[LEAPSECS] Reliability

[M. Warner Losh]

In message: <AE6C787C-1725-48CB-BBA8-4A1BBE06DDEB at noao.edu>
            Rob Seaman <seaman at noao.edu> writes:

: Adi Stav wrote:

: 

: > what problems could exceeding the tolerance(s) cause?

: 

: Well covered in the archive.  For astronomy, 1 second of time is 15  

: seconds of arc on the equator.  This is a large error (colossal for  

: some purposes).  It doesn't appear that any other industry has  

: actually performed a coherent risk analysis.  For some reason this is  

: asserted to be the astronomers' responsibility.


There were three groups of people identified as caring where the earth
is pointing to a high degree of accuracy.

	(1) Astronomers so they know where to point their telescopes.
	(2) Space Engineers wishing to know where their satellites or
	    other objects of interest are.
	(3) Navigators that still rely on astral navigation.

The first group needs time accurate to a few milliseconds to point the
largest of their telescopes, less accurate for the smaller ones (with
1s being a very large error for all but the smallest scapes).  Having
time broadcast available, and having that time within 1s of UT allows
these smaller errors to be corrected in the mid to small telescopes.
The larger ones need daily updates of DUT1 to get the job done.  The
mid sized ones can cope well enough with the DUT1 broadcast to 100ms
in things like WWVB.  There's also software that's used in the
astronomy world that benefits from the DUT1 < 1s rule that would need
to be retooled.

The second group has even tigher tolerances than the first, since
their birds are moving much faster than planets and the servos have to
be both fast and accurate to track them.  These folks usually are in
the classified world, so little can be said for sure about them.  What
is known is they have very high precision timing gear and software
that takes the DUT1 difference into account, and may have 'back door'
information to the raw measurements that go into the daily numbers
that are published.  This group is most likely to be able to cope with
DUT1 since they have lots of $$$ to track these things.  Also, 'their'
might not imply titular ownership, merely interest.

Most astral navigation is done with GPS these days.  There are a
vanishingly small number of folks that do it by hand, listen to
shortwave broadcasts to get the time, etc.  The US Navy doesn't even
have the gear on board to do the astral navigation anymore, I'm told.
Most of the folks in this forum have written them off as not being a
constituency worth caring about, but they are mentioned here for
completeness.

All other users of time, it is widely agree, basically want everyone
to agree on a time, have the sun basically overhead around noon, and
do what they are told.  There's debate over what each of these loosey
goosey terms means, and what the boundaries are for them.


: > (Especially problems that time zones far from their reference  

: > meridians, DST switches twice a year, and the difference between  

: > mean and apparent solar time don't already cause).

: 

: This confuses periodic with secular effects, also in the archive.


Well, yes and no.  If you permanently shift an hour to account for the
drift of time, then it is no different than permanently shifting 1s.
The question is who does it and when.  An interesting debating point
as well.

There is an important point to be made here.  The reason that DUT1
matters to most people is for the sun overhead at noon feature.
Sliding time zones solves that issue for many people, although there
is much debate about the aesthetics of doing this.


: > A good parallel would be adding leap hours and using the existing  

: > DST mechanism

: 

: 

: Reasons why leap hours won't work are in the archive.  There was a  

: clear consensus from both sides of the aisle that the notion of leap  

: hours is absurd.  Alternately, by relying on shifting timezones, there  

: would be no underlying stabilized civil timescale permitting  

: commonsense timekeeping inferences by humans.


Leap hours in UTC.  Let's be clear what we're talking about.  Also, I
don't think that your assertion that there's no stabilized civil
timescale causing issues has a firm foundation, let alone one to draw
the conclusion that it is a problem.  If DUT1 and the timezone info
were known in a database (like timezones and leap seconds are today),
then historians would be able determine when things happened, where
the sun was etc with more or less the same precision they have today.
The computations would be harder, and it has been debated as to the
extent of the hardness (eg does it really matter or not).


: By contrast, interval time is important to computers.  Computers are  

: good at computing.


True, but they are only as good at computing as the programmers are at
writing code, and the test organizations are at validating that code.


: > I don't understand :)

: 

: Imagine a version of the Gregorian calendar that interpolates leap  

: days only every 400 hundred years.  That would amount to about 3  

: months at a time.  Since this is a whole season, it is equivalent to  

: not stabilizing the calendar at all.

: 

: Leap hours or tweaking timezones can be interpreted the same way.  If  

: intercalary adjustments are the width of a timezone, no practical  

: stabilization is occurring.


For the seasons, it is clear that this drift is intolerable.  At least
from the historical technological level of most farmers.  They rely on
the calendar to do their planing, harvesting, etc.  To have it drift
that much wrt the seasons is a big deal.

Contrast this with our daily life where apparent solar time drifts by
dozens of minutes over the course of a year (granted, around a mean).
DST moves time of day for most people by an hour forward in the
spring, back in the fall.  In such an environment, a 1 hour shift once
every thousand years or so doesn't seem like such a horrible idea.  It
wouldn't be that disruptive.  It also wouldn't be a leap-hour in the
UTC time scale, but rather just a DST without end once in 50
generations.

Of course, I doubt there'd be more than a couple of these shifts
before people realize that something else is needed.  There may never
be a shift, but instead a change to a whole new time system as well
that suits the needs of future generations better.  One that we cannot
imagine from this vantage point in time.  Can you imagine being alive
at the time of Christ and thinking you'd be able to measure the length
of the day so accurately that you'd detect variations at the 1e-8
level?  And even if you did, would you have the skills necessary to
work out all the implications of that in advance?  Or that there'd be
a standard written for it in a language that wasn't even around at the
time?  This suggests, at least to some, that predicting what people
will need and want over such long periods of time is difficult at
best.  Both sides use this as part of their argument: the
pro-leapsecond folks to say that it keeps things in sync, which gives
future generations more options.  The anti-leapsecond folks to say
that things will be so different, it just might not matter.

Warner