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Since the earth rotates around its axis once every 24 hours, there are 24 time zones around the earth, with 1-hour difference between them. Since the earth rotates through 360 degrees in 24 hours, it must rotate through 15 degrees of longitude in 1 hour (360° ÷ 24 = 15°). Therefore, each of the 24 time zones is 15 degrees wide. Every place within a time zone has the same time, referred to as its standard time.
With the development of rapid railway transportation, in the second half of the 1880s, the need for standard time zones became obvious to Sir Sanford Fleming, Canada’s most experienced railway surveyor and civil engineer. He wrote to the major governments of the world proposing the use of time zones. In 1884, an international conference was held in Washington, D.C. to approve Fleming’s system. It adopted the meridian that runs through the Royal Observatory in Greenwich, England as the prime, or zero, meridian.
The prime meridian is the centre of a time zone that extends 7.5 degrees on either side. Time in this zone is called universal time (UT – Universal time was formerly called Greenwich Mean Time (GMT). Some people still refer to time in this zone as GMT.) and the standard times in other zones are compared to it. Every 15 degrees, on either side of the prime meridian, is the centre of a time zone.
Since the earth rotates from west to east, time zones that are east of the prime meridian have local times that are ahead of UT. Time zones that are west of the prime meridian have local times that are behind UT. For example, if the sun is directly above the prime meridian, the local time in the city of Berlin, which is one time zone east of UT, is 1 p.m. In Ottawa, which is five times zones west of the prime meridian, the time is 7 a.m. It will be five more hours before the sun appears directly over the 75°W meridian, which is the centre of the time zone where Ottawa is located.
Countries, however, may modify the shape of the time zones and the standard) or legally recognized) time they use. For example, China has one standard time for the whole country despite the fact that it covers more than 60° of longitude. Canada, on the other hand, has six standard time zones. The boundaries of the zones, however, do not follow exactly the meridians of longitude.
Countries modify the shapes of the time zones for political reasons. For example, it may be more convenient to have all of a province in one time zone. Time zones may also be adjusted so that they do not pass through a city. Imagine the confusion that would arise if half a city was in one time zone and the other half was in another time zone.
Some places are located where time zones meet. St. John’s, Newfoundland, at 52.18°W, is located between the third and fourth time zones west of the prime meridian so its time is 3.5 hours behind UT.