GMT vs UTC | Greenwich Mean Time and Coordinated Universal Time | Atomic Time

Difference between GMT and UTC GMT (Greenwich Mean Time) and UTC (Coordinated Universal Time) are the two references…

Difference between GMT and UTC

GMT (Greenwich Mean Time) and UTC (Coordinated Universal Time) are the two references for telling time all over the planet. GMT is the time that the Royal Observatory located in Greenwich, London and is referred to as mean solar time. The UTC, however, is based on the International Atomic time or TAI. Some might wonder how Coordinated Universal Time came to be becomes UTC in acronym. This is because the English would give us the acronym CUT while the French Temps Universel Coordonné gives us TUC. As a compromise, the two parties agreed to adapt UTC which seemed fair to both.

The GMT is based upon astronomical observations and is used mainly by those connected to the UK like BBC World Services, the Met Office and the British Royal Navy. UTC though is the time scale that is proposed by the International Bureau of Weights and Measures or BIPM to be the official basis for international time. To determine the ITC, the BIPM in Paris utilizes data gathered from atomic clocks which are strategically positioned in timing laboratories all over the globe. Leap seconds are then added to compensate for Earth’s irregular shape, rotation and other factors.

Several countries such as Belgium, United Kingdom, Canada and Ireland make use of GMT. For the World Wide Web and Internet, however, UTC is more preferred. Global positioning systems or GPS also use UTC as their time reference.  Even the Network Time Protocol which synchronizes clocks of several computers connected through the internet utilizes UTC. Although GMT and UTC differs in a mere fraction of a second, this difference is considered very important when it comes to scientific matters.

It can therefore be assumed that UTC is a standard that is more internet-based, while GMT is more country-based.  In some situations where fractions of second are not really that important, GMT may be safely assumed as equivalent to UTC. Most people often think world time zones are negative or positive offsets from UTC. There is significant proof that UTC has already replaced GMT when it comes to being the chief time scale used as the reference in several regions.


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