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28 Apr 2014, 07:01 UTC
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Question: I read what you have on your Local & UTC page, but I’m still a little confused about the difference between GMT and UTC. Could you expand on that please? — DV, Tucson, AZ
Answer: Absolutely. Time measurement systems are not the easiest thing to understand. Let’s start with GMT, which was the first global time system.
GMT stands for “Greenwich Mean Time” and it’s measured using a telescope at the observatory in Greenwich, England. Officially known as the “Royal Observatory, Greenwich,” it was founded in 1675 by Charles II for the purpose of improving time measurement and navigation.
What they do at Greenwich is point a telescope straight up, and time the passage of stars overhead. When the same star reappears in the view, Earth has rotated once, and a “day” has elapsed. Averaged over time, this is what defined the length of a mean day (average length of one day). England had a strong incentive for measuring time accurately.
During the Age of Discovery, England was the preeminent seafaring nation. Portugal and Spain were up there too, but England had the clear lead in terms of ships, colonies, and technology. To claim a territory on the globe, ...

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