[LEAPSECS] UTC Redefinition Advanced

Sanjeev Gupta ghane0 at gmail.com
Mon Oct 25 00:00:27 EDT 2010

Snipped some stuff.

This is the best definition of the problem I have seen for quite some time.

I thought I was firmly in the camp of keeping leap-seconds, but put this
way, it seems to me that I was staying within an error bound, and now
(taking Jonathan as an example), all that is being done is to increase the
error bound. And since everyone who really cares about solar/pulsar time
would have to be applying DUT1 now, the cost to them is ensuring that their
software (and processes) do not have variables that chuck out values of DUT1
greater than 1s. This _is_ a real cost, no doubt, but it seems less of a
problem than I assumed.

I am still opposed, in principle, to letting NTP time (for example) diverge
from The One True Cosmic Time; but my principles are cheap, and my
engineering work does not suffer under Jonathan's formulation.

Sanjeev Gupta
+65 98551208 http://www.linkedin.com/in/ghane

On Mon, Oct 25, 2010 at 07:42, Jonathan E. Hardis <jhardis at tcs.wap.org>wrote:

> 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 you?


> 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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