[LEAPSECS] BBC article

[Warner Losh]

On Nov 15, 2011, at 2:21 AM, Nero Imhard wrote:


> 

> On 2011-11-15, at 04:43, Doug Calvert wrote:

> 

>> Why is redefinition of UTC / end of leap seconds not just another

>> routine change?

> 

> Because it is not simply a refinement of how UTC is kept near UT, but a rather fundamental change in semantics.


The debate really is about do those semantics really matter or not.  Some say yes and point to telescopes and sextants.  Others say no and point to computers that do regular things well, but irregular things like leap seconds poorly.

It all depends, really, on what "near" means.  Discontinuing them now means that "near" will stay within a minute for the foreseeable future (whatever that means).  We'll drift off a while minute by 2075 (give or take 10 years).  I doubt more than a handful of people posting to this forum will be alive then.  Of course, the delta will grow more in the future, but a few hundred years after that the slowing rate of the earth will mean a growing delta.  That's where people start to think that this definition of "near" might not be good enough, so why even go down this path.

It is disheartening that the middle ground remains unexplored.  Most of the difficulty of the current system could be solved by allowing DUT1 to grow as large as 10s, but still keep it bounded.  If we know there will be about 60, then schedule one every 18 months for the next 10-20 years.  On the average we'll stay in sync, computers will know well enough in advance to update tables.  Exceptions could be announced 10 years in advance, if they are needed if that rate turned out to be really 55 or 65 since the earth's rotation is slowing, but also sometimes speeding up a bit.

Heck, even without relaxing DUT1 too much, studies have shown that we can predict at the 95% level of certainty, the leaps we'll need to stay under the 1s limit out 3 years.  Predicting it out 2 years can be done quite a bit better (to like 200ms).  Exact numbers are in the archives.  But even a 2-3 year time frame would allow easier updating of tables and such to give systems a better chance at working, and also increase the testability of the leap second.  This would also make the costs for leap seconds more predictable for business.  Right now, many businesses have unexpected costs associated with leap second compliance when a leap is announced.  They need emergency budget on a sub-year time-scale which is disruptive.  If we know there's one in June 2012, the appropriate managers can put that into their budgets in the normal process, rather than making it be an emergency (and possibly causing them to say screw it, we'll take our chances).  True, most businesses don't worry about this, but if we're talking about improving the current system, a change like this will allow businesses, mostly government contractors, a more predictable cost structure around this.

There's nothing magical about the current leap seconds.  They are but one of many ways to realize a mean solar time (as opposed to the one true way of realizing Mean Solar Time from Newcomb's Equations of Time).  It isn't a great system, but it is the one we have today.

Warner