----- Original Message ----- From: "John Carmichael" <[EMAIL PROTECTED]> To: <[email protected]> Sent: Tuesday, February 15, 2000 12:27 PM Subject: Dali dial rings & gnomons
> HI all: >...<cliipped> >Apparently, ring order and number of > rings are not important. Am I correct? >...<clipped> > I know we discussed the Dali azmuthal dial before, but the type of gnomon > one can use still confuses me. How does a vertical rod with no nodus and no > set height, function just like a polar axis gnomon? John, I think you're trying too hard! Image a short vertical pole in your garden, next to a long one. The length of the shadow cast by the sun will differ for the two poles, but the direction of the shadows is the same for both poles. Hence the direction of the shadow indicates the sun's azimuth, irrespective of the pole's length. Since the azimuth is related to time of day, so the shadow's direction can indicate time. You build your dial with an azimuth scale forming a circular arc, like a giant protractor. Since the length of the shadow has no bearing on the azimuth, so the radius of the protractor does not matter. For convenience, instead of marking the scale with degrees you mark the corresponding hours. But the azimuth for a given hour varies throughout the year so you have to have lots of individual protractors, each one giving a time scale for one day. In the diagrams you have seen, these 365 scales are simply concentric arcs - lots of protractors sitting inside each other.. The dial designer is exploiting the fact that radius is not relevant to time of day, and is thus available for indicating date. When reading the dial, you choose the radius according to today's date and thus you are selecting which protractor to use. They could be in any order, but for convenience they are arranged in date sequence so that you can easily home in on the one for today. The shadow coming from the pole is crossing all the arcs, but we ignore that and just choose the position where it crosses today's arc. We don't have to indicate the date by varying the radius. I have designed a dial with a vertical style, where the dial face is an upright semi-cylinder. The date is selected by varying the height from the ground at which the shadow is read. Then the time is read from the shadow angle. Steve
