[time-nuts] Why 9,192,631,770 ??

paul swed paulswedb at gmail.com
Fri May 11 18:53:42 UTC 2012

So what you are saying is every 30 years select a new leap CS reference.
Dispense with everything in between.

On Fri, May 11, 2012 at 2:21 PM, Tom Van Baak <tvb at leapsecond.com> wrote:

> > Are there better estimates of the ET second nowadays (relative to the
> > SI second)?  It would be interesting to know what the cesium frequency
> > "should have been" if much better estimates of the ephemeris-time
> > second were available at the time.
> Hi Peter,
> Everyone should take ten seconds and look at this animated GIF:
> http://leapsecond.com/pages/ut/ut-ani-v2.gif
> It shows what would have happened to "earth time" vs. "atomic time" if the
> cesium frequency had been defined to be other than 9192.631770 MHz. As
> you can see a slightly higher number would have meant less deviation
> between the two timescales.
> However it should also be clear, even with this short 40-year plot, that
> no number is the best or correct or right choice. It all depends on which
> year(s) you choose to base your earth rotation rate calibration on (the
> astronomers doing the calibration in the 1950's selected the year 1900 as
> their baseline).
> To see each page at your own pace here is it as a multi-page PDF file:
> http://leapsecond.com/pages/ut/ut-ani-v2.pdf
> If you wanted a near perfect match between atomic time and the rotation of
> the earth during the 1970's hindsight tells you the frequency should have
> been 9192.632080 MHz. Similarly if your crystal ball said to use
> 9192.632010 you would have been very close for three decades. If you
> wanted the best time accuracy from the year 1972 to present you should have
> picked 9192.631950.
> /tvb
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