[LEAPSECS] Reliability

[Rob Seaman]

Tom Van Baak wrote:


> why in your opinion, are leap seconds OK but leap tenth-seconds, or  

> leap minutes, or leap hours not OK?  Each of these preserve, to one  

> degree or another, the notion of stationary wrt solar time.


I'll refrain from references to current practice.  We often get  
tangled in assertion of "you can't do that!"

I would say that a leap tenth-second or leap minute are within the  
bounds of possibility simply based on size.  A leap hour is far too  
big a jump.  The point of intercalation is to make a succession of  
changes which are individually of small enough amplitude to be ignored  
(or, at least, ignorable).

The latency between intercalation events must be short enough to  
permit smoothing over historical periods - say, a few decades.  Too  
short is also not good.  I think tenth-second events would be needed  
too frequently.  Clearly some here believe one-second intercalation  
events occur too frequently :-)

On the other hand, permitting a long delay between events - or rather,  
between scheduling opportunities for events - risks losing the  
corporate knowledge to handle the events properly.  Others here  
believe leap seconds are already demonstrating that :-)

One great benefit of leap seconds is that there is a simple mechanism  
for introducing them into the flow of time marks.  (The original  
design is pretty clever, actually.)  A leap minute would likely be  
added as an additional last minute to the UTC day, basically 60  
straight leap seconds.

I suppose a leap tenth-second would correspond to a slightly longer  
final second to a day.  One advantage of this is that (for the next  
few hundred years) a rigid schedule could be instituted.  Something  
like add (or subtract) an integral number of tenth-seconds each and  
every 31 December.  The schedule remains fixed, the amplitude varies.  
This has similarities to the various timeslicing schemes that were  
mentioned - oh - five years into the discussion.

There are prior posts on why it would be very difficult to interpose  
an additional hour, basically because hours are individually tagged in  
each time zone, whereas each minute can cleanly add a 60th second and  
each hour a 60th minute.  That is, in Greenwich the leap hour could be  
a 25th hour, but in Tucson it would fall between the end of the 17th  
and the beginning of the 18th hours.  (Among other logistical issues.)

So, with the caveat that I really do believe that we should be  
focusing on collecting requirements to characterize the problem,  
rather than speculating on possible solutions, here is a score card:

leap tenth-seconds:
	small amplitude (too small?  some might see these as rubber seconds)
	too frequent operationally?
	not so infrequent we could ignore them
	split-second mechanism needed to implement

leap seconds
	small amplitude
	marginally too frequent (meaning people obviously disagree)
	marginally too infrequent (to force programmers to handle correctly :-)
	mechanism already deployed (obviously people disagree :-)

leap minutes
	marginally acceptable amplitude (for some purposes, DUT1 would have  
to adapt)
	not frequent enough operationally (but not outside the bounds of  
reality)
	too infrequent to maintain corporate knowledge
	mechanism possible

leap hours
	unacceptable amplitude (waayy unacceptable)
	not frequent enough operationally (by any interpretation)
	too infrequent (whole civilizations would come and go)
	mechanism is impossible

Like I said, if the alternative is the ITU giving up on civil  
timekeeping entirely, we can likely reach some sort of consensus to  
extend scheduling based on a relaxed approximation of some sort.

My personal preference is either to maintain the current status quo  
(although extending the schedule without relaxing DUT1 should also be  
possible) OR to redesign the system entirely from the ground up, eg.,  
Steve Allen's zoneinfo concept.

There clearly is resistance to admitting that there are two different  
underlying concepts of civil timekeeping that must both be honored.   
Embracing this will be the quickest way to reach a new equilibrium.

Rob