What Is Coordinated Universal Time

Golden clock worked by the sun - on a pedestal in a garden in the West Country in England with trees behind in the valley and rising onto the hills behind

What Is Coordinated Universal Time? Greenwich Mean Time (GMT) is the time measured by reference to London. Eastern Standard Time (EST) is the time measured by reference to the east coast of the United States. And so on for places around the world.

Coordinated Universal Time (UTC) is different. It is not the time measured in a specific place at all.

It is the time measured by the vibration of caesium atoms in atomic clocks and it was adopted as an international standard in 1967.

Of course, 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 is the place that all other times around the world were measure against. So, two hours ahead or eight hours behind, means so many hours ahead of or behind 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 more than happy to adopt Greenwich as the meridian or ‘zero’ line because of what is on the opposite side of the world from Greenwich.

If you are a few paces 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 paces to the other side of the meridian line at Greenwich.

Time Zones

And as you move around the Earth clockwise, you see that the Earth is divided in Time Zones. The time in Paris being one hour ahead of the time in London. And the time in New York being five hours behind the time in London, and so on around the world. Tokyo is eleven hours ahead of Greenwich Mean Time. And then we get out into the Pacific Ocean.

The International Date Line is on the other side of the world, opposite London.

Now suppose it is midday in London. Then on the International Date Line, when you go a few paces further west, it’s one day later; cross back and it’s a day earlier.

How inconvenient that would be if there were a lot of messages flying back and forth across the International Date Line. The reply could be sent before the original message was sent – at least according to how we measure time around the Earth.

But of course when you follow the meridian line of longitude around the world to the International Date Line, you are 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.

Atomic Clocks and Coordinated Universal Time (UTC)

Everything changed with atomic clocks, because now mankind had a way of measuring time that had nothing to do with where you were in the heavens.

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 caesium clocks all over the world. There are more than 70 caesium clocks in the United States alone.

Why Coordinated Universal Time (UTC) Was Developed

UTC was developed as a means of measuring time independent of the Earth’s rotation and the sun and the stars.

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.

Since the first observation of the vibration of caesium in 1958, the Earth has moved 33 seconds out of synchronisation.

In 1884, Greenwich Mean Time was adopted as the international standard for the Meridian. And in those days, a few fractions of a second difference between the time measured by an independent means and the time measured by reference to the sun and the stars, was insignificant.

But in the world of high-speed electronic communications, thousandths of a second matter. So to correct the discrepancy leap-seconds are added. What this means is that UTC is retarded periodically in order to maintain agreement between UTC and the apparent day length.

The most recent leap-seconds that were added to UTC were on December 31, 2008 and on December 31, 2016.


As Time Goes By

How different were the days when the sun was the closest approximation for measuring the passage of time. Who would have had a clock in the garden of their house? This one is in the West Country in England, in a grand house as befits the timepiece.

If you are a subscriber to Half Moon Cards, this photograph is also available as an ecard. Click the photo to go straight to the Compose page.

Golden clock worked by the sun - on a pedestal in a garden in the West Country in England with trees behind in the valley and rising onto the hills behind

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