[LEAPSECS] MEAN! SOLAR! TIME! (was re: something else)

[Rob Seaman]

  Poul-Henning Kamp wrote:


> Rob Seaman writes:

>

>> 3) My own point of view focuses on the requirements for "wall

>> clocks".  Civil timekeeping has (heretofore) been mean solar time

>> [...]

>

> You mean "has been within a couple of hours of mean solar time" ?


No.  I mean "mean solar time".  As explained, for instance, in 2005 (I  
could go back farther):

	http://www.mail-archive.com/leapsecs@rom.usno.navy.mil/msg00676.html

and

	http://www.febo.com/pipermail/time-nuts/2005-July/018991.html

and in 2006:

	http://www.mail-archive.com/leapsecs@rom.usno.navy.mil/msg00954.html

and in 2007:

	http://catless.ncl.ac.uk/Risks/24.79.html#subj3

and in 2008:

	http://six.pairlist.net/pipermail/leapsecs/2008-January/000184.html

Searching on these various lists (or via google or in my mail folders)  
hasn't uncovered a message I remember sending to one of the lists that  
addressed the question even more explicitly, but the plots from that  
message are in the first link above.  In any event, I've attempted to  
answer this question over and over and it keeps popping up.  My  
apologies for failing to find the words to make the point clear.

Briefly:  Apparent solar time is a red herring (http://www.nizkor.org/features/fallacies/red-herring.html 
).  Mean solar time is simply sidereal time (the "real" length of day  
on Earth) offset by about 4 minutes to make up for the Earth lapping  
the Sun once a year.  The issue is the stability of the rate, not some  
imagined average over lots of separate measurements of the Sun's  
apparent position in the sky.  After all, we circle the Sun, not the  
other way around.  Local time is an offset from mean solar time, not  
the other way around.

The first plot in the first link:

	http://www.mail-archive.com/leapsecs@rom.usno.navy.mil/msg00676/daylength.pdf

shows that the sidereal rate (adjusted for one lapped day per year) is  
quite stable, even when the shape of the Earth's orbit and the  
inclination of its axis are accommodated. (I'll keep looking for that  
message, it discussed all this.)  Rather, the issue is confused by the  
sundial corrections shown in the second plot, grandly named the  
"equation of time".  The EOT is nothing special, just the integral of  
the modest annual LOD excursions.  Static timezone offsets and  
periodic DST zero point adjustments are similarly beside the point.

Even briefer:  The real length of the day on the planet earth for  
human purposes is the mean solar day.  Introducing a secular trend  
into this is a poor idea with a limited lifespan.  Eventually the  
embargoed leap seconds (or equivalent) will have to be released, else  
it is the definition of "day" that will be broken.  Any viable  
proposal has to include some plan for what happens then.


>> 4) The ITU proposal is basically an assertion that people don't care

>> about mean solar time.

>

> The fact that the majority of the earths population use a legal time

> that is more than 15 minutes different from mean solaer time, would

> seem to support the ITU in this.


Local time is not the issue, has never been the issue, has nothing to  
do with the issue.

The issue is stabilizing the daily rate of the underlying "universal"  
time scale.


> Have you or others provided any documentation for the opposite claim ?


Over and over and over.

Here's another go:  As you say, it is likely that most people rely on  
local standard time that is more than 15 minutes different from local  
mean solar time at their location.  In addition, their local apparent  
solar time has excursions from their local mean time of around 15  
minutes in amplitude (although most of the year, they will be closer  
than this).  In addition, their local government may institute DST  
adjustments of an hour or more.  The citizens may also be jet lagged,  
for that matter.

However:  Everybody everywhere, throughout history, has observed a day  
whose LENGTH throughout the year is within +/- 30 SI seconds of the  
mean solar day length:

	http://www.mail-archive.com/leapsecs@rom.usno.navy.mil/msg00676/daylength.pdf

The ITU is attempting to tilt the baseline of this plot.  (Or rather,  
remove the controls that periodically accommodate the natural tilt.)   
Whether or not you believe this is an acceptable kludge, it most  
definitely is a kludge.  Such a secular trend cannot accumulate forever.

Rob
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