In message <68AAE0F0-15A4-440B-A87F-0869AD25AD78_at_noao.edu>, Rob Seaman writes:
>On Jan 8, 2006, at 9:09 AM, Poul-Henning Kamp wrote:
>
>> Doing so would once and for all have to divorce earth orientation
>> from that unified time scale, leaving it to governments to align
>> civil time with daylight as they see fit (just like today).
>
>Without further debating the meaning of "civil time", consider the
>implications of this two stage system. The first stage conveys TAI
>or something related to it by a constant offset.
Yes, too bad about the offsets (GPS etc) but as long as they don't change
with short notice, they can be dealt with.
>The second stage at
>any location (correct me if I misunderstand you) would be a secondary
>clock disseminated at the direction of the local authorities.
Yes, just like now.
The DCF77 transmitter for instance sends out "German legal time"
which means that if you want UTC from it, you need to know the UTC
offset for summer/winter in Germany.
>Governments and technical users would subscribe to the first stage
>clock. Businesses and civilians would subscribe to the second stage
>clock(s). Correct so far?
Almost.
What you overlook here is that computers tend to trancend governmental
boundaries.
Sensibly designed operating systems keep time in the form of the
first stage clock, and at the representation layer, knowing all the
worlds governmental decisions about getting from 1st to second stage
applies the appropriate conversions.
Badly designed operating systems keep time in local time which makes
interchange of information a nightmare across timezones.
Windows have got it right now I belive, but it used to be that a
file created and transmitted from Denmark at the end of the business
day would be older than a file created at the start of business day
in California, despite a strict ordering of the events.
>For the sake of argument, let's discount the risks associated with
>confusing one stage's clock with the other.
That's actually the good thing about the constant offset, it should
make it much easier to see if timestamps mix things that shouldn't be.
>I won't belabor the many worldwide systems that must interoperate for
>the benefit of all. But these systems must interoperate not only
>between themselves, but with natural phenomena.
Sure, and you can timestamp then on either timescale, because there
is a 1 to 1 translation between the two timescales [1].
You mention sunrise and sunset.
Since the introduction of timezones, one of the things which were
given up was the concept that sunrise/sunset happened on the same
numerical time at any given lattitude.
Denmark spans only a few hundred kilometers from east to west (not
counting Greenland this time), yet sunrise and sunset varies about
30 minutes from one side to the other.
Most people get the sunrise/sunset numbers out of the "Almanac from
the Copenhagen Universitys Observatory" [2] which lists sun rise/set
times for the observatory in Copenhagen and prints a table of
approximate geographical adjustment factors.
So already today, sunrise & sunset can only be determined using
auxillary tables of correction factors, tables which could trivially
absorb the DUT correction in addition to the longtude corrections.
>The question is: how precisely does this differ from the situation
>now or in the past? Whether by fiat or not, some common worldwide
>"stage two" clock must exist.
BZZZT wrong.
The definition we started out with is:
The second stage at any location (correct me if I misunderstand
you) would be a secondary clock disseminated at the direction
of the local authorities.
Conversion from stage two to stage one (and back) is perfect, so
if I measure a supernova in Denmark on Danish Civil Time, I can
mail you my observations and you can convert it first to stage 1
and then to your local stage 2 to compare with your own observation.
Or more likely, convert your own stage 2 to stage 1 and compare
in the "scientific time domain".
If Denmark or Elbonia decides to use a timezone which is offset from
stage one by 1h3m21s, then it still works, (but people travelling
abroad will probably vote differently in the next election)
>I have heard no response to my discussion of techniques for achieving
>synchronization - of the difference between naive "fall back" hours
>and 25 hour days. But how in practice is it envisaged that a scheme
>for migrating time zones versus TAI would work, precisely?
The same way all changes in timezone seems to be carried out: by
_not_ adjusting the clock when going to summer or winter time.
In a couple of hundred years, the Danish Parliament (or its successor
in interest) will simply decide "from YYYY-MM-DD HH:00, the Danish
Civil time will use offsets -3h and -2h (instead of presently
-1h/-2h) and the transition will happen on the switch from summertime
to wintertime by _not_ adjusting the clock".
That's been done many times throughout the world already.
If you look in NPL's decription of the Rugby timegrams:
http://www.npl.co.uk/time/ctm001v05.pdf
You will find the following interesting bit of text:
Summer Time
When UK civil time is subject to a one-hour positive offset
during part of the year, this period is indicated by setting
bit *58B to '1'. Bit *53B is set to '1' during the 61
consecutive minutes immediately before a change, the last
being minute 59, when bit *58B changes.
In the event of UK civil time undergoing an additional
permanent offset, bit *58B will need to be changed once
without any corresponding change in UK clock time.
I suspect this is because an proposal to move UK to UTC-1h/2h like
the rest of EU has been bubbling in House of Commons for many years.
>Note, for
>instance, that nothing short of redefining the second can avoid the
>quadratic acceleration between the stage one and stage two clocks.
Does not follow. It's only a matter of how often timezones change.
A far better way would be to redefine the length of the day to a
more suitable value a couple of thousand years from now.
>Time zones (and the prime meridian?) would race more-and-more rapidly
>around the globe.
In the case of moving timezones, this effect wouldn't make it self
felt until 1200 years from now (the second "leap hour"), and we are
in no position to prescribe how people a millenia from now will
measure time. If we preposterously try, they will at best get a
good laugh about their quaint forefathers preposterousness.
The scheme you propose is eminently workable, and more or less exactly
what we advocate. I'm happy that you now see the merits of it.
Poul-Henning
[1] Well, if you don't record _everything_ that goes in the timestamp,
there can be confusion when going from summertime to wintertime. In
Denmark the law specifically specifies a 'A' and 'B' suffix to resolve
this.
[2] Published in its current form since 1685, but goes all the way
back to the foundation of the university in 1479. Oldest surviving
copy is from 1549.
http://www.ku.dk/publikationer/?content=http://www.ku.dk/publikationer/aarlige_udgivelser/almanak.htm
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
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Never attribute to malice what can adequately be explained by incompetence.
Received on Sun Jan 08 2006 - 11:59:10 PST