"waggy"  wrote in message
<web.45c9603e92dd103d442464bf0@news.povray.org>:
> I managed to find a spreadsheet (link to descriptive page of it below) to
> calculate what I hope will be all the corrections needed for sunpos to
> estimate the location of the Winter Solstice sunrise a few millennia ago at
> the location of interest.  These data, combined with the links y'all have
> provided to explain the formulas, should be enough to figure out how to
> modify the macro.  It will be interesting to see how much the time of day,
> perhaps even the day of the year, has changed as well.

As I said earlier, I doubt you will have a correct precision for such a
distant date. But in your case, it is probably not a problem. Let me
explain.

The course of the sun in the sky can pretty accurately be decomposed in two
components: the course of the sun along its daily path through the sky, and
the position of that path, low in winter and high in summer.

The former defines the local time of the place. And that is the point where
you lack accuracy: if you try to calculate it with earth-rotation models in
absolute time, you will have an offset of several minutes.

The latter is much slower, and therefore can bear a few minutes error.

Now, what you ask is the "sunrise". You can not make more localtimeish than
that.

Therefore, what you can do is:

- Compute the instant of the winter solstice in the year you are interested
  in. You can probably achieve better than a few seconds accuracy.

- Find the nearest sunrise at the place you are interested in. You will have
  a few minutes error, but this error would only visible through the
  position of the sun along its path through the sky, which is precisely the
  parameter you have fixed.

Well, all this is theory, and you probably can not do it with sunpos.inc.
The first step can probably be done with ephemerides programs. As for the
second, I have no idea.

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