John Carmichael wrote:
> Hello all:
>
> As you know, horizontal and vertical sundials have hour lines that are close
> together at noon and further apart in the early morning and late afternoon.
> This unequal spacing makes interpolation of the time between hour lines
> somewhat difficult. Out of curiosity, I tried to see if I could make a
> drawing of a standard horizontal dial (not an equant dial) that has equally
> spaced hour lines.
>
> First I drew a simple horizontal dial showing local solar time with one hour
> timelines, then I picked a point at random on the 12 noon line. I drew a
> small circle with this point as center and made it big enough so that it
> intersected the 1 o'clock line at a single point. Then using this point on
> the 1 o'clock line I drew another circle of the same radius as the first
> circle and marked the point where it intersected the 2 o'clock line. If you
> repeat these steps for all the hour lines and connect the center points of
> all the circles with lines (or splines), you will see that all the hour
> lines are equally spaced. The dial face that is produced by this method
> looks like an indian arrowhead or a philodendron leaf, pointed at 12 noon.
>
> Fred Sawyer gave a talk at the conference about similar dials which have
> equally spaced hour lines. He descibed formulas (incomprehensible to me!)
> used to design such dials and showed several examples of these. He called
> the shape of these "Oudaman's curve". If I recall his talk, he said that
> the arrow shape I came up with is wrong, and that a proper Oudaman curve
> produces a dial which is more eliptical in shape. I don't understand why my
> drawing is wrong if it obviously works!
>
> Any ideas on this subject?
>
> John Carmichael
> Tucson Arizona
>
> If anyone would like to see my drawing, let me know and I'll e-mail it to you.
I'd like to see your drawing, please.
But I'd like to quibble that although the hour points produced by this method
are
equally spaced, the hour lines are not. They are the original hour lines of the
horizontal dial. When we speak of equally spaced hour lines we usually refer to
lines radiating from a common point with 15 degrees between each, or else lines
on
a circular ring every 15 or 30 degrees around its circumference.
To achieve 30 degree hour mark spacing, try an equatorial dial with the gnomon
placed not on the axis but on the circumference.
Chris Lusby Taylor
Newbury,
England
51.4N 1.3W
