Ron, et al,
Yes, I like (and now think I understand) your approach, alas, I wish I
were patient enough. Namely, the hour angle of the Sun and its
projection will be what they are and no correction need be applied since
the EoT is zero and the hour angle delta between the local and standard
meridians is constant.
The approach I've taken in the past is to project the Analemma itself
from computed local hour angle values. One might consider this approach
for at least one of the hour-lines, as it is adds the satisfaction of an
analytical approach to the measured approach that Ron has mentioned.
Also, and IMHO, using the hour-line of the Analemma provides a more
intuitive understanding.
Regards,
Luke Coletti
Ron Anthony wrote:
>
> Mike, Luke,
>
> I was saying, on a few of the days that the EOT is 0, look at your watch and
> mark the ceiling right on the hour. And then connect all of the like hours
> dots you will build a dial that works and is corrected for longitude, but not
> EOT. The hour lines are simple straight ones, (that is of course if all the
> hour dots are on the ceiling).
>
> Someone had told her to use the winter and summer soltice as the days to mark
> the hour dots. That would be somewhat close, but not as accurate as the 4
> days given.
>
> ++ron
>
> >
> > This is a bit confusing because I'm not sure I can tell what type of
> > hour-lines are being considered here. Perhaps Ron was referring to the
> > fact that if one constructs the hour-lines from the projection of the
> > Analemma itself AND includes the delta in local longitude from the
> > longitude of the standard meridian, then, the solar projection will
> > indeed cross zero four times per year(?). I think this may be the more
> > intuitive way of constructing such a sundial too, for the very reason
> > that this thread makes clear.
> >
> > Regards,
> >
> > Luke Coletti
> >
> >
> > The Shaws wrote:
> > >
> > > Ron wrote:
> > > <<I've never built a ceiling dial, but something doesn't sound right about
> > > the advice that you got. The days that you choose should be days where the
> > > "equation of time is 0 (sun time and watch time agree). This happens 4
> > > times a year. April 15, June 14, Sept 1, Dec 25.>>
> > >
> > > But clocks and dials DON'T coincide on these dates UNLESS you just happen
> > > to
> > > live on your prime meridian.
> > >
> > > If you want to apply the "normal" Equation of Time correction, you will
> > > have
> > > to work out which days coincide at your own longitude.
> > > Where I live (3 degrees West of Greenwich) clock and dial only coincide
> > > on 2
> > > days a year.
> > > If you live more than about 5 degrees away from your meridian, they will
> > > never coincide.
> > >
> > > See:
> > > http://homepage.ntlworld.com/jmikeshaw/
> > > and follow the link to "Equation of Time slide rule" first,
> > > and then "Noon Mark"
> > >
> > > Mike Shaw
> > >
> > > N 53º 21' 24"
> > > W 03º 01' 47"
> > > Wirral, UK.
> > >
> > > ---
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