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. > -
