Re: [LEAPSECS] Where the responsibility lies

From: Rob Seaman <>
Date: Tue, 3 Jan 2006 14:02:12 -0700

All right - I guess we can go another round or two while waiting -
perhaps indefinitely - for reports of leap second related
catastrophes to filter in.

First, an apology for posting my previous reply publicly. It escaped
my notice that I was replying to a private message.

On Jan 3, 2006, at 12:32 PM, Poul-Henning Kamp wrote:

> If you already have to cope with DUT1 anyway, how much difference
> can it possibly make if the definition says
> |DUT1| < .9
> or
> |DUT1| < 10sec
> or
> |DUT1| < 1 hour

The amplitude of the effect is 3600 times as great. The interval
between each event much longer (though quadratic, not proportionate,
of course). The larger amplitude implies a larger impact on human
activities. The longer interval implies that future
instrumentalities will be less prepared to deal with the impact.

> If we can increase the tolerance to 10sec, IERS can give us the
> leapseconds with 20 years notice and only the minority of computers
> that survive longer than that would need to update the factory
> installed table of leapseconds.

No. Rather all computers that exist during such an event are
obligated to deal with it. The number of deployed systems follows
some increasing trend similar to Moore's law. By delaying the
adjustments, you guarantee that more systems will be affected when
they do occur. And, unless you can guarantee that a particular
deployed system (and systems derived through various upgrade
pathways) will be retired prior to the adopted horizon, prudent
policy would require remediation in any event.

Would like to see a proposed architecture a little more detailed than
a "factory installed table".

> As far as I know, less than 1% of people on this planet actually
> have the sun straight south at 12:00 local time today and a quite
> sizeable minority (a lot of China) lives perfectly happy with the
> sun being further than 15 degrees from south at local 12:00.

Once again confusing secular and periodic effects. Leap seconds are
the result of a monotonic trend. Time zones and daylight saving
repeat seasonally or represent geographic constants.

> It follows from this, that a proposal for a 1hour tolerance on
> DUT1 is perfectly feasible without odd things happening to the
> cows milk etc.

Got me. Might it not be prudent to ask a farmer? We interviewed a
guy last year who networks dairy cattle via instrumented capsules
that monitor pH and communicate wirelessly from cow to cow back to
the barn. Farmers are likely among the more technologically literate
community for temporal data products. Why not benefit from their

> I think the leap hours is a political tool to make the proposal go
> through without commiting anybody to anything for the next couple
> of hundred of years.

Of course. We differ on whether it is: A) ethical, or B) viable.

> There are three orders of magnitude difference between a leap second
> and a leap hour, and consequently the need for leap hours will grow
> less rapidly than the need for leapseconds.

Bzzz! Nice try, but an incorrect answer. Thanks for playing our
home game.

I've appended Steve Allen's excellent plot of the long term behavior
of length of day. LOD grows linearly over scales pertinent to this
discussion (discounting wiggles due to interesting geophysics). This
is due, of course, to angular momentum transfer from the Earth's
rotation to the Moon's orbit. Mitigating factors include the rebound
of the continents since the last ice age.

Leap seconds accumulate as the integral under this curve (thus
quadratically). The last hour's worth took 1200 years. (Hence the
600 year estimate for issuing the first leap hour to limit excursions
to +/- 0.5 hours.) The hour before that, only four centuries. It
does not matter whether these are binned as leap seconds or leap
hours, they will arrive when they arrive. That being the case, one
argues that bleeding off the accumulation in smaller, more frequent
doses is a better choice.

Rob Seaman
National Optical Astronomy Observatory

Received on Tue Jan 03 2006 - 13:03:53 PST

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