[LEAPSECS] Straw men

Rob Seaman seaman at noao.edu
Mon Jan 9 19:23:50 EST 2012

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:


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.


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