For most of human history, time was estimated using the position of the sun, with noon defined as the sun being directly overhead. A precise method for telling time was not available, nor was it needed, and towns and cities determined their own times, usually based on sundials. For those areas without access to a sundial, time could also be estimated by the sun’s position in the sky, and that was usually good enough.
A sundial uses the shadow produced when the sun moves around an upright stick, known as a gnomon. The shadow falls on a calibrated circle, which tells the apparent time. The ancient Sumerians (part of Mesopotamia, now southeastern Iraq) refined the sundial so that six time divisions were possible, and by 800 BC, the Egyptians were using sundials divided into 12 equal segments. Still, they were considered little more than a curiosity, since most people had no need to know what the time was. They simply went to bed when the sun went down, and got up when the sun came up.
With the rise of industrialization in the early 19th century there was increased use of mechanized clocks and watches, but still there was no standard, and time differed from town to town. This all changed with the invention of the telegraph and the beginning of the railroad industry.
In the 1850s observatories began to use the telegraph to communicate precise meridians and distances; this dovetailed nicely with the railroad’s need for time standards.
The Great Western Railway in Great Britain was the first to use a standardized time, ordering in November 1840 that all its stations use London time. Many other railways followed, and in November 1852, the Greenwich Observatory began sending out daily telegraphs to railways to assist in standardizing time.
The first standard time in America was introduced by railways in New England following an August 1853 fatal head-on collision in Rhode Island that occurred because the conductors of the two lines had set their watches with two different times.
Over the next several decades, U.S. railways adopted their own standardized times. Unfortunately each line thought its scheme should prevail, and once a few months went by, passengers discovered that there were more than 50 railway times and hundreds of local times. Stations displayed multiple clocks showing the local time and the times for the various railroads, creating unbelievable confusion for passengers. It could be officially 9AM on the clock in Chicago, but at the station 18 miles down the line the official time might that be anywhere from 8:50 to 9:10. And even in Chicago, where the official time might be 9AM, the Chicago & Northwestern clock might read 9:10, while the Chicago, Burlington & Quincy clock might say its 8:55.
In 1880, the British government ordered that Greenwich Mean Time become the standard time to be used throughout the country. The following year, U.S. railroads adopted a plan that called for four U.S. time zones, each measuring 15 degrees of longitude. Planners tweaked the map to keep existing N-S train lines within the same time zone so that railways would not have to leave one time zone, be in another time zone for ten minutes, then go back to the first. This is the reason time zones have somewhat odd shapes.
At noon, November 18, 1883, all over the United States and Canada, people changed their clocks and watches in synchronization with their zone’s standard time. Many towns, however, continued using their own local times, so, in March 1918, Congress passed the Standard Time Act, which officially established standard time throughout the United States.
There are literally dozens of time zones spread across the world. Here’s the list. Most zones are separated by an even hour, although there are several that are offset by 30 minutes, or 45 minutes, such as Venezuela (GMT – 4:30,) India (GMT + 5:30,) and Nepal (GMT + 5:45.)