I agree with what I understood to have been the point of Claude’s initial comment: Analemmatic sundials are not very good for pedagogical purposes. It is true that they are often pitched to children and put in parks or playgrounds. A child can be briefly diverted by the fact that if he stands on the name of the month, the shadow of his head will point more or less toward the same time as his watch – or these days, maybe his cell phone—but the mechanics and geometry that makes it so are quite daunting. Without a polar gnomon, there is no understandable link to the daily rotation of the celestial sphere that carries the rising and setting sun. Moving the base of the shadow seems like an arbitrary trick designed to make the numbers come out right. I barely understand how they work myself, and can only do so by moving to an uncomfortable level of geometric abstraction where I lose track of the north pole and the ecliptic. I think it is way too much to expect a child to be able to grasp the relationship between an analemmatic dial and the basics of celestial motion that should be the point of the exercise.
I think Tony’s demonstration dial gives an excellent explanation of how the sky and the sun relate to the sundial and also how different types of dials relate to each other. Instead of adjusting the dial to take account of daylight saving time it is more instructive for a child – or an adult – to learn that there are three reasons that solar time is not the same as clock time. It is good to be reminded that clock time is arbitrarily shifted twice a year for summer time, and maybe to reflect on whether this is useful or not. Similarly, the longitude adjustment – or lack thereof – is a good introduction to time zones and why we have them. Both of these concepts are interesting, straightforward, easily understood, and worth knowing. The final equation of time adjustment to go from solar time to mean time can be taken on faith. Most people are satisfied by a vague explanation that this is a consequence of the earth’s tilt and elliptical orbit without wanting to understand the exact mechanics. Jack From: sundial-boun...@uni-koeln.de [mailto:sundial-boun...@uni-koeln.de] On Behalf Of Claude Hartman Sent: Sunday, April 24, 2011 5:40 PM To: Willy Leenders; sundial@uni-koeln.de Subject: Re: Analemmatic dials at schools ? 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 ▼ [Vertaal] 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 Fonetisch lezen 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 <http://www.fransmaes.nl/zonnewijzers/en/zw-analem2-e.htm#principe> principle of the analemmatic dial 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) --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
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