Re: Introduction of long term scheduling

2007-01-07 Thread Daniel R. Tobias
On 8 Jan 2007 at 0:15, Tony Finch wrote:

 How did you extend the UTC translation back past 1972 if the undelying
 clock followed TAI? I assume that beyond some point in the past you say
 that the clock times are a representation of UT. However TAI matched UT in
 1958 and between then and 1972 you somehow have to deal with a 10s offset.

Formulas for UTC, as actually defined at the time, go back to 1961
here:

ftp://maia.usno.navy.mil/ser7/tai-utc.dat

It appears the offset was 1.4228180 seconds at the start of this.

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Re: how to reset a clock

2007-01-04 Thread Daniel R. Tobias
On 4 Jan 2007 at 10:53, Peter Bunclark wrote:

 Indeed isn't this Rob's ship's chronometer?

Captain's log, stardate 30620.1...

http://en.wikipedia.org/wiki/Stardate


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Re: A lurker surfaces

2007-01-02 Thread Daniel R. Tobias
On 2 Jan 2007 at 12:40, Warner Losh wrote:

 The interval math in UTC that's hard today would be significantly
 harder with rubber seconds.  But it is just software, eh?

 In short, it is an interestingly naive idea that was tried in the
 1960's and failed when there were only dozens of high precision time
 users rather than the hundreds of thousands there are today.

Actually, rubber seconds were what were in use for centuries, as
the time calibrated to astronomical observations, with the second
defined in terms of the length of a solar day, was what was in use
(or, actually, a very rough approximation of it given the lack of
accuracy of timepieces in the pre-atomic era).  What was tried
unsuccessfully in the 1960s was to actually define such timekeeping
in a rigorous scientific way allowing conversion to and from atomic
time.



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Re: Introduction of long term scheduling

2007-01-02 Thread Daniel R. Tobias
On 2 Jan 2007 at 19:40, Poul-Henning Kamp wrote:

 Has anybody calculated how much energy is required to change
 the Earths rotation fast enough to make this rule relevant ?

Superman could do it.  Or perhaps he could nudge the Earth's rotation
just enough to make the length of a mean solar day exactly equal
86,400 SI seconds.

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Re: A lurker surfaces

2007-01-02 Thread Daniel R. Tobias
On 2 Jan 2007 at 11:47, Ashley Yakeley wrote:

 The obvious solution is to transmit rubber time on a rubber frequency.

Are rubber duckies involved?


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Re: A lurker surfaces

2007-01-02 Thread Daniel R. Tobias
On 2 Jan 2007 at 11:56, Ashley Yakeley wrote:

 GPS is TAI. I'm not proposing abandoning TAI for those applications
 that need it.

It's a few seconds off from TAI, isn't it?  It was synchronized to
UTC in 1980 (I think), but without subsequent leap seconds, so it's
now different from both TAI and UTC.  They probably should just have
used TAI if they wanted a time scale without leap seconds, rather
than ending up creating a different one.

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Re: Mechanism to provide tai-utc.dat locally

2006-12-27 Thread Daniel R. Tobias
On 27 Dec 2006 at 20:57, John Cowan wrote:

 Very true.  And adopting the Egyptian-Roman calendar redefined
 the concept of a month.  Somehow civilization survived.

Keeping months in sync with phases of the moon apparently turned out
to be insufficiently important to civilization to require it as a
feature of the calendar.  I'm doubtful that keeping clocks in
approximate sync with the rising and setting of the sun is likely to
be judged equally unimportant, however.

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Re: what time is it, legally?

2006-12-14 Thread Daniel R. Tobias
On 13 Dec 2006 at 21:43, Steve Allen wrote:

 http://gauss.gge.unb.ca/papers.pdf/gpsworld.january01.pdf

One quibble with that article is that it gives the Global Positioning
System as an example of how humanity has been obsessed with knowing
what time it is.  Actually, GPS arises from our obsession with
knowing what *place* we're at; its need for precise time is a mere
technical detail of its implementation.  (Some of the earlier
historical needs for precise time also arose out of navigation, where
knowing one's position in space necessitated also knowing something
about time.)

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Re: Risks of change to UTC

2006-01-21 Thread Daniel R. Tobias
On 21 Jan 2006 at 10:11, M. Warner Losh wrote:

 I maintain that for human activity, there's no need for leap seconds
 at all.  In each person's lifetime, the accumulated error is on the
 order of a few minutes.  Over generations, the problems with noon
 drifting to 1pm can trivially be solved by moving the timezones that
 civilian time uses.

What about when that accumulated difference is over 24 hours, so the
offset between solar-based time and atomic time is actually on the
order of days?  Will people be able to deal with a civil time
standard that is based on an offset from a UTC that says it's
Monday when all actual points on Earth have the local date at
Saturday or Sunday?  Many Web sites (including Wikipedia) use UTC as
the standard for date/timestamps; will this be a reasonable thing
when this causes the date of postings to be far off from what is
being used locally?  And when, at some future point, the Gregorian
calendar itself needs adjustment to handle the fact that it doesn't
get the length of the year precisely correctly (and the length of the
year in terms of solar days is changing due to the lengthening of the
day, anyway), will this adjustment be done to the UTC standard (why,
when it doesn't follow astronomy anyway?), or as an additional offset
to local times (which could result in different countries having
different dates as in the Julian/Gregorian transition period)?
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Re: Risks of change to UTC

2006-01-21 Thread Daniel R. Tobias
On 21 Jan 2006 at 15:15, M. Warner Losh wrote:

 For some perspective, we've been using UTC for only ~50 years and the
 gregorian calendar for only ~1500 years.  I'd anticipate that
 something would need to be done about the slowing of the day well
 before 4300 years have passed.

