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What's the meaning of the word "mean" in "Greenwich Mean Time"?

Shouldn't we simply say something like "Greenwich Time"? I don't understand what the word "mean" is doing there.

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3 Answers 3

up vote 34 down vote accepted

According to Wikipedia, Greenwich Mean Time refers to the mean solar time measured at the Royal Observatory in Greenwich.

This article has more information about mean solar time, but it is essentially the mean (in the sense of average) angle of the sun in the sky at any particular time of day. At least insofar as I understand it.

So the mean in Greenwich Mean Time is "average."

As a note, mean solar time is contrasted with apparent solar time, which is based on how long it actually takes for the Sun to return to the same position in the sky. Apparent solar time varies because the Earth's orbit is not perfectly circular, and the Earth speed of the Earth varies according to its distance from the Sun. I think mean solar time is the average of apparent solar time over a solar year.

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+1 I'll try to expand on this answer a bit. If you took noon to be the time when the sun was highest in the sky each day (solar noon), then there wouldn't be exactly twenty-four hours each day, because of the eccentricity of the Earth's orbit. When this discrepancy is averaged out, you have mean time rather than solar time. Solar noon and mean noon can differ by as much as fifteen minutes, depending on the time of year. –  Peter Shor Jun 9 '11 at 13:38
@Peter Thank you. That is much clearer. I always have trouble explaining astronomical time (like sidereal, yeesh). –  KitFox Jun 9 '11 at 13:41
I upvote Peter Shor's comment explaining how solar noon drifts by a small time each day according to the Earth's orbit. I would like to downvote Matt Ellen's answer (but that would be unkind) for introducing the non-scientific definition of mean. In 2011, GMT is largely outdated. It is worth understanding that scientists and engineers are likely to refer to Universal Time or Universal Coordinated Time (abbreviated as UTC) which is closely related. This removes the geopolitical loading of "Greenwich" Mean Time. –  Dizzley Jun 9 '11 at 18:01

In addition to its normal usages, ("not nice", and "has the meaning of"), mean can also be used as a mathematical term to mean the average of a set of numbers. I always figured this was roughly the sense in which the word is being used in GMT. So basically it ..er.. means "this is the base or middle of all the time zones that exist". That is why you don't see that word in the other time zones. It is telling you that this particular one is special.

You can also think of it as "Mean (average) Time", with the word "Greenwich" tacked on to tell you where this time exists.

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It's only in the modern era that GMT is the only timezone to be "X Mean Time", since others are defined as offsets to GMT. For example, Ireland was on "Dublin Mean Time" (~25 minutes behind GMT) from from 1880 until 1916. –  Random832 Jun 9 '11 at 14:45
Time zones are defined by Coordinated Universal Time these days. GMT is a throwback to the 1970s, I think. –  KitFox Jun 9 '11 at 17:58
In the UK the term GMT is still commonly used to describe the local time zone during winter. I think time zone selection screens (eg in Windows) often put the offset from GMT alongside each time zone. –  Caltor Nov 6 '12 at 23:37
@KitFox Yes, sadly, for us sentimental British, the Greenwich Time Signal (the six pips broadcast by BBC on the hour) no longer come from Greenwich, but from an atomic clock in the basement of Broadcasting House, connected to the National Physical Laboratory, in Teddington (I suppose linked to similar devices worldwide). The other thing that nowadays messes up the 'pips', is that with digital radio there is a three second delay, versus analogue. To set your watch perfectly you need to be listening to the pips on an analogue radio. –  WS2 Nov 7 at 12:39

As has been stated mean in the context of GMT is a type of average.

There are three main types of average: mean, median and mode.

The mean average is where you add up all the number in a list of numbers and then divide the total buy the number of numbers. E.g. the mean of 4,5,6,7,8,9,10 is (4 + 5 + 6 + 7 + 8 + 9 + 10) / 7.

According to Etymonline the meaning is

that which is halfway between extremes

and is from the same word as median.

The median average is where you sort the numbers in a list in ascending numerical order and pick the one in the middle. So the median of 4,5,6,7,8,9,10 is 7.

The mode average is where you pick the number that occurs most frequently in a list. E.g. in 1,1,2,3,4,5,5,5,5,6,7 the mode is 5.

The meaning of mode is related to the French word for fashion, and can be approximately understood to mean most popular.

For more information see this website: http://www.bbc.co.uk/schools/ks2bitesize/maths/data/mode_median_mean/play.shtml

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Hardly relevant to the question! –  Colin Fine Jun 9 '11 at 17:06
@Colin: It's a perfect answer to the question: What's the meaning of the word "mean" in "Greenwich Mean Time"?, which was asked. It doesn't answer Why do we care about Mean Time?, as Kit's answer does, but pedantically speaking, Tom didn't ask that. –  Ben Voigt Jun 9 '11 at 17:25
@Colin I think this is a good answer. It is a nice explanation of mean. @Ben Pedantically speaking, Tom didn't ask for median and mode either. –  KitFox Jun 9 '11 at 17:55
My undergrad degree is in math, and I must admit that I have never heard of either a "median average" or a "mode average." In contrast, "mean," "median," and "mode" were covered extensively. I have been taught that "mean" and "average" were synonymous ("mean average" redundant and not used), and that "median" and "mode" were as @Matt Ellen described (without the "average" affixed). My cursory Google search seems to place "median average" or "mode average" on the European side of the pond. I'm inferring from the BBC site that British children are taught "median average" at a tender age. –  rajah9 Jun 9 '11 at 18:12
They are all measures of central tendency as I recall. Interesting to know that they have different phrasings in BrE and AmE. –  KitFox Jun 9 '11 at 18:57

protected by RegDwigнt Jun 9 '11 at 21:21


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