On Mon, 3 May 1999, Phil Pappas wrote: > **** Does this mean that there is no upper limit for the size of a > sundial? ***** > > Assuming that the fuzziness of the shadow is not a limiting factor for > maximum size, because you can get precise readings using the fuzz zone's > center, then a huge sundial could be built that could have extremely small > time divisions. Couldn't it?
Well, sort of. There is "penumbral shadow" and there is what I would really call "fuzz", due to scattering of the light. In space, the shadow would exhibit a precise trigonometric function (what is it, anyway? Secant?), from full sun to full shadow, in the angular diameter of the Sun. On Earth, the atmosphere adds more distortion and "fuzzes" the shadow too. > If this is true, then one second time line markings could be placed on the > dial face, couldn't they? I haven't done the math, but if the one second > lines at high noon ,when they are closest, were spaced at an easy to read > distance of about a 1/2 inch apart on a giant horizontal sundial, then the > height of the style and the diameter of the face could be determined. It > would be a large sundial indeed! For an equatorial dial, or approximately, for a horizontal dial near noon, 1 Second marks 1/2" apart imply the distance to the style about 573 feet. Now, that *is* a big sundial!! > It has long been my dream to design and construct such a sundial, maybe not > with one second markings, but with 30, 20, or 10 second time lines. > The diameter of available stranded metal cable may be the limiting > size factor here because if the sundial were too large and the cable too > narrow then the shadow would completely disappear (like telephone lines do > on the ground). Agreed. 10 Second marks give you only 57 feet to the cable, which is more manageable. However, the cable would have to be greater than 6" diameter to cast a shadow at all, at noon. Earlier or later in the day, the distance from the cable to the dial surface is longer, so you would need even larger cable. The counterweight needed to hold that big a cable straight, is mind-boggling! > John Carmichael > Tucson > website: http://www.azstarnet.con/~pappas Dave Bell
