meaning - Why is there a "mean" in "Greenwich Mean Time" (GMT)?


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.



Answer



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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