[ntp:questions] Re: Newbie questions, UTC vs. Greenwich

Peter Bunclark psb at ast.cam.ac.uk
Tue Jan 24 15:10:27 UTC 2006

Markus Kuhn wrote:
> alkruse at gmail.com (Alan Kruse) writes:
> |> What is the difference between Universal Coordinated Time (UTC) and
> |> Greenwich Mean Time?
> Greenwich Mean Time is a historic term that is no longer used among
> time-keeping experts or in many languages (other than English). There
> is no modern official definition of what GMT is exactly, but astronomers
> generally will tell you that GMT is pretty much the same as the modern
> astronomical definition of time they call UT1 today.

Well actually, that's putting words into the mouths of astronomers that
don't really belong... professional astronomers whose work involves an
understanding of timekeeping fully understand that GMT went the way of
the dodo. Rather, it is the civil authorities and the media who assume
that the pips on the BBC are "GMT" whereas in fact they are UTC.


UTC is a time
> produced by precision clocks (mostly cesium resonantors) that is
> guaranteed to be within 0.9 seconds of UT1.
> So the difference between UTC and GMT is: < 0.9 s
> For non-astronomical purposes, e.g. computer timekeeping,
> UTC is the modern-day replacement for GMT. UTC is today widely
> announced with far greater accuracy over radio signals than
> any time called GMT ever was.
> http://en.wikipedia.org/wiki/Universal_Time
> http://en.wikipedia.org/wiki/Coordinated_Universal_Time
> http://en.wikipedia.org/wiki/GMT
> Markus

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