fer j. de vries wrote:
>
> Someone wrote : ( I don't remember who it was, I don't save all the
> messages)
>
> > > Suppose a horizontal sundial at latitude 4ยบ S. His gnomom is a very
> > acute
> > >square triangle. What happens with the shadow when sun's declination get
> > the
> > >other side of celestial equator? What happens at the july solstice, does
> > >the shadow
> > >make redable, correct indication?
> >
>
> The subject "What happens when" started with a question about a
> horizontal sundial at 4 degrees south.
> Well, such a sundial will operate perfectly from sunrise to sunset all
> the year around, as any horizontal sundial at any latitude.
> It doesn't matter if the sun is north or south of the equatorial plane.
>
> Reading some answers on the list I got the impression that the subject
> was changed into "What happens with a vertical south or north facing
> sundial at 4 degrees southern latitude".
> No problem to discuss about this question, but it must be seen as a
> separate problem.
>
> Because of the fact that the latitude is nearly 0 degrees, we roughly
> may say that a vertical dial facing south or north at 4 degrees southern
> latitude is equal with a plane equatorial sundial.
> One side of it will operate correctly during one half of the year and
> the other side during the other half of the year.
> In real it will not always show all the hours between sunrise and
> sunset, but this hardly will be noticed at this latitude.
>
> At higher latitudes it is easier to see what happens.
> Suppose a vertical sundial at 45 degrees south, facing north.
> At the december solstice the sun rises behind the sundial plane and will
> shine on the north facing plane a litle before 8h suntime.
> The dial will operate till a little after 16h suntime.
>
> The southern part of such a vertical plane will catch sun from sunrise
> till nearly 8h and also from a little after 16h till sunset, in 2
> separate periods at the same day.
>
> The best is to calculate for whole a year during what time ( 1 or 2
> periods each day ) a certain plane can be used as a sundial.
>
> Fer de Vries, Netherland.
