From rkellogg@comcast.net Sat Jul 30 07:58:51 2005 Date: Mon, 18 Jul 2005 22:28:23 -0400 From: Robert Kellogg To: Walter Sanford , Mac Oglesby Cc: Fred Sawyer Subject: Re: It's About Time - Sundials // Rules of the Event [ The following text is in the "iso-8859-1" character set. ] [ Your display is set for the "US-ASCII" character set. ] [ Some characters may be displayed incorrectly. ] I believe that a heliodon is too complicated for the average judge to create and set up at his/her school. Beside the accuracy of Serle's ruler estimate of the plate lines, (a) closeness of the style edge of the gnomon to the proscribed latitude angle (protractor here) ... 1 1/2 deg tolerance (b) closeness of the gnomon being perpendicular to the 6-6 line and the point of the gnomon just touching the 6-6 line. For an 8" diameter dial, it must be less than 1/8". How close can you measure?? I can do 1/16th inch. Can a judge? (c) straightness of the gnomon (d) alignment of the 12 local noon with the edge (edges of the gnomon). Again for the 8" dial, 1/8 or 1/16 inch. Try laying out a dial and see how accurate you are... then multiply by 3/2 for judging criteria of students. (e) gnomon perpendicular to the dial plate ~ 1 1/2 degree tolerance? Bob ----- Original Message ----- From: "Mac Oglesby" To: "Walter Sanford" Cc: "Fred Sawyer" ; "Robert Kellogg" Sent: 07/18/2005 11:37 AM Subject: Re: It's About Time - Sundials // Rules of the Event > > Walter, > > I see a difference between evaluating a sundial, which is a system, and > evaluating a dial plate, which is only part of the system. For a > horizontal sundial, the sundial system consists of a delineated dial > plate, a shadow caster, proper assembly, and correct installation. > > To check the accuracy of a sundial you'll need either sunshine (very rare > when needed), or a heliodon. We had the pleasure of watching a complicated > heliodon in action at the San Francisco NASS meeting in 2000. See > http://arch.ced.berkeley.edu/resources/bldgsci/bsl/heliodon.html [period] > However, it's not very hard to make a heliodon suitable for "home" use. I > made good use of a homemade one when working on my Foster-Lambert dial. > Pictures and diagrams available on request. > > Since it's unlikely the judges will have a heliodon available (at least > not this year, but might be a great project in itself), and since it's > probably not workable to use the Sun (even if it chooses to shine), we're > left with measuring the precision with which the dial plate is marked off, > and perhaps the assembly of the gnomon with the dial plate. > > If we talking about judging the "on-the-fly" dial, and if the students > only have 30 minutes to work, then I think hour lines alone will be all > that the time allows. > > Has it been determined exactly what tools and procedures the teams may use > to delineate the dial plate? (I vote for allowing only geometry tools, but > maybe dialing scales.) What size plate? Will each team do the same type of > dial and for the same latitude? (I'm thinking here about the problems the > judges face when measuring the dials.) > > So, questions beget questions. > > Mac > > > > > > > > > >>Gentlemen: >> >>A couple of key questions remain unanswered; need to reach closure on >>these issues before we can finalize the rules for the event. >> >>1. How *precise* do we expect the sundials to be? That is, the dials >>should measure time to the nearest [insert time unit here]. "Hour" seems >>too coarse (maybe not); "minute" seems too fine. >> >>That criterion established, then how *accurate* do we expect the dials to >>be? That is, how close to the true value should the dial read? For >>example, if it's exactly 12 noon L.A.T., then how close is good enough to >>receive full credit? >> >>2. What is the plan for connecting coaches & teams with NASS members? >> >>More later as I discover loose ends.... >> >>================================================================= >>Walter Sanford, Director Carl Sandburg Middle School >>Center for Sky Awareness 8428 Fort Hunt Road >>Sandburg Planetarium Alexandria, VA 22308 >>Fairfax County Public Schools Work: 703-799-6169 -6197 (fax) >>E-mail: wsanford@wsanford.com Home: 703-765-9392 >>AMS Project ATMOSPHERE Atmospheric Education Resource Agent & >>Water in the Earth System (WES) Resource Teacher >>SCSA, Geosystems, & Camp T-Equity - URL: http://www.wsanford.com/ >>================================================================= > >