[LEAPSECS] Inside GNSS published an update of my CGSIC talk

[Warner Losh]

On Mon, Mar 20, 2023 at 1:37 PM Michael Deckers via LEAPSECS <
leapsecs at leapsecond.com> wrote:

>
>     On 2023-03-20 07:54, Jürgen Appel via LEAPSECS wrote:
>
>
> > In your Conclusion, you say "the CGPM resolution also stipulates that no
> > change to current practices can occur before 2035."
> >
> > This is not how I read read the CGPM document on the BIPM website:
> > "The General Conference on Weights and Measures (CGPM), at its 27th
> meeting
> > [...] decides that the maximum value for the difference (UT1-UTC) will be
> > increased in, or before, 2035,"
> >
> > So in case the negative leap seconds become a real threat, according to
> my
> > interpretation is is an option to increase the tolerance value earlier
> than
> > 2035 to avoid trying out negative leap seconds a last and first time.
> >
> > Can someone confirm my view?
>
>
>
>      You read correctly, the French (official) version has
>
>         ..."décide que la valeur maximale pour la différence
>             (UT1 - UTC) sera augmentée au plus tard en 2035,"....
>
>      which means "in 2035 at the latest".
>
>      Note also that the definition of UTC as approved by the
>      CGPM never mentions _any_ explict bound for |UT1 - UTC|; it
>      only says that (TAI - UTC) is an integral multiple of 1 s
>      as determined by the IERS. It is the IERS who state that
>
>         "Coordinated Universal Time (UTC) a measure of time
>          that conforms, within approximately 1 s, to the mean
>          diurnal motion of the Sun and serves as the basis of
>          all civil timekeeping."
>
>      quoting the IAU "Nomenclature for Fundamental Astronomy (NFA)"
>      found at http://syrte.obspm.fr/iauWGnfa/NFA Glossary.html.
>
>      This seems to be lenient enough to allow for not scheduling
>      a negative leap second even in the case that the difference
>      (UT1 - UTC) should go a bit below -1 s before 2035.
>

So is "approximately" here to be read in the "astronomer" sense that it's
that, give or take an order of magnitude, or some more close reading :) For
astronomy, often times things that are approximately the same can vary
quite a bit, and that's fine.

More seriously even 2s is approximately 1s if there's some kind of effort
to keep it from freewheeling to 10s, 100s, or 1000s of seconds.

For example, the Gregorian calendar approximates the year over the
centuries, but any given year can deviate up to 2 days (worst case) from
the idealized solstice dates.

I'd also note that if GLASNOS can't be fixed by 2035, then a fallback to a
schedule of leap seconds to keep the answer approximately 1s in the long
haul could also be on the table. Having it be scheduled, rather than
observational, has advantages: everybody gets leap years right, and will
for the next few hundred years (maybe with a glitch at 2100 and 2400). A
much lower percentage get leap seconds right because leap second knowledge
propagates imperfectly, even after all these years of trying (my first
anti-leapsecond screeds date back maybe 20 years). So my first choice is
always 'none, cope with shifting civil time on the scale of centuries' but
my second choice is 'schedule for the long-term average and don't worry
about going > 1s' .

Warner


>      Michael Deckers.
>
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