Luke Coletti wrote:
> Hello,
>
> I have found the best method of correcting for the periodic
> variability
> of the EoT relative to the calendar is to take the EoT as a four year
> average.
How do you do that?
Do you pick a 12 month year which straddles the mid point between two leap
years,
like Jun 30, 97 to Jun 30, 98?
Or do you calculate the "average" of something(s) and if so what? Perhaps the
average of the 4 results of the noon EoT's for each day of the year. I.E.,
calculate the average of the noon results of the EoT for Jan 1,96; Jan 1,97; Jan
1,98 and Jan 1,99. Then do this for every day of the year and list the average
across every day. Leap day?
Or, is there a clever way of just "slipping" one day in the tabular values
between
the results of the EoT and the calendar day 4 times and then averaging. This
would
seem to work and the algorithm wouldn't be too daunting.
> The temporal variability, as has been discussed in earlier
> threads, is chiefly due to the phase relation between obliquity and
> eccentricity as caused by the precession of the equinoxes and the
> shifting of perihelion. The magnitude of obliquity and eccentricity
> changes too but on a slower time scale.
Does this mean that the daily values in the EoT, irrespective of the "calendar
problem", generate some sort of periodic function when taken over the Period of
a
Year (of what sort?) but not quite? That is, on the anniversaries the EoT isn't
quite where it was a Year ago due to the slow change in the absolute values of
the
obliquity and eccentricity?
I missed the earlier thread that you referred to. Perhaps you could direct me to
an URL which quantifies your two marvellously concise sentences above. I think
that I'm getting in over my head and need to do some figuring!
Thank you
Tom Semadeni
>
>
> Best Regards,
>
> Luke Coletti
>
> Chris Lusby wrote:
> >
> > Daniel Wegner ([EMAIL PROTECTED]) is only partly correct in
> > saying that an analemma must have an error due to leap years. The error can
> > be avoided.
> >
> > It is true that tables of the Equation of Time are slightly inaccurate
> > because they take a mean value for the solar longitude on a named date (such
> > as February 17th), whereas the 4 year and 400 year cycles should be
> > allowed-for to be totally accurate. Fortunately for us, the peak error is
> > less in the next few years than at any other time in the 400 year cycle. How
> > convenient. The worst case is in 1903+400n and 2096+400n, when the longitude
> > is 7/8 of a day different from its mean value. But even 7/8 of a day
> > accounts for less than 30 seconds of EoT, so still allows a sundial to be
> > less than a minute out. Around the year 2000, the worst case is half this -
> > about 14 seconds.
> > If an EoT table is drawn graphically to allow a sundial reading to be
> > converted to mean time, then this too must have an error with the same 4 and
> > 400 year cycles.
> >
> > But if the sundial is marked with figure-of-eight hour lines, then there
> > need be no such error, since the sun's declination and longitude are related
> > by geometry, not by what we call the date. Even if we lost another 11 days
> > in a calendar reform (I am from England), such a sundial would continue to
> > read correct mean time. Therefore, I suggest that this is a purer and
> > altogether more satisfactory solution than an EoT table or figure. Except
> > for the little point that the EoT changes rather a lot, and the longitude
> > does not, at the solstices. Pity.
> >
> > By the way, if you are ever making a circular date scale - to calibrate a
> > declination scale, for instance - you should divide it into 365.25 and make
> > February 29th be just the .25. This is the best simple way to allow for one
> > February 29th every four years.
> >
> > Chris Lusby Taylor
> >
> > =======================================================
> > Email: [EMAIL PROTECTED]
> > (Formerly [EMAIL PROTECTED])
> > =======================================================
--
Tom Semadeni O
[EMAIL PROTECTED] o
aka I (Ned) Ames .
Britthome Bounty ><<((((*>
Box 176 Britt ON P0G 1A0
'Phone 705 383 0195 fax 2920
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