Re: questions about UTC time, definitions, leap seconds, and measurement.
- From: nospam@xxxxxxxxxxxxxxxx (J. J. Lodder)
- Date: Mon, 30 May 2005 05:20:18 +0000 (UTC)
robert bristow-johnson <rbj@xxxxxxxxxxxxxxxxxxxx> wrote:
> i've been looking into the NIST site on their standard expression of the
> time:
>
> http://tf.nist.gov/pubs/bulletin/leapsecond.htm
>
> this brings me back several decades when i was a young ham radio operator
> and i tuned into WWV to set my clock and to calibrate my 100 kHz frequency
> calibrator.
>
> i think i understand sufficiently what astronomical time is and what atomic
> time is.
>
> my first question was this:
>
> by 1983, they must have expected about 1 leap second each mean year (1 part
> per 31556952) why didn't they define the second to be that much longer (say
> 9,192,632,060 cycles instead of 9,192,631,770 cycles of this cesium
> radiation? they would still have to add or subtract leap seconds
> occasionally, but much less often than they did in the late 20th century.
> why not, when you're *defining* the standard of time (with a much better
> standard), make that initial definition agree as much as possible with the
> known astronomical unit, and then stick to it?
They opted for best backwards compatibility with the second that was.
This had been defined in terms of the tropical year at 1900.
Anyway, it won't matter in the long run.
The earth is a clock that is steadily decelerating,
so the error will ultimately grow quadratically with time.
This will swamp any linear term due to a less fortunate choice
of the definition of the second.
> my second question is:
>
> what happened after 1999??? why are there no leap seconds after 1999 on
> that web page? certainly it must have been updated since then? were there
> *no* leap seconds added in the last 5 years?
Right, the earth has spun up a bit, temporarily one assumes.
So the shortish choice of the second may be fortunate after all.
Otherwise negative leap seconds might have been needed by now.
Best,
Jan
.
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- questions about UTC time, definitions, leap seconds, and measurement.
- From: robert bristow-johnson
- questions about UTC time, definitions, leap seconds, and measurement.
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