[LEAPSECS] Cheating means more planning, not less
seaman at noao.edu
Mon Dec 29 01:45:07 EST 2008
Poul-Henning Kamp wrote:
> Rob Seaman writes:
>> Focus on the SI second and we see the world through atomic
>> eyeballs. Focus on the primacy of the definition of the day in
>> civil timekeeping, and Earth orientation pops out.
>> Both timescales are necessary.
> It is well documented that the SI second based timescale has
> precision and stability requirements on the order of microseconds
> for telecoms and 10 orders of magnitude smaller for scientific tasks.
So I guess we're agreed that this is one of the necessary timescales.
Good thing it is already widely available in GPS. (The 10^-16 is a
bit extreme :-) A lot of good science happens at a precision less
than a second, for that matter.
> In contrast to this, nobody, including you, seem to be willing to
> even hazard a guess what level of presision is required or
> sufficient for the "earth orientation clock".
The current UTC standard is precise to 0.1 SI seconds. I will hazard
a guess that this is sufficient. I believe we both have long since
expressed the opinion that this could be relaxed a bit. If the ITU
proposal didn't explicitly deprecate DUT1, a new-and-improved UTC
could even improve the precision for corrected values while loosening
the fit for uncorrected values.
> The current UTC definition says "better than one second",
> but relative to an abstract definition of earth rotation angle which
> only astronomers can figure out.
I'll spare everybody my familiar harangue on the mean solar day just
corresponding to the sidereal day plus a little bit extra to correct
for the Earth lapping the Sun once a year.
Oh! I guess I won't. Raise your hand if you can't figure out what I
> Emperical evidence show that most of the earths human population is
> perfectly happy with local time that is within a couple of hours of
> "proper" earth rotation time.
And I really will spare folks my other screed on civil timekeeping
having nothing to do with local apparent solar time. Since everybody
seems to agree on this point, I'm not sure why it keeps coming up.
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