Roger You and Fer, more than anyone else, have taught me to visualize the movements of the sun, earth and planets. When I first saw that telescope with all those possible styles it was very confusing and I didn't understand how I would make it work because it had so many possible styles. I thought, Oh no, there are going to be different dial face scattered everwher and it's going to be really confusing for the user. But then I just stood there, looking at the telescope, and ran a speeded-up vision in my mind of the sun's movement during the day. When I did this, it all became clear. It was then that I realized that I could use thre shadows of smoothly shifting styles on a single sundial face. As soon as I got home, I grabbed one of those long cardboard boxes that hold aluminum foil. (This kind of long square box is the same shape as the telescope/sundial gnomon.) I held it perpendicular to the sun and very slowly rotated it to imitate the sun's movement, closely observing the shadow edge at the moment of style change. This is when I saw that the style change was smooth but with a slight change in the width of the penumbra.
John John L. Carmichael Jr. Sundial Sculptures 925 E. Foothills Dr. Tucson Arizona 85718 USA Tel: 520-696-1709 Email: [EMAIL PROTECTED] Website: <http://www.sundialsculptures.com> ----- Original Message ----- From: "Roger Bailey" <[EMAIL PROTECTED]> To: <[email protected]> Sent: Sunday, May 19, 2002 8:56 PM Subject: Shadow Sharpener > Hi John, > > Isn't it amazing what you can learn about shadows and time with a shadow > sharpener, a simple pinhole in a card. I learned the dark edge of the shadow > that we typically use for sundials of all sizes does not represent the > middle of the sun that we use for our calculations. Things don't get dark in > the penumbral shadow until most of the sun is covered. This means that our > readings are typically more than half of half a degree or one minute off. > > Your heliochronometer with the wire gnomon offers a solution. For the right > gnomon diameter and distance, here we see the diminishing cone of the umbral > shadow. As you have found, the centre of this shadow accurately projects the > position of the centre of the sun to give accurate time readings. This > applies to real solar time not that averaged, adjusted, homogenized, > uncivilized civil, daylight savings, central meridian, Cesium atomic time > that needs all the longitude, EoT and other corrections. Trust the sun and > your shadow sharpener for the correct time. Never trust anything atomic or > run by the government. At High Noon the sun is due south at its highest > elevation. > > Roger Bailey > Walking Shadow Designs > N 51 W 115 > > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] Behalf Of John Carmichael > Sent: May 18, 2002 10:20 AM > To: Edley McKnight; Sundial List > Subject: Re: Multiple styles (and Penumbra Width) > > > Hi Edley (and all) > > It's really neat how the telescope sundial shifts from style to style. I've > never seen a sundial work like this before. The styles shift from west to > east as the sun goes from east to west. One takes over where the other one > leaves off. In the mornings, The time scale starts with the bottom > underneath style at sunrise, this will shift into the east side style at > 9:26 am which takes over and works until 3:27 pm. shifting to the top style > that functions until sunset. And it does this so smoothly that the observer > doesn't even know that it happens (unless he's closely observing the > penumbra's width). > > Speaking of penumra width, I read with great interest the article by the > Virendra Nath Sharma, India in the June Compendium (pg.19). It "shows how > subtleties of the solar penumbra lead to a discrepancy in time-reading > between the two quadrants of the Samrat, which is generally acknowledged to > be the world's largest sundial." > This weird unexpected effect is something I had never heard of before, but > it makes a lot of sense (read the article and look at his drawings). In > essence, this effect is only noticeable on very large sundials and it WILL > cause significant errors especially if you design a huge sundial and neglect > to consider it in your equations. > > To avoid this problem and others when laying out the hourlines on a giant > sundial, it is far easier and more precise to use the "time method" and a > good clock than to use surveying methods and/or drawings. > > When using the time method he also talks about a little trick he used to > determine the edge of the wide penumbra. He"... superimposes the shadow of a > taut string 2 cm long parallel to the shadow edge, about 1 cm or so above > the instrument's surface, and reading the scale where the string's shadow > merges with the shadow of the gnomon's edge, we could repeat our readings > with an accuracy of =?- 3 sec. or better." > > In other words, he's saying that if you slowly move the taut string east to > left and right, you can see on the face the place where the string's shadow > appears and disappears. I'm sure this method works, but in my experiments on > Kitt Peak I'm finding that a pinhole "shadow sharpener" works even better > because it is a spot rather than a line. (when he's holding the string > parallel to the shadow, how does he know it is really parallel? The pinhole > doesn't have to be parallel). > > With a pinhole shadow sharpener I'm sure we can get 1 second accuracy on > Kitt Peak because I've already tested it. In fact, a little display of a > shadow sharpener could be set up on what I call a "High Noon table" for > visitors. This would be a waist-high table about a meter wide located on the > meridian underneath the east gnomon. It would incline south like a polar > sundial and would have minute markings a little before High Noon until a > little after. A metal shadow sharpener would be permanently attached on the > meridian above the High noon line. A little before High Noon, the pinhole > will produce a circle of light on the table dial time. At the instant the > sun crosses the meridian, the shadow of the gnomon will cover the pinhole > and the circle of light will disappear. Don't you think this would be fun > for visitors? It might even serve as a date marker with a little analemma on > the time scale (you can try this technoque yourself using the shadow from a > tall building). I haven't tried using a shadow sharpenner at the moment of > the style shift. Can't wait to try it. This is cool stuff!!! > > John > > John L. Carmichael Jr. > Sundial Sculptures > 925 E. Foothills Dr. > Tucson Arizona 85718 > USA > > Tel: 520-696-1709 > Email: [EMAIL PROTECTED] > Website: <http://www.sundialsculptures.com> > ----- Original Message ----- > From: "Edley McKnight" <[EMAIL PROTECTED]> > To: <[EMAIL PROTECTED]> > Sent: Friday, May 17, 2002 11:44 PM > Subject: Multiple styles > > > > Hi John, > > > > I've seen one site that had multiple edges as styles where the hour marks, > not lines > > were extended over a restricted range for each style and colored a > different color > > as well. In this case the styles overlapped in coverage, but the correct > time coule > > be read from each one. i.e. the hour lines don't have to radiate from > ground level > > but from a spot underneath a leaning style. Fer's monofiliment program > might shed > > some better light on this. Ask if both interested and if it is not clear. > As an > > example of two styles one using a lower section of the first style and > the other > > using an elevated section of the second style. This would group the > shortened hour > > lines into two groups where the style to use could be made obvious. > > > > Anyway, great fun huh?! > > > > Edley. > > > > - > > > - > -
