Ian Batten wrote:
> I know you really, fervently believe that to be not only true currently but
> inevitably and essentially true, but it just isn't.
No, rather I assert it as a definition of terms. A mathematical identity if
you like.
> For a start off, as the examples of Penzance,
"Dear father, why leave your bed
At this untimely hour,
When happy daylight is dead,
And darksome dangers lower?
See, heaven has lit her lamp,
The twilight hour is past,
And the chilly night air is damp,
And the dews are falling fast!
Dear father, why leave your bed
When happy daylight is dead?"
The current worldwide civil timekeeping system is "stationary" with respect to
mean solar time whatever meridian one chooses. Local zone offsets and apparent
solar time variations are interesting and important, just not pertinent to the
issue at hand. Let's see, there was a plot I tried to upload to leapsecs at
some point. Searching on the bounce from the moderator points to 4 January
2009. Ah! Right after the last leap second. Rummage, rummage - here's where
I put the plot at the time:
http://iraf.noao.edu/~seaman/images/HowLongIsADay.pdf
And let's see…ah! The first message from Ian appears 2 Sep 2010, well after
that discussion. Whether searchable or not, there's a lot of good stuff in the
archives. If sometimes some of us (meaning "me" :-) seem short-tempered and
snarky, please factor in that (literally) nothing is new under the Sun on this
list:
http://www.ucolick.org/~sla/navyls/ (2000 - 2007)
http://six.pairlist.net/mailman/listinfo/leapsecs (since 2007)
The equation of time varies by a few tens of minutes (on Earth - a couple of
hours on Mars). The length of the day is much more tightly constrained with a
maximum excursion of about half a minute - less than a minute peak-to-peak.
The dashed line on the plot is mean solar time. The length of the synodic day
is just an offset from the length of the sidereal day (due to annually lapping
the Sun). This can all be expressed in very technical language with caveats on
top of caveats, but the point is that even a tiny fraction of a percent
difference in length of day would be vastly far outside of any talking points
on this list over the past dozen years. A day for civil timekeeping purposes
is a mean solar day. By all means discuss fiddling the approximations to
implement this.
> And I think this is at the heart of your failure to convince people.
Have I failed to convince people in general, or just you guys in particular?
> I think civil time is a distant relative of mean solar time, which in order
> to reflect some long-standing cultural assumptions is maintained as something
> approximately aligned to mean solar time.
"Distant relative" is a statement of an engineering requirement. Over lo these
many years that's all I've been trying to say. "Approximately aligned" is a
statement of an engineering trade-off. We obviously disagree about the
necessary tolerances.
> for everyone else, it just doesn't matter.
This is an unsupported assertion. Nobody has looked.
> Were UTC not to be hitched to UT1, either DUT1 will become larger in
> magnitude (which you foresee presenting problems), or an extra variable
> representing the integer portion of DUT1 (which will obviously change more
> slowly, having a resolution of only 1s, but will change over the lifetime of
> a given system) needs to be distributed somehow, or you're screwed. We
> understand that.
More the latter.
> There's also some merit --- but, as I suspect you deduce from the vibes, a
> merit not everyone is convinced of --- in the idea that the status quo is
> changing and therefore those the onus is on those that wish to see the change.
Vibes? What we have here are email selection effects :-)
> But the argument that there is an inherent need for civil time to be coupled
> to mean solar time is debatable
So you are saying that the word "day" could be set to mean some duration
distinctly different from 86,400 SI-seconds? Please consider the implications
should a civil day be taken as 86,408 SI-seconds - less than one-hundredth of
one-percent different from the synodic day. Eight leap seconds per day or one
leap hour per 450 days or timezone shifts (assuming this is practical - I'm
still waiting to be convinced) every year-and-a-quarter. Is this practical?
The civil day must continue to mimic the mean solar day to a close tolerance.
This really isn't news. Nothing to see here citizens, move along.
> and your continued assertion that it's necessarily true and there's no point
> in debating it is putting people off the rest of your argument.
Rather than debating physical parameters, debate the details of permissible
approximations.
> You either need to justify it a lot more carefully, including explaining why
> the ~2 hour offsets routinely seen today don't cause anyone any difficulty,
Because they are not offsets in the underlying time scale, which remains
stationary with respect to the synodic day.
Rob
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