#AtoZChallenge2015: Greenwich Mean Time (GMT)

As a lover of navigation history I was delighted when I fist visited Greenwich, in East London, once the home of the Royal Navy College, and, of course the locus of the meridian that sets the time for the whole world! Greenwich is close to our home, we often visit its beautiful park and old streets. If you happen to visit London, don’t miss Greenwich!

Greenwich MeantimeGreenwich Mean Time (GMT) refers to the mean solar time at the Royal Observatory in GreenwichLondon, which became adopted as a global time standard. Coordinated Universal Time (UTC) is arranged so that it runs no more than 0.9 seconds fast or slow of Greenwich Mean Time. The name Greenwich Mean Time is especially used by bodies connected with the United Kingdom, such as the BBC World Service,[1] the Royal Navy, the Met Office and others particularly in Arab countries, such as the Middle East Broadcasting Center and OSN. It is a term commonly used in the United Kingdom and countries of the Commonwealth, including Australia, New Zealand, South Africa, India, Pakistan and Malaysia, and many other countries in the Eastern hemisphere.

Greenwich Mean Time is the same as Universal Time (UT), a standard astronomical concept used in many technical fields. In the United Kingdom, GMT is the official time during winter; during summer British Summer Time (BST) is used. GMT is very close to Western European Time. Some countries using Western European Time may base their standard on UTC.[2]

Noon Greenwich Mean Time is rarely the exact moment when the sun crosses the Greenwich meridian and reaches its highest point in the sky there, because of Earth’s uneven speed in its elliptic orbit and its axial tilt. This event may be up to 16 minutes away from noon GMT, a discrepancy calculated by the equation of time. Noon is the annual average (i.e. mean) time of this event, prompting the inclusion of “mean” in “Greenwich Mean Time”.

Historically the term GMT has been used with two different conventions, sometimes numbering hours starting at midnight and sometimes starting at noon. The more specific terms UT and UTC do not share this ambiguity, always referring to midnight as zero hours. Astronomers preferred the latter GMT convention to simplify their observational data so that each night was logged under a single calendar date.”

Image: “Greenwich clock” by Alvesgaspar – Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons – https://commons.wikimedia.org/wiki/File:Greenwich_clock.jpg#/media/File:Greenwich_clock.jpg

3 thoughts on “#AtoZChallenge2015: Greenwich Mean Time (GMT)

  1. Right on time to read this! Nobody in the US speaks of the Meridien de Greenwich as I used to hear when I lived in France. You are right about the place being intersting. I went years and years ago when I lived in Paris, before the channel was even built. Beautiful place. See you tomorrow.


  2. I have an entry coming up where I talk about UTC and GMT (they’re used interchangeably these days, although there is the difference you mentioned), and I’m going to send people over to see this. This is excellent!


  3. Pingback: Noon (#atozchallenge) | The Sound of One Hand Typing

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