[fitsbits] Re: leap second alert

[Paul Schlyter]

In article <838ofn$7v at post.gsfc.nasa.gov>,
William Thompson <thompson at orpheus.nascom.nasa.gov> wrote:
 
> Patrick Wallace <ptw at star.rl.ac.uk> writes:
> 
> >On 15 Dec 1999, Paul Schlyter wrote:
> 
> >>       Usually one means UT2 when saying UT.
> 
> >I think UT1 is the more usual meaning.
> 
>         (rest deleted)
> 
> There's a good explanation of the different kinds of UT time at 
> 
>         http://www.maa.mhn.de/Scholar/times.html
> 
> which contains the statement
> 
>         Because of the availability of time standards that are more precise and
>         easier to obtain (atomic clocks) UT2 has hardly any practical use. The
>         Universal Time commonly adopted in astronomy is therefore the UT1
>         scale.
 
Thanks for pointing this out.  Yes, I should update that text to reflect the
fact that UT2 nowadays is obsolete.
 
> Personally, I would expect UT to mean either UTC or UT1.  This is one reason
> why letting UTC drift relative to UT1 is so troubling.
 
If one requires an accuracy large enough that the difference between
UT1 and UTC matters, then one should not just write "UT" (or "GMT"),
but instead explicitly specify if UTC or UT1 is used.
 
> The computer industry tends to use the acronym GMT as a synonym for UTC, but
> there's currently no exact recognized definition for GMT.
 
There have been several different definitions of GMT.  Earlier, GMT was
often considered equal to UT1.  Thus, the term "GMT" should be used only
when the difference between UTC and UT1 doesn't matter.
 
> If they're so
> worried about handling leap seconds in their software, they can define
> 
>         GMT = TAI - 32
>
> (or whatever the value of TAI-UTC is when they freeze it).  The values
> of GMT and UTC would start out in sync, but then would start to diverge
> as soon as the next leap second was implemented in UTC.
 
No they shouldn't -- there are enough different definitions for GMT already,
no need to invent yet another one!
 
If one needs to use a uniform time scale with no leap seconds, there are
already two such time scales available:  TT and TAI.  Choose one of those!
 
-- 
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