Actually, that's more like ~400 years for the Gregorian calendar
(first instituted in 1582; adopted in different dates in different
countries, as late as the 1920s in some).  Its predecessor, the
Julian calendar, goes back ~2000 years.

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Re: The real problem with leap seconds

2006-01-11 Thread Daniel R. Tobias
On 11 Jan 2006 at 0:08, Tim Shepard wrote:

 If humans spread out to other places besides the earth, an
 earth-centric time scale might begin to seem somewhat quaint.
 Distributing leap second information to a Mars colony seems kind of
 silly.

As I recall, the NASA Mars missions are using Mars-centric time
scales, which include a Martian second that has a different length
from the SI second in order for the different-length Martian day
(called a Sol) to be subdivided into a familiar 24 hours composed
of 60 minutes each with 60 seconds.

If, however, this Martian second is actually defined as a particular
multiple of the SI second, then the use of leap seconds on Mars would
ultimately be necessary to account for any future changes in the
length of the Martian day.

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Re: interoperability

2006-01-08 Thread Daniel R. Tobias
On 8 Jan 2006 at 15:04, Tom Van Baak wrote:

 You cannot divide timekeeping, time dissemination,
 into neat stages. In the 1960s if ten labs were told
 to offset their phase or frequency it affected only a
 handful of people or systems. Today when IERS
 announces a leap second, millions of machines,
 systems, and people are affected. Thankfully, most
 of them handle it OK.

Although, even now, the majority of consumer and business equipment
is not directly affected in any noticeable way; such machines usually
run on a local clock considerably less accurate than an atomic clock,
periodically re-synced (perhaps manually, perhaps automatically) to
an external time standard.  At each such re-syncing, the time may
need to be adjusted by a few seconds, or even a few minutes, due to
inaccuraccies in the local timepiece, so any leap second that may
have occurred since the last syncing will merely result in a 1-second
difference in the magnitude of this adjustment, not particularly
noticeable to the end users.  If some application (e.g., a database)
requires a timescale without discontinuities, the application might
need to be shut down for a few seconds to perform the time adjustment
(whether or not there is a leap second in the mix) in order to
prevent data corruption at the moment of the change.

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Re: a system that fails spectacularly

2005-12-09 Thread Daniel R. Tobias
On 9 Dec 2005 at 10:42, David Harper wrote:

 On the other hand, the idea of ISO 9000 compliant Morris dancers is a
 very funny one. Presumably, they'd have to standardise the size of
 their pig's bladders. There's a Monty Python sketch just waiting to be
 written.

 I'm guessing that their level of ISO 9000 compliance falls in inverse
 proportion to the amount of beer they drink. As does their likelihood
 of observing leap seconds correctly.

But if they fail to observe the leap second properly, the timing and
synchronization of the dancers will be off, and they might collide
catastrophically into one another!  We must fix this danger right
away!

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Re: Comments on Civil Time decision tree

2005-09-26 Thread Daniel R. Tobias
On 26 Sep 2005 at 16:09, Poul-Henning Kamp wrote:

 Other more laid back parliaments like the Danish have not been able
 to find time to revisit the issue since 18xx and still use solar
 time at some more or less random coordinate.

You mean like the U.S. Congress?
http://tycho.usno.navy.mil/260.html

...the standard time of the first zone shall be based on the mean
solar time of the sixtieth degree of longitude west from
Greenwich... (and so on for all the other zones)

 Imagine for instance that we send a probe out of the solar system
 at seriously high speeds and it manages to get as far as 6 light
 months away:  Under the current UTC rules we would be unable to
 upload a leap-second warning and get it there before it is too late.

I would suppose that such a space probe would have little need to be
synchronized with earthly solar time, and thus might be best off
operating on TAI, with any adjustments to UTC for the sake of humans
observing it on Earth being done at the Earthly end of things.

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Time after Time

2005-01-22 Thread Daniel R. Tobias
I sure hope that the future of mankind's timekeeping systems doesn't
get decided by an Internet flame war between contending groups of
geeks...

As I see it, the dispute comes from the fact that people want two
different, irreconcilable types of time, time of day (earth/solar
angle) and constant interval time.

Traditionally (over all of human history), civil time has always
related in some way to solar-angle time, originally directly, and now
in a complex, artificial way with confusing politically-imposed
irregularities such as daylight saving time and wildly gerrymandered
time zones.  It still does relate to solar time, however, with the
local clock time at a given point on Earth at a particular time of
year generally fixed at a constant increment from solar mean time for
that spot (but sometimes changing to a different increment for part
of the year).  There's no prospect that eventually, due to
discrepancies in the system, noon will come when it's dark (except
perhaps very near the north or south poles).

Some of the proposals, however, seek to decouple civil time
altogether from solar time, an unprecedented step which would
possibly lead to day and night being completely reversed; any leap
hours that prevented this would, if ever implemented, be even more
traumatic than leap seconds are now.

In addition to being historically unprecedented, such a move would be
illegal in the United States and some other countries, which have
laws explicitly defining their time zones based on solar mean time,
unless such laws were changed.

Now, if a time standard is to be defined based solely on constant SI
seconds, with no reference to astronomy, then why even include all
the irregularities of the Gregorian Calendar, with its leap year
schedule designed to keep in sync with the Earth's revolutions?  It
really makes no sense that TAI includes days, years, and so on at
all, and this will seem particularly senseless when the current date
by TAI is a day or more removed from Earth-rotational time, as will
happen in a few millennia.

What is really needed is two different time standards:  a fixed-
interval standard consisting solely of a count of SI seconds since an
epoch (no need for minutes, hours, days, months, and years), and a
civil-time standard that attempts, as best as is practical, to track
the (slightly uneven) motions of the Earth.  When other planets are
settled, they'll need their own local time standards too (NASA is
already doing this for Mars).

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