[LEAPSECS] MEAN! SOLAR! TIME! (was re: something else)
seaman at noao.edu
Mon Dec 22 11:39:53 EST 2008
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):
and in 2006:
and in 2007:
and in 2008:
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:
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"
> 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:
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.
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