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

David Forbes dforbes at dakotacom.net
Wed Jul 13 12:59:56 EDT 2005

O time-nuts,

I realize that this subject was beaten to death here last week, but 
here's a new twist.

This is a little missive from an astronomer on the delicate subject of 
the divergence of UTC from UTx. It seems that those bastards in the 
precision timing community want to abandon UTC's leap seconds entirely 
because they are too much trouble, and he's hopping mad.

[His most amusing argument against modifying UTC is that astronomy 
software tends to use UTC not UT1 etc.]

PLEASE direct all reasoned replies to Rob, NOT this list. Otherwise 
the febo server will melt.

Begin forwarded message:

From: Rob Seaman <seaman at noao.edu>
Date: July 12, 2005 4:19:16 PM MST
Subject: 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 quasi-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:


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


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


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
 >> (daniel.gambis at obspm.fr), IERS EOP Center.

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

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

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