Coordinated Universal Time (UTC) was adopted as an international standard in 1967 when it became apparent that the world needed to know what time it was to a greater degree of exactitude than ever before.
UTC was developed as a means of measuring time independent of the Earth’s rotation and the sun and the stars.
Yet UTC is one of the least understood measurements of time.
That’s because unlike, for example, Greenwich Mean Time (GMT) in London or Eastern Standard Time (EST) on the east coast of the United States, UTC is not the time measured in a specific place.
Rather, it is the same time all over the world because it is the time measured by the vibration of caesium atoms in atomic clocks.
That said, caesium clocks had to be set by reference to time somewhere, and Greenwich Mean Time (sometimes called ‘London Time’) was chosen.
What is Greenwich Mean Time?
Greenwich Mean Time was adopted as the time standard around the world at the Washington Meridian Conference of 1884.
It is the time calculated by the Greenwich Observatory in London by reference to the Earth’s rotation and by the position of the sun in the heavens.
The signatories to the Washington Conference were happy to adopt Greenwich as the meridian or ‘zero’ line because of what was on the other side of the world.
You see, if you are a few feet to one side of the meridian line at Greenwich in London on a particular day of the year then it is still the same day of the year as it is a few feet to the other side of the meridian line at Greenwich.
However, the Earth is divided in Time Zones, with the time in Paris being one hour ahead of the time in London, the time in New York being five hours behind the time in London, and so on around the world.
But there comes a point on the other side of the world that when you are standing on one side of the Date Line you are a whole day ahead of the date on the other side of the Date Line.
You can imagine how inconvenient that could be if there were a lot of messages flying back and forth across the Date Line. The reply could be sent before the original message was sent.
The thing is that if you draw the meridian line of longitude around the world, the line on the other side of the world from Greenwich in London lies mostly over the open water of the Pacific Ocean.
And everyone at the 1884 Washington Meridian Conference agreed that the open water of the Pacific Ocean was a convenient place for the Date Line. Therefore Greenwich Mean Time was an equally convenient place to use as a reference for UTC.
Atomic Clocks
The first successful caesium atomic clock was built in the Teddington laboratory in England in 1955. It works by measuring the vibrations of caesium atoms.
Once the reliability of these clocks was proven and established, the time measured by them was adopted internationally, and now there are many caesium clocks in various parts of the world. Today, there are approximately 70 caesium clocks in the United States alone.
Why UTC Was Developed
For all practical purposes, UTC is the same as Greenwich Mean Time, but they are not exactly the same because the Earth is slowing down in its rotation. As it slows, it puts everything out of synchronisation.
Of course in 1884 when Greenwich Mean Time was adopted as the international standard for the Meridian, a few fractions of a second difference between the actual time and the time measured by reference to the sun and the stars was insignificant.
Since the first observation of the vibration of caesium in 1958, the Earth has only moved 33 seconds out of synchronization, so the discrepancy is small.
To correct the discrepancy leap-seconds are introduced every few years. For example, a leap-second was added to Greenwich Mean Time at the end of 2008 to coordinate it with UTC.
In the latter part of the 20th century, those fractions of a second started to become important for electronic communications. Now that the measurement of time for very practical uses is measured in milliseconds, measuring time as exactly as possible is all the more critical.
For example, some stock trading networks have moved their servers nearer to the point of origin of the stock trading prices so that they can shave milliseconds off the time it takes them to gather and act on the information.
You can check times throughout the world at Time And Date.