UTC - A Cautionary Tale

On July 4th, the International Earth Rotation Service (IERS) announced
that the first leap second in seven years will be issued at the end of
December 2005.

On July 5th, the head of the Earth Orientation Center of the IERS,
Daniel Gambis, made the first official pronouncement that the next leap
second may be the last leap second.

What would this mean? This is such a drastic change in the philosophy
of timekeeping that it is hard to express. This would mean that civil
time worldwide - the clock on the wall, the watch on your wrist, the
time provided by your cell phone and laptop and television and by the
time signals on the radio - that civil time literally everywhere would
cease to have any connection to the rotation of the Earth. An
alternate interpretation could be that the Prime Meridian would begin
to drift from the observatory at Greenwich England. For most purposes
time-of-day would become just a polite fiction. For others,
time-of-day would become a nightmare to calculate and correct for from
tables downloaded from the internet or corrections typed in manually
with all the usual ambiguity of sign and magnitude.

To summarize the main points of the change proposed by the US
delegation to a committee known as the ITU-R WP-7A which appears to
have been indirectly delegated the authority to make this decision:

1 - Maintenance of a time scale called UTC.

2 - Suppression of the leap seconds adjustments which maintains UTC
close to UT1, a time scale based on the Earth's rotation (currently
UT1-UTC .9 s)

3 - The difference of UT1 from UTC should not exceed 1 hour.

4 - The change should take effect at 21 December 2007, 00:00 UTC

The full text of Gambis's message is available from:

http://www.ucolick.org/~sla/leapsecs/gambis.html

A link to the official proposal is available at the bottom of that
page, along with a list of committee members who can be contacted with
comments. I have a hard time expressing how much I detest both this
proposal and the process through which it has been made. Gambis is to
be applauded for finally bringing it to light after six years of
furtive discussions within the precision timing community. Belgian
astronomer and mathematician Jean Meeus comments on the proposal:

http://www.ucolick.org/~sla/leapsecs...vent2005-07-08

Steve Allen of Lick Observatory provides an excellent page of UTC and
leap second resources:

http://www.ucolick.org/~sla/leapsecs

There would be a significant expense (software, hardware, and
operations) to the astronomical community should leap seconds cease,
but my visceral rejection of this proposal is rooted more deeply. We
should not so blithely discard the ties between our clocks and the
rotation of the Earth. As Steve Allen points out, time has always
meant Earth rotation because civil time has always been a subdivision
of the calendar. This proposal is simply goofy - but it is no less a
real threat for that.

Daniel Gambis suggests interested parties (potentially everybody on the
planet) take action:

"If your activity is affected by the content of the US proposal
which will be discussed in November 2005 at the WP-7A, you are urged to
react. This could be the last opportunity before a recommendation is
issued by the WP-7A.

If you wish you can express your opinion to your representative(s)
at the WP-7A of ITU (for the list see the ITU website,
http://www.itu.int/home/index.html) with a copy to Daniel Gambis
), IERS EOP Center."


Finally, a few comments on the points of the proposal to provide
context:

Note the absence of any suggestion of what affected parties might do
about such a change. Note the absence of any discussion of what
changes or improvements might be forthcoming from WWV, NTP, GPS and
other systems currently used to distribute Universal Time. The last
six years would have better been spent discussing how to improve the
systems we already have, not how to dismantle them.

1 - Maintenance of a time scale called UTC.

Universal Time was designed to be equivalent to Greenwich Mean Time.
The original wording of the standard governing the distribution of UTC
via radio signals (e.g., WWV) actually strengthens this statement by
reversing it:

"GMT may be regarded as the general equivalent of UT"

This wording has recently been removed.

There are many flavors of UT - UT0, UT1, UT2 - which form progressively
better approximations. The precision timing community has explicitly
rejected the idea of ceasing the issuance of leap seconds by rather
constructing a new civil time standard (to be called "TI" for
"International Time"). They insist on continuing to call it UTC.
Thus, under this proposal, one flavor of Universal Time, UTC, would
diverge in meaning from all others.

2 - Suppression of the leap seconds adjustments which maintains UTC
close to UT1, a time scale based on the Earth's rotation (currently
UT1-UTC .9 s)

The precision timing community appears to assume that all usage of UTC
takes advantage of the correction signal, DUT1, which is the difference
between UTC and UT1, provided to a tenth second accuracy. Most
astronomical applications, however, appear to use UTC as an
approximation accurate to 1 second. Software that does account for
DUT1 may not be able to support a value greater than the 0.9s allowed
by the current standard. Software that does not account for DUT1 will
simply be wrong.

The maximum excursion allowed by the current WWV correction signal is 3
seconds. Even if our WWV clocks handle DUT1, they will soon need to be
replaced. The Network Time Protocol that sets the clocks on our
computers provides only minimal support for leap seconds (and none for
DUT1) now. It would provide none later. It is likely that any new
system that emerges to provide access to UT1 (or other flavor of
Universal Time) would never be implemented on the legacy computers that
drive many of our telescopes and instruments. Only new hardware is
likely to be responsive to our future needs.

Y2K was a non-event for the astronomical community, like many other,
only because the astronomical software community worked hard to make it
so. Some of our telescopes tracked backward until the software was
fixed. An IRAF release was required. The FITS standard was modified.
Many other changes were made throughout all our systems. Even a minor
change to UTC might require large - and large numbers of - changes to
our software and hardware (and operating procedures). This would not
be a minor change.

3 - The difference of UT1 from UTC should not exceed 1 hour.

The width of a time zone is one hour. The idea of leap hours to be
issued every half millennium or so is equivalent to no civil
time-of-day at all.

4 - The change should take effect at 21 December 2007, 00:00 UTC

The current standard is good for at least the next half millennium.
The rate of leap seconds will increase quadratically. (We don't need
leap seconds because the Earth is slowing down - we need leap seconds
because it has already slowed down since the 1 Jan 1900 epoch.) The
current standard allows one leap second per month. There is plenty of
time to make this decision prudently and with public comment from all
affected communities (everybody, everywhere).

Law suits resulted in the 19th century from disputes over the
interpretation of the precise beginning and ending times of contracts -
twice. First when the world community switched from local apparent
time (sundial time) to local mean time (clock time). Second when it
switched from local time to standard (zone) time.

This is not only (or even primarily) a technical issue. Since this is
a proposal originating with the US delegation, it is important that the
WP-7A understand that they don't represent a US consensus on this
issue. One has to also wonder how such a narrow working group (or even
the larger ITU) believes itself empowered to make a unilateral change
that would affect so many other interested parties (literally
everybody, everywhere).

Rob Seaman
NOAO

In sci.astro.research Rob Seaman wrote:
On July 5th, the head of the Earth Orientation Center of the IERS,
Daniel Gambis, made the first official pronouncement that the next leap
second may be the last leap second.
[[...]]

Steve Allen of Lick Observatory provides an excellent page of UTC and
leap second resources:

http://www.ucolick.org/~sla/leapsecs

I also found the following article to be interesting:

R. A. Nelson et al,
"The Leap Second: Its History and Possible Future"
Metrologia volume 38 (2001), pages 509-529
http://www.iop.org/EJ/article/0026-1...6/6/me1606.pdf

ciao,

--
-- "Jonathan Thornburg -- remove -animal to reply"
Max-Planck-Institut fuer Gravitationsphysik (Albert-Einstein-Institut),
Golm, Germany, "Old Europe" http://www.aei.mpg.de/~jthorn/home.html
"Space travel is utter bilge" -- common misquote of UK Astronomer Royal
Richard Woolley's remarks of 1956
"All this writing about space travel is utter bilge. To go to the
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