Paul Schlyter Sep 19, 2005
> --- In guide-user@yahoogroups.com, Bill J Gray <pluto@p...> wrote:If the planets had any noticeable influence on the Earth's rotation, it
>
>>Hi Larry, all,
>>
>> As Denis points out, one can (in theory) go to the Web site and
>>dig through the source code (delta_t.cpp, in this case) and see
>>exactly how Delta-T is computed. As source code goes, this is
>>(in my humble opinion as its author!) reasonably clear and easy
>>to understand... _if_ you happen to be a C programmer. It's
>>probably rather opaque to the rest of the world, I'm afraid.
>>
>> Anyway. 'delta_t.cpp' has a table of values for Delta-T from
>>1618 to 2004, at two-year intervals, and interpolates within
>>that table. After 2002, it just does a linear extrapolation.
>>That is, for 1 January 2002, it has Delta-T = 64.30 seconds;
>>for 1 January 2004, it has Delta-T = 64.57 seconds, i.e.,
>>Delta-T increased by .17 seconds over those two years. So,
>>for a given time t (expressed in years),
>>
>>Delta-T = 64.57 + (t - 2004) * .17 / 2
>>
>> That rate of increase corresponds to 8.5 seconds/century, and
>>to Delta-T = 64.13 seconds on 1 January 2000. So in the more
>>conventional system where t_cen = centuries from J2000,
>>
>>Delta-T = 64.13 + 8.5 * t_cen
>>
>> Usual warning: Delta-T becomes increasingly ill-defined as
>>you extrapolate it. For example, a decade or so ago, it
>>looked as if Delta-T was going to keep increasing by about a
>>second or so each year, requiring a new "leap second" each
>>December. Then it stopped increasing so briskly, and we've
>>not had a new leap second since 1999. The above formula is
>>there simply because Guide has to say _something_ about what
>>Delta-T is apt to be, even if that something isn't very
>>meaningful.
>>
>>-- Bill
>
>
> One way to understand Delta-T better is to spot the cycles which may
> be incorporated within it. One of these may be solar luminosity -
> which may have a ca. 190-year cycle, as manifested in the delta-T
> values evidenced between ca. 1700 and ca. 1890.
>
> As written at
> http://www2.nature.nps.gov/geology/caves/newsletters/ie7_1/ie7_1.pdf
> "A Master's thesis on a broken stalagmite in the main cave
> indicates a temperature cycle of 190 years between ~13,500 and 9,500
> years ago, most likely a solar luminosity cycle."
>
> This in turn may relate to the cycles in sun spots
> http://personal.inet.fi/tiede/tilmari/sunspot5.html
> which in turn are related to geomagnetic activity
> http://www.sec.noaa.gov/info/SumSept.html.
>
> In my opinion, these variations are caused by planetary perturbations
> viz. planetary positions varying between perihelion and aphelion. See
> http://www.stjarnhimlen.se/comp/tutorial.html
>
> If that were true, then primarily responsible for the ca. 190-year
> cycle would be Pluto (actually, the orbit here is 248 years as such),
> which was at perihelion until 1999, when it was still within
> Neptune's orbit. See
> http://science.nasa.gov/headlines/y2001/ast04jan_1.htm
> Now it is headed back out to be the 9th planet again.
>
> Accordingly, if the above theory be true, Delta T should now slow
> down as it has, stay constant at ca. +65 and then diminish, rather
> than increase. Of course even if we were right on this prediction, we
> could be wrong, since current theory is that the planets can not
> exert this kind of influence, but I am not so sure.
> Andis Kaulins--
> The LexiLine Group at Yahoo
> http://groups.yahoo.com/group/LexiLine/ with a posting relating to
> Delta-T at http://groups.yahoo.com/group/LexiLine/message/1340
>
>
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