[LEAPSECS] UTC Redefinition Advanced

Jonathan E. Hardis jhardis at tcs.wap.org
Sun Oct 24 19:42:01 EDT 2010

On Oct 24, 2010, at 3:29 AM, Rob Seaman wrote:

> But this isn't the discussion we've been having for ten years - and

> we aren't the ones to convince. On this list we have speculated

> widely on possibilities of all sorts, but the entire time a

> relentless and inflexible and closed-door campaign has been carried

> out by nonparticipants of this list to redefine UTC without leap

> seconds.

So, let me parse this. You have no idea what's been discussed in
"closed-door" meetings, so your only point of reference is what been
bantered about on this mailing list for ten years. That becomes the
reference truth.

I have no idea what's been discussed in the closed-door meetings
either, but at least I've discussed the subject with some of the
experts on it, so I've heard what's in their heads. There is no
thoughtless and nefarious scheme to decouple civil time from solar
time for the remainder of civilization, nor is there a move afoot to
salt the earth so that there could never be leap seconds again. There
is merely a reexamination of the presumption that DUT1 has to be kept
to less than 0.9 seconds, which -- when you stop to think about it --
is a rather arbitrary requirement.

Let's get quantitative. What magnitude of DUT1 would be tolerable to

Based on your postings here, I presume that you're fine with 0.9
seconds. One could also, arbitrarily, limit it to 0.6 seconds by
having the IERS declare leap seconds every few months, both + and -.
However, since I haven't heard you suggest that, may I presume that
you would agree with me that DUT1 need not kept that small? Can we
agree that reasonable people can discuss what an appropriate bound on
DUT1 should be for the purposes of civil time?

What about the public at large? In most of the world, we're long past
the point of arguing that sundials are divine and time zones are the
work of the devil. Time zones with the width of an hour are generally
acceptable, which indicates that the public might accept DUT1 as large
as 1800, or so. Certainly no one today gets bent out of shape because
sundial time in Boston is 1200 seconds different than sundial time in
Washington, DC, or that sundial time in Los Angeles is 1000 seconds
different than sundial time in San Francisco.

So, hypothetically, what would happen if we had no leap seconds for
the next 100 years? There are people who have analyzed how the
deceleration of the Earth's rotation will affect the need for leap
seconds -- and I'm not one of them. Let me make a simple guestimate
that with leap seconds occurring about once every 18 months, in the
next 100 years there would need to be about 80 of them to maintain
DUT1 at 0.9 seconds or less. This means that, without any leap
seconds, sundial time at JFK Airport in New York would become what
sundial time at Liberty Airport in Newark is now, and sundial time in
Long Beach would become what sundial time in Santa Monica is now.
DUT1 up to 100 isn't likely to cause the public much if any heartburn.

The point is that, even if we went 100 years and did nothing, the
magnitude of the effect would be so small as to be inconsequential for
most if not all purposes of civil time. However, I don't hear people
talking in terms of doing nothing in the next 100 years. In
international metrology definitions tend to change with a natural time
constant (tau) of about 30 or 40 years -- a unit of time called a
"career." And 100 years is about 2 or 3 tau. The people at the ITU
today couldn't stop their successors from changing UTC even if they
wanted to. Their successors can and will have a reasoned debate on
the best way, in their time, to take account of the fact that -- to
use your expression -- the SI second is not 1/86,400 of a day.

The debate today is not about setting a course for all future human
history. It is only about whether -- in our time -- keeping DUT1 to
0.9 seconds is worth the grief that it causes.

So, to repeat what I told you at the outset of this thread, if you
believe that leap seconds are the best technical approach, long term,
for keeping DUT1 below some threshold, your challenge is to make the
world safe for leap seconds.

- Jonathan

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