Re: [LEAPSECS] What problems do leap seconds *really* create?

From: Rob Seaman <>
Date: Wed, 29 Jan 2003 15:43:24 -0700 (MST)

Those of us with a day job may be having a hard time keeping up with
the messages as they arrive fast and furious :-)

> # The need for leap seconds is not caused by the secular slowdown
> # of Earth's rotation (which is less than 2 milliseconds per century)
> # but by irregular variations in this rotation and by the fact that the
> # definition of the SI-second is fixed on the duration of the year 1900
> # which was shorter than average.

Basically we don't have leap seconds because the Earth's rotation is
slowing down (by transfering angular momentum to the Moon). Rather,
we have leap seconds because the Earth has *already* slowed down since
1900. See the rather consistent slope of about 7 seconds per decade
on the plot of UT1-UTC (with leap seconds removed) versus date:

The current dynamical effects are the subtle wiggles imposed on this
trend. This is actually a fairly useful bias since it guarantees (short
of asteroid impact or armageddon) that there will be no negative leap

Again, please note how consistent the divergence is between atomic
and Earth timescales over decade long periods. Where precisely is the
urgency to adopt a quick fix?

Ken Pizzini says:

> I realize that the astronomical community has evolved to a consensus
> that UT1 (approximated by UTC) is a highly useful way to mark time,

Rather we've evolved a consensus that different problems require
different systems of time - not surprising, since we invented most
of them.

> with the additional feature that it is usable as a civil time standard,

It isn't just usable - it is preferable to many alternatives.

> but there is so much of that evolution which is based on historical
> accident rather than purely technical requirements

"Historical accident" makes it sound like the practice of timekeeping
was some afterthought to events. The reality is that timekeeping has
often been central to other, bloodier, battles - from Augustus Caesar
appropriating an extra day from February into "his" month - to Harrison's
chronometer that was instrumental to the building of a later empire.

Is there nobody on this list who was present at the birth of UTC?
It is a good, solid, pragmatic standard. The ability to issue leap
seconds monthly clearly represents a recognition that these would be
needed to preserve the standard over hundreds or thousands of years.

> that I find it hard to believe that there would be no possible way to
> improve upon it, even after non-astronomical constraints are factored
> in, if only it were possible to start anew with a clean slate.

The astronomical community isn't afraid to discuss a successor to UTC.
The mere presence here of several vocal members of that community should
demonstrate this. There is always room for improvement - although the
elegant simplicity of UTC will be hard to rival.

What we object to (if my friends don't mind my saying so) is not the
idea of *improving* UTC. What we object to is this current process
which appears to be an attempt to discard the standard entirely -
and to do so with minimal consent.

Please! Let's talk about ways to improve UTC and civil timekeeping.
And let's take the appropriate amount of time to reach a decision -
say - 40 or 50 years. In the mean time, let's pay attention to the
real question, which is how to build an infrastructure that will
dramatically improve the dissemination of all time signals.

NTP is a lovely mechanism. Astronomers are likely among its most
passionate users. The limitations of NTP or of any other general
purpose mechanism for disseminating time signals should not limit the
definition of the standards behind those signals. Rather, NTP, WWV,
GPS - and heavens to Betsy, GLONASS - should be built in a fashion that
can handle a general parametrized time standard. This should include a
static offset as used by GPS, as well as frequently or infrequently
introduced time jumps of variable sizes in either direction as required
by daylight saving or leap seconds. These systems should perhaps be
parametrized to support epsilon schemes as described by Calabretta or
by Kuhn's UTS. And a general purpose time distribution mechanism
should support differing rates as required by sidereal time, for

Similarly, personnel associated with various projects are expected to
know enough engineering to build incredibly complicated radio equipment
and other devices. Why aren't these same projects expected to handle
time issues with similar professionalism? If they need unsegmented
time - they should use some variation of TAI - and if they don't require
an Earth fixed time scale - they shouldn't use UTC. And if they do use
UTC, well then, they should figure out how to handle leap seconds.

Leap seconds are discussed very prominently in a very short document.

Rob Seaman
National Optical Astronomy Observatory
Received on Wed Jan 29 2003 - 14:43:33 PST

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