Thank you Willy. That is indeed the type of graphic that is needed.
Frans Maes is to be complimented for this diagram approach to explaining
the principle.
In contemplating this diagram, I think it is important to point out that
the ellipse is not a shadow.
What then is the reason that the shadow of the vertical rod falls on the
projection of the hour point on the ellipse?
I think the answer is best pointed out by noting that the sun's ray and
the vertical rod define a plane.
The shadow of the vertical rod must also lie in this plane (it is the
"shadow plane" of the vertical rod).
Hence the vertically projected hour point and the shadow of the vertical
coincide.
It can also be pointed out with the aide of this diagram that the shadow
of the polar gnomon does not lie in this plane.
Furthermore its shadow would not pass through any hour point other than
noon.
Thus the difference in operation of these two dials is made obvious.
This can be used to find true North if both are combined in some way.
This is done in the many "Sun Compass" devices discussed just recently.
I wonder how many teachers would be able to demonstrate these
relationships on the site of a analemmatic dial?
On 4/24/2011 1:05 PM, Willy Leenders wrote:
Engels
▼
Een tekst of webpagina vertalen
Bedoelde u: Clude beweert dat de zonnewijzer de */productie/* is van
een polaire zonnewijzer
Typ tekst of een websiteadres of vertaal een document.
<http://translate.google.com/?tr=f&hl=nl>
Annuleren <http://translate.google.com/?tr=t&hl=nl>
Luisteren
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Vertaling van het Nederlands in het Engels
Clude claims that the sundial is the projection of a polar sundial
See on the website of Frans Maes, click on The principle of the
analemmatic dial
<http://www.fransmaes.nl/zonnewijzers/en/zw-analem2-e.htm#principe>
Op 23-apr-2011, om 21:24 heeft Claude Hartman het volgende geschreven:
I would like to question the instructional qualities of analemmatic
sundials.
Unlike Tony Moss' 'Multi-dial', it is my view that there is only one
relationship easily illustrated - the projection of a polar dial on
to a horizontal surface is an ellipse with corresponding hour points.
It is extremely difficult why (or how) a vertical person or staff
casts a shadow to these points.
I am afraid that the instruction falls into rote learning of
procedure like so many sundial kits (and much of pedagogy). I do
not know what the underlying motivation was for "that part of the UK
National Curriculum". I assume it was to understand something of the
natural world and not to acquire the skill of making a sundial.
Under that assumption I think the analemmatic falls short and could
produce confusion for inquiring minds.
With that said I certainly would NOT join any ban on such dials as
"dangerous for children"! Indeed, both children and adults interact
with them quite well and they can make a beautiful asset in a school.
However, expect difficulty with a persistent inquiring person
seeking explanations.
Perhaps someone on this list can show me a way to explain that to
anyone lacking much geometric skill such as a 10 year old (or me).
Claude Hartman
(retired science teacher basking in the sunshine of 35 N, 120 W)
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