[time-nuts] Re: UTC - A Cautionary Tale
seaman at noao.edu
Thu Jul 14 12:33:47 EDT 2005
John Ackermann says:
> By the way -- Rob's message was held as a non-member submission
> which I approved. Unless he's subscribed to the list in the
> meantime, he won't see any responses unless you separately cc him.
Thanks for approving the message - it wasn't clear from the list's
web home whether members only could post. I did try to join, but
think I hosed the form. In any event, there didn't appear to be a
way to turn off email list delivery - I much prefer to read list
archives online. Thanks for making your archives public, BTW.
Poul-Henning Kamp says:
> I find it surreal that astronomers cannot tell the difference
> between precision time and the Earth rotational orientation.
Astronomers are among the largest users (per capita anyway :-) of TAI
as well at UTC - I think TAI is a triumph of modern technology and
international cooperation. Both timescales represent precision
timing - as well as do numerous other timescales that perhaps only
astronomers and space scientists are ever likely to use. Astronomers
were the first precision time keepers, of course. This continues today.
But note that this includes millions of amateur astronomers as well.
If you haven't looked recently at commercially available amateur
telescopes, they are paragons of civil time handling. GPS is used to
report UTC which is used to calculate local sidereal time which is
used to gauge the orientation of the Earth which is used to point the
telescope. GPS alone is sufficient to locate a couple of bright
stars in the finder. Center them on the crosshairs and the thing
will place object after object in a high powered eyepiece all night
long. Why do I mention this? Well, there hasn't been word one of
discussion about how DUT1 will be transported worldwide - let alone
to remote mountaintop sites - after WWV ceases to transmit the DUT1
signal (which is indeed part of the proposal).
Shouldn't we explore the requirements and use cases before making a
change to the standard? The argument has been that the discontinuity
of leap seconds is too difficult for various projects and systems to
handle. I would more likely buy this argument if any level of
appropriate project management or standards discussion had occurred.
The crystal clear intent is simply to cease issuing leap seconds.
Then what? Many systems and many users for many purposes certainly
won't care. Shouldn't we try to identify ones (oh - air traffic
control springs to mind) that might *possibly* care (if so, big time)
and perform a Y2K-like inventory of ramifications? I am skeptical
that projects that have inappropriately selected UTC as a timebase
and have inappropriately implemented support for what is a
straightforward standard will make any better choices in the future
or will ever implement systems that better conform to any new
standard. Ostriches don't actually put their heads in the sand - nor
This has never been a discussion about whether TAI is "better" than
UTC - for some purposes, of course it is. TAI is about the
definition of the second. UTC is about the definition of the day -
something astronomers have been involved in for literally thousands
of years. Civil time should continue to reflect time-of-day. The
headaches from retiring leap seconds will prove larger than those
from continuing to issue leap seconds.
Astronomers are among the few communities whose line of work is
absolutely guaranteed to be interrupted by a leap second occurring at
midnight on New Year's Eve. Major observatories usually close only
two nights a year - Christmas Eve and Christmas night. We don't give
a pass to grad students who want to party with Dick Clark.
Observations throughout the western hemisphere will pause before
midnight and resume afterwards. Others will simply continue with the
thought that the effects of the leap second (if any) will calibrate
out during reductions. We tolerate these complications because they
are the price to pay for retaining sanity in our clocks.
I perused a number of the back threads from this list. A recurring
topic appears to be the complexity of systems that are used to
transport time. The problem is not that the POSIX (or what have you)
time model is too complex. The problem is that it is too simple by
far. The appearance of time is deceptively simple. It is the
reality of time that our technological systems have to support. The
goal in designing systems to transport time signals should not be to
try (and inevitably fail) to impose an artificial simplicity - it
should be to accurately model the complexity that is inherent in time.
This is a task worthy of our best efforts.
National Optical Astronomy Observatory
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