[time-nuts] Re: UTC - A Cautionary Tale

Rob Seaman 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  
should we.

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.

Rob Seaman
National Optical Astronomy Observatory

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