[LEAPSECS] Schedule for success

Nero Imhard nimh at pipe.nl
Tue Dec 30 17:41:11 EST 2008

On 2008-12-30, at 20:57, M. Warner Losh wrote:

> Why is it a fundamental change?

Because the coupling between UTC and UT is abandoned.

> Only one parameter of UTC is being

> changed, and we all knew that this day was coming when the length of

> the second was fixed in 1956 to be the mean second as of 1900 (well,

> it turned out to be as of 1820, but I digress).

Of course. And that's why we have a time scale (actually several) that
counts SI seconds, and one that follows UT. The latter is a
compromise. It counts SI seconds and sometimes seconds have to be
inserted (or skipped) to keep it in sync. Kind of kludgy, but the best
solution we have come up with so far. Better solutions would be

> Time used to be strongly coupled to the earth. Each day was divided

> into hours minutes and seconds. The earth used to rotate through a

> fixed angle each second, because of the definition of the second as

> 1/86400th of a day.


> However, that is no longer the case. We now know that each day is

> 86400.002 seconds long. Part of this is that the length of the second

> was poorly chosen, but it really doesn't matter: the earth is slowing

> down and the length of the second will continue to degrade as a

> measure of the earth's rotation.

It hurts that bad huh? Well, there may be ample reason to base telco
or geek time on something else than UTC. Fine. What about TAI? I mean,
it's a familiar and established time scale, and has just the
properties the ITU seems to be after. Really, why not TAI?

> As an expert in the time keeping field, I believe that it is. DUT1

> only matter to astronomers, satellite trackers and ocean navigators.

> Since GPS has replaces almost all celestial navigation, the primary

> motive for DUT1 to be kept small is now gone.

How big can DUT1 become then? Would it be reasonable to wish for the
upper bound to stay below the accuracy of a sundial? What does the ITU
proposal say about the new limit of DUT1? I haven't seen any mention
of it (but I do tend to overlook things).

> Put another way, who would care if clocks were 34s ahead of where they

> are today.

Probably nobody. Although it would definitely require an adjustment to
a Bernhardt-type sundial. I'm not sure about the interval over which
these sundials (which last eternally) can safely be adjusted (if at
all). Proper adjustment would require a rotating scale, which they
don't have.

> So what's so important about having small DUT1 anyway?

Hell, I don't know. It's just that UTC is the time scale that promised
it in the first place.


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