[LEAPSECS] Schedule for success
M. Warner Losh
imp at bsdimp.com
Tue Dec 30 14:57:06 EST 2008
In message: <EA6B0AE7-7375-4AB4-8F0C-005E5A5183AA at pipe.nl>
Nero Imhard <nimh at pipe.nl> writes:
: >> I still don't get why you are insisting that UTC could be changed.
: > It was changed in 1958 and 1972, I see nothing preventing it from
: > being changed in 2008, -9, -10 or any other year.
: Like the redefinition of the meter that you mention, these were not
: such fundamental changes as the one now proposed.
Why is it a fundamental change? 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).
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.
: >> switching to something like TI [...]
: > We have millions of documents which mandate, directly or indirectly,
: > the usage of UTC. Starting with legislation about local timezones
: > over POSIX to international treaties about transport, communication
: > and power-generation.
: And if I understand you well, you are saying that it is reasonable to
: second-guess the motivation behind all these documents (and those we
: haven't thought of) and conclude that it is safe to change a
: fundamental property of UTC.
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.
: > Taking leap seconds out of UTC has a clear implementation plan, it
: > is one document that needs changed and ratified in a UN forum and
: > we are done with the problem.
: Keeping civil time in line with mean solar time is the key issue that
: brought forth various schemes of rubber seconds, leap seconds, etc.
: These mechanisms testify that closely following UT has always been the
: very goal of UTC. Redefinitions of UTC that refine or reform the
: mechnism for following UT may be reasonable. Abandoning the
: relationship completely just isn't.
Actually, the human race has been moving away from mean solar time for
some time now. First the timezones decoupled man from local solar
mean to a mean time around an arbitrary meridian. Next, daylight
savings time has shown that we can shift that about by an hour and
people cope. Finally, now that we can measure the second more
accurately, we can see the slow drift.
Put another way, who would care if clocks were 34s ahead of where they
: > Changing all the other documents and teaching people about proper
: > choice of timescales is a proven path to failure.
: Making a proper and deliberate choice will be quite impossible once
: the idea is accepted that time scales can be fundamentally altered in
: > UTC is a definition for the advancement of international
: > communications
: > not one of the 10 commandments.
: If "the advancement of international communications" were the only
: goal and use of UTC, I would have no objection at all. Let the ITU
: have a ball with it! But that's just not realistically the case. UTC
: always seemed, and has been used, like something from a standards body.
So what's so important about having small DUT1 anyway?
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