Hello,
It is true that the Analemma is a graph of the EoT vs. Dec but I would
recommend that EoT values that are the computed points of an Analemma be
based on a four year average if one is, as I am guessing, correlating
the imaged solar position along the Analemma to an exact date. One
should also consider computing the four year average of declination too.
The change in the shape of the Analemma i.e., the secular change of the
EoT, is as you say, on a rather slow time scale but don't sell it short,
the Vernal Equinox is shifting clockwise at 50 arc-secs/year and
Perihelion is shifting counter-clockwise approx. 12 arc-secs/year and it
is the phase relation between the two events (i.e., the maximum of
eccentricity at Perihelion and the min(s) of obliquity at the Equinoxes)
that drive the shape of the Analemma.
Best Regards,
Luke Coletti
Chris Lusby wrote:
>
> Daniel Lee Wenger <[EMAIL PROTECTED]> writes:
>
> > On my globe the significance of the analemma is apparent. Each analemma
> > represents the geographical
> > position of the sun at mean time 6, 7, 8, 9, etc. for each day of the year.
> > In fact the geographical
> > postions of the sun at those mean time hours would be a collection of 365
> > dots but the analemma
> > is interpolated to generate a semi continuous curve. Since the set of dots
> > that would be generated during the following year would be slightly
> > different the curve used represents some sort of average
> > of the dots over a four year period.
>
> The set of dots that would be generated during the following year would lie
> on the same curve. The reason the dots are not in the same places is that
> the year is not a whole number of days long. The curve is not an average
> over a four year period. If you were to draw four years' curves separately,
> you would see that the curves are identical, but the points used to plot
> them are not. The curves are the graph of Equation of Time against
> declination, both of which are functions of the solar longitude. The
> particular values of solar longitude at, say, noon every day in 1999 are
> different from those in 2000, but the analemma points lie on exactly the
> same curve (ignoring only very long-term drift).
> To use straight lines as an illustration, the points (1,100) and (6,105) lie
> on the same line as the points (2,101) and (7,106) or the points (3,102) and
> (8,107) or the points (4,103) and (9,108).
>
> Chris Lusby Taylor
>
> Email: [EMAIL PROTECTED]
> (Formerly [EMAIL PROTECTED])
