Re: [LEAPSECS] Mechanism to provide tai-utc.dat locally

From: Rob Seaman <seaman_at_noao.edu>
Date: Fri, 29 Dec 2006 00:04:18 -0700

M. Warner Losh wrote:

> vague rumblings about astronomical software needing to be rewritten,

Unlike Y2K, there is no solid public proposal for astronomers to cost
against, but the cost is likely to dwarf Y2K in our community, since
algorithms and deployed services would have to change, not just
retrofitting two digit years.

Folks keep fretting here about retrieving lists of leap seconds
autonomously, although no specific use case is proffered about why
one needs to use UTC to measure intervals across various and sundry
leap second events. On the other hand, astronomers have quite
reasonably coded their systems to take advantage of the original (and
current) definition of UTC: "GMT may be regarded as the general
equivalent of UT." One second of time is 15 seconds of arc on the
celestial equator - this is many times greater than the resolution of
any O/IR telescope.

The proposal really amounts to trading the need to track leap seconds
to convert from time-of-day to intervals, for the need to track the
equivalent of leap seconds to convert from interval time to time-of-day.

> Daylight savings time and time
> zones prove that society at large has a very high tolerance for
> variations between the mean solar time at an arbitrary location, maybe
> hundreds of miles away, and the local time.

This is a static offset. Leap seconds are one mechanism to address a
secular drift - a rate, that is, not a constant. Local time -
daylight, standard, mean, apparent - has been raised as an issue
innumerable times - it has been irrelevant every one of them. We're
not talking about mucking with local time, we're talking about
subverting the mother of all solar time.

> Only specialized users of time would be affected.
> Who are they and how do we find out the cost of change?

If this is true, you can find out by actually seeking them out,
rather than hiding a proposal with worldwide implications away in
some squirrelly little bureaucratic committee.

However, the suggestion has been implicitly made that everybody is a
potential victim of leap seconds - that airplanes will drop from the
sky, even though they haven't done so through 23 prior
opportunities. That suggestion opens the possibility that
"generalized users", i.e., "people" might be affected by civil
timekeeping issues in subtle ways. The way to find out the cost of
change is to spend some time and money characterizing this.

For example, I set my workstation's clock forward eleven years (to
match Gregorian calendars) for a couple of years prior to Y2K to
provide a platform for our Y2K remediation activities. We set clocks
forward at the telescopes and some of them started to track the sky
backwards. Surely a Y2K-like inventory could be performed for
certain key industries to get a sense for what dependencies might
lurk? We've long since established that different countries have
different legal civil time standards, e.g., GMT versus UTC. What
happens should this discrepancy continue as UTC diverges from GMT?

The proposal on the supersecret ITU-R table is to discard an
international standard that has been in effect for more than a
century and that pervades every aspect of our technical, cultural,
legal, economic, etc., world. One might expect somebody might have
thought to submit a proposal for a few hundred thousand dollars to
conduct a proper pilot study before even beginning to speculate about
discarding mean solar time as a basis for civil timekeeping.

> These little devices have obviated the need, in many cases, for
> celestial navigation.

In many cases? That will be comforting to the folks that really need
a backup when their GPS goes out. Whatever the solution should be,
the one thing it should not be is brittle.

> Given that change, the cost benefit analysis
> that was done in 1972 likely needs updating.

It sounds like we agree on this point.

> Seems like a logical leap for leap seconds to follow, if the costs
> aren't
> prohibitive. Chances are that one person knows all the users of time
> that still need DUT1 to be less than 1s.

"Seems like"? "Chances are"? Pick some other random technical issue
- say, automobile airbags, standardized educational testing, the lead
content of pigment in children's crayons, and so forth and so on.
Would "seems like" and "chances are" be phrases you would want to see
in a white paper discussing the costs, benefits and risks of these?
Oh yeah - that's right. After seven years there is not one single
white paper discussing issues related to the decision-making of the
ITU-R WP-7A. (And if there were, we presumably would not be able to
read it.)

There are several thousand professional astronomers in the world and
many times this number of amateurs. All of them ultimately care
about the subtle concepts underlying the notion of DUT1. It seems
bizarre to dismiss their concerns precisely because they might have a
more personal interest than some others.

It may be naively convenient to assume that the only risks are with
poorly designed, non-conforming UTC implementations failing to handle
leap seconds - and none with civil time-of-day falling further and
further away from actual mean solar time. One might rest easier,
however, knowing that any effort whatsoever had been invested in
searching for potential risks on both sides.

> so maybe some other means of distribution is
> necessary... And is 100ms really good enough?

Excellent questions. Might I suggest that they be appropriately
answered before UTC is removed from life support?

Rob Seaman
NOAO
Received on Thu Dec 28 2006 - 23:04:39 PST