Northern Arizona astronomy: why we have time zones

October sky chart.

Barry Malpas

October sky chart.

Before clocks were invented, people kept time based on the position of the sun. When the sun rose, it was time to get up and work. When it was highest in the sky, crossing the local meridian, it was noon. And when the sun set, it was time to go to bed. It was pretty straightforward.

As civilizations advanced and became more complicated, devices were developed that would cast the shadow of a vertical rod onto the ground, or other flat surface, which divided the day into smaller parts to better manage time. These sundials were used well into the Middle-Ages.

However, things began to change when the mechanical pendulum clock was developed during the 17th century and cities set their town clock by measuring the local position of the sun. Every city would be on a slightly different local time depending on their longitude since the sun will appear at its highest point in the sky at different times because of each town’s location east or west of one another. This really did not matter to the average citizen since most people rarely travelled any great distances beyond the town in which they grew up.

As countries like Britain, became sea faring, pendulum clocks were insufficiently accurate to be used at sea to determine longitude for navigational purposes. So in 1764, when the chronometer was invented, time could be determined accurately in spite of a ship’s swaying motion or other varying conditions. For the average person, dawn and dusk occurring at different times between distant locations were still barely noticeable because of long travel times and the lack of long-distance communications. Towns and cities still set their clocks based on their local solar position.

Britain was the first country to set the time throughout a region to one standard time. The railways cared most about these inconsistencies in local mean time, and forced a uniform time on the country. The original idea was suggested by Dr. William Hyde Wollaston, and the Great Western Railway was the first to adopt London time in 1840. By 1847, most railways in Britain used Greenwich Mean Time, though there were many hold-outs until the Statutes (Definition of Time) Act of 1880 took effect.

Meanwhile, every city in North America still used a different time standard, resulting in more than 300 local solar times from which to choose. The first person in the U.S. to realize the growing need for a time standard was William Lambert, an amateur astronomer, who in 1809 presented to Congress a recommendation for the establishment of standard time meridians which was promptly ignored.

As the 19th century advanced, the railroad lines needed a time plan offering a uniform train schedule for departures and arrivals. Time zones finally became realized as the possible compromise. It relaxed the complex geographic dependence, while still allowing local time to be approximate with mean solar time. Originally, railroad managers tried to address the problem by establishing 100 railroad time zones, but this was still too complicated. In 1883, four standard time zones for the continental U.S. and Canada were introduced, though not exactly the same ones we have today.

It was many years before this new method of telling time was actually accepted by the people. However, the popularity eventually increased because of the practical advantages for communication and travel. Standard time zones were finally established by U.S. law with the Standard Time Act on March 19, 1918.

In 1884, the International Meridian Conference in Washington DC adopted a proposal stating that the Prime Meridian for world longitude and timekeeping would be one that passes over the center of the transit instrument at the Greenwich Observatory in England, which established 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.

So, be happy that we live in the Mountain Standard Time Zone becasue otherwise, every time you travelled between Williams and Flagstaff you would have to reset your watch, plus or minus, 2 minutes 10 seconds.


Comments are not posted immediately. Submissions must adhere to our Use of Service Terms of Use agreement. Rambling or nonsensical comments may not be posted. Comment submissions may not exceed a 200 word limit, and in order for us to reasonably manage this feature we may limit excessive comment entries.

Use the comment form below to begin a discussion about this content.