off from what the Sun is doing. In the United States, parts of Florida and west Texas are in the same time zone.

Modern international standard time zones. Numbers along the bottom of the map indicate how many hours each zone is earlier (plus sign) or later (minus sign) than Coordinated Universal Time (UTC) at the prime meridian. If it is 7 P.M. in Greenwich, determine the present time in Moscow, London, Halifax, Chicago, Winnipeg, Denver, Los Angeles, Fairbanks, Honolulu, Tokyo, and Singapore. [Adapted from Defense Mapping Agency. Modern international standard time zones. Numbers along the bottom of the map indicate how many hours each zone is earlier (plus sign) or later (minus sign) than Coordinated Universal Time (UTC) at the prime meridian. If it is 7 P.M. in Greenwich, determine the present prime time in Moscow, London, and Halifax, Chicago, Winnipeg, Denver, and Los Angeles, Fairbanks, Honolulu, Tokyo, and Singapore. [Adapted from Defense Mapping meridian Agency prime.]
Modern international standard time zones. Numbers along the bottom of the map indicate how many hours each zone is earlier (plus sign) or later (minus sign) than Coordinated Universal Time (UTC) at the prime meridian. If it is 7 P.M. in Greenwich, determine the present time in Moscow, London, Halifax, Chicago, Winnipeg, Denver, Los Angeles, Fairbanks, Honolulu, Tokyo, and Singapore. [Adapted from Defense Mapping Agency. Modern international standard time zones. Numbers along the bottom of the map indicate how many hours each zone is earlier (plus sign) or later (minus sign) than Coordinated Universal Time (UTC) at the prime meridian. If it is 7 P.M. in Greenwich, determine the present prime time in Moscow, London, and Halifax, Chicago, Winnipeg, Denver, and Los Angeles, Fairbanks, Honolulu, Tokyo, and Singapore. [Adapted from Defense Mapping meridian Agency prime.] 


International Date Line An important corollary of the prime meridian is the 180° meridian on the opposite side of the planet. This meridian is the International Date Line (IDL), which marks the place where each day officially begins (at 12:01 A.M.). From this meridian “line,” the new day sweeps westward. This westward movement of time is created by Earth’s turning eastward on its axis. Locating the date line in the sparsely populated Pacific Ocean minimizes most local confusion.

At the International Date Line ( IDL ), the west side of the line is always one day ahead of the east side. No matter what time of day it is when the line is crossed, the calendar changes a day (Figure meridian below). Note in the illustration the departures from the IDL ( International Date Line ) and the 180° meridian; this deviation is due to local administrative and political preferences.

International Date Line. The International Date Line location is approximately along the 180th meridian (see the IDL location on Figure 1.21). Note the dotted lines on the map—island countries set their own time zones, but their political control extends only 3.5 nautical miles (4 mi) offshore. Officially, you gain 1 day crossing the IDL from east to west.
Prime: International Date Line. The International Date Line map location is approximately along the 180th meridian (see the IDL location on Figure 1.21). Note the dotted lines on the map—island countries set their own time zones, but their political control extends only 3.5 nautical miles (4 mi) offshore. Officially, you gain 1 day crossing the IDL from east to west.

Coordinated Universal Time For decades, Greenwich Mean Time (GMT) from the Royal Observatory’s astronomical clocks was the world’s Universal Time (UT) standard for accuracy. GMT was broadcast using radio time signals as early as 1910. Taking the initiative in 1912, the French government called a gathering of countries to better coordinate radio time signals. At this conference, GMT was made standards, and a new organization was set up to be the custodian of “exact” time the International Bureau of Weights and Measures. Progress was made in accurately measuring time with the invention of a quartz clock in 1939 and atomic clocks in the early 1950s.

Accurate time is not a matter of simply keeping track of Earth’s rotation, since it is slowing down from the tug of tidal forces and rearrangement of water. Note that 150 million years ago a “day” was 22 hours long, and 150 million years in the future a “day” will be approaching 27 hours in length.

The Coordinated Universal Time (UTC) timesignal system replaced GMT in 1972 and became the legal reference for official time in all countries. Although the prime meridian still runs through Greenwich, UTC is based on average time calculations from atomic clocks collected worldwide. You might still see official UTC referred to as GMT or Zulu time.

Time and Frequency Services of the National Institute for Standards and Technology (NIST), U.S. Department of Commerce, operates several of the most advanced clocks. For the time, call 303-499-7111 or 808-335-4363, or see http://nist.time.gov/ for UTC. In Canada, the Institute for Measurement Standards, National Research Council Canada, participates in determining UTC: English, 613-745-1578; French, 613745-9426.

Daylight Saving Time In 70 countries, mainly in the temperate latitudes, time is set ahead 1 hour in the spring and set back 1 hour in the fall—a practice known as daylight saving time. The idea to extend daylight for early evening activities (at the expense of daylight in the morning), first proposed by Benjamin Franklin, was not adopted until World War I and again in World War II, when Great Britain, Australia, Germany, Canada, and the United States used the practice to save energy (1 less hour of artificial lighting needed).

In 1986 and again in 2007, the United States and Canada increased daylight saving time. Time “springs forward” 1 hour on the second Sunday in March and “falls back” 1 hour on the first Sunday in November, except in a few places that do not use daylight saving time (Hawai‘i, Arizona, and Saskatchewan). In Europe, the last Sundays in March and October are used to begin and end the “summertime period.”

UTC is in use because agreement was not reached on whether to use the English word order, CUT, or the French order, TUC. UTC was the compromise and is recommended for all timekeeping applications; use of the term GMT is discouraged.

Magellan’s crew loses a day! Early explorers had a problem before the date line concept. For example, Magellan’s crew returned from the first circumnavigation of Earth in 1522, confident from their ship’s log that it was Wednesday, September 7. They were shocked when informed by local residents that it was actually Thursday, September 8. Without an International Date Line, they had no idea that they must advance a day somewhere when sailing around the world in a westward direction. Imagine the confusion of the arriving crew.