What is Universal Time (UTC)? Why is it for? How does UTC relate to Greenwich Mean Time (GMT) and Eastern Standard Time (EST)? What is an Atomic clock? What is a leap-second? What is the Meridian and what is the International Date Line? Why are fractions of a second and UTC important in the 21st century?

1. Universal Time (UTC) Explained
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David Bennett
One of the less understood measurements of time is Coordinated Universal Time
(UTC).
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 it is not the time measured
in a particular place.
Instead, it is the same time all over the world because 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 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 that meant for the time and date 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.
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.

2. 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.
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
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.

3. Out of Synchronization
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.
Nonetheless, to correct the discrepancy leap-seconds are introduced every few years.
A leap-second was added to UTC at the end of 2008 to coordinate it with Greenwich
Mean Time.
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.
But in the latter part of the 20th century, those fractions of a second started to become
important for electronic communications.
You can check times throughout the world at Time And Date.
These facts about UTC were brought to you by The Quillcards Team