Hi Jim: I've always thought a sphere would be best for most nodi (or is it noduses?) A sphere produces a shadow that is usually elliptical (except when the sun is directly overhead, then it's a circle) so it is fairly easy to guesstimate where its shadow's center is, producing more accurate time and date readings. Whereas if the nodus is any other shape, its shape becomes distorted especially when the sun is at a low altitude in the am and pm, making it more difficult to read.
It's debatable, I guess, whether the tip of a pointed rod gnomon or the tip of a triangular sheet gnomon might be better than a sphere. If you used a shadow sharpener with these pointed tip gnomons, they would of course work better than a sphere gnomon. A shadow sharpener would be useless on a shadow cast from a sphere because it would not help you to find the center of the elliptical or circular shadow. Also, I just like the look of a sphere better. Anyway, those are my thoughts on the subject, for what it's worth.... John p.s. Don't forget to calculate the minimum diameter of the sphere so that it's shadow will still produce an umbra. To do this, calculate the maximum distance from the sphere to the dial face that the shadow will have to be projected. Then use the "Fuzz Factor Ratio" (1/107) to calculate it's minimum diameter. We found out at Kitt Peak that you will want to make this diameter slightly larger than the ratio indicates. For example: If the maximum projection distance is 107meters, then the minimum sphere diameter is 1.07 meters. But I'd make the sphere a little larger than that. ~ 1.5 meters or so. It's hard to be exact, because when you look at these shadows, it's really a judgment call. ----- Original Message ----- From: "J.Tallman" <[EMAIL PROTECTED]> To: "Sundial List" <sundial@rrz.uni-koeln.de> Sent: Monday, July 12, 2004 5:35 AM Subject: Re: Turtle Bay Sundial Bridge opens > Hello All, > > > > > If they just use the tip of the angled pieceas the gnomon (or nodus) and > > forget about using the edge of angled piece, then it could work. > > > I have been wondering about the practical realities of the use of a nodus > that is 200 - 300 feet away from the dial face. Wouldn't the umbral effect > blur the shadow of a nodus to the point of it being difficult to see? > > John, I suspect that you may have the best practical experience with this > after working on the Kitt Peak project. What do you think the best > configuration would be for such a nodus? > > > Best, > > Jim Tallman > Sr. Designer > FX Studios > 513.829.1888 > > > > > - > -
