Leap Seconds

JRS: In article <N7v2ZEGtyQtDFwdh@xxxxxxxxxxxxxxxxxxxxxxx>, dated Fri,
30 Dec 2005 10:32:13 local, seen in news:uk.sci.astronomy, Jonathan
Silverlight <jsilverlight@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> posted :
>
>Sorry, but over geological time (which is what MDJ was talking about)
>the Earth _is_ slowing down. There were more days in the year in
>Jurassic times, and there's no reason to think (and no way to show) that
>the year was any different. I know this has nothing to do with "leap
>seconds".

Well, we cannot both be right - see <URL:http://www.merlyn.demon.co.uk/
leapsecs.htm> and its links, and see what you then think; and
<URL:http://www.merlyn.demon.co.uk/leapyear.htm>.

The SI second is constant in length, and was chosen to match 1/86400 of
an average day, of the sort for which there are on average about
365.2425 per year. Leap Seconds accommodate the discrepancy between
1/86400 and the actual value, keeping UTC & GMT within 0.9 seconds of
each other.

With the Earth then spinning appreciably faster, the Jurassics would
have has to have negative Leap Seconds at a great rate, if they had SI
atomic clocks.

Leap Seconds deal with the rate of the Earth's rotation, and currently
accommodate a varying discrepancy of the order of 0.5e-8.

Leap Years deal with the period of the Earth's orbit as a multiple of
its rotation, and accommodate a virtually constant discrepancy of close
to 0.2425 parts in 365.2425.


>BTW, am I right in thinking that the UK is no longer part of the network
>of atomic clocks that maintain UTC? I know that was proposed at one
>time.

That should not be so; NPL has in the past had considerable experience
and a number of clocks. Check on www.npl.co.uk. Last I heard, the
world ensemble had around 200 clocks. Try the IERS site.

"IERS" always makes me picture a few Frenchmen, equipped with striped
jerseys, berets, vin ordinaire, & Gauloises, in a field near Paris,
supervising a multinational team of labourers turning a *very* big
handle.

--
© John Stockton, Surrey, UK. ?@merlyn.demon.co.uk Turnpike v4.00 MIME. ©
Web <URL:http://www.merlyn.demon.co.uk/> - w. FAQish topics, links, acronyms
PAS EXE etc : <URL:http://www.merlyn.demon.co.uk/programs/> - see 00index.htm
Dates - miscdate.htm moredate.htm js-dates.htm pas-time.htm critdate.htm etc.
.

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