shadow velocity & shadow tracing
Hi once more: Just read all your e-mails on drawing hour lines by shadow tracing. Chris Lusby Taylor pointed out that it would be difficult to trace the whole hour line because the shadow is constantly moving; so he suggested drawing a point on the hour line first and then visually guestimating the entire hour line's position on the ground. This seems like it might cause small errors in the hour lines because it is based on a guesstimate and not an actual shadow tracing. I have an idea, though, that might facilitate drawing an hour line from the face edge to the center: How about first marking only the hour points as Chris suggests. Then, at night, fasten a high power, tightly focused flashlight to a ladder to simulate the sun. Position the light so that the style's shadow falls on the hour point that was marked earlier using the sun. The wiggly hour line could then be traced at one's convenience using an unmoving "sun". Whadaya think? John
re: shadow velocity & shadow tracing
John, If you are not to concerned with getting the hourline traced across dips and hills on the ground, use a long straightedge (yardstick, meterstick, 8' - 16' extruded strip of aluminum flashing, etc.) to mark the line. You can maintain it on the shadow edge until the top of the hour and then mark the line. Larry Bohlayer All MAIL: Celestial Products, Inc. P.O. Box 801 Middleburg, VA 20118-0801 All shipping: Celestial Products, Inc. 20659 St. Louis Rd. Philomont, Virginia 20131 U.S.A. USA (Eastern Time Zone or -5 UT) EMAIL: [EMAIL PROTECTED] VOICE: 540-338-4040 FAX: 540-338-4042 Order Line: 800-235-3783 On-line Catalog: http://www.celestialproducts.com
Re: shadow velocity & shadow tracing
Good idea John. But when you say "tightly focused" I take it you mean a flashlight with a wide beam emanating from a small aperture. This is what gives a sharp edge to the shadow. Conversely, a flashlight with a narrow beam from a large reflector would be poor. The poor man's laser trigon, indeed. Chris John Carmichael wrote: > Hi once more: > > Just read all your e-mails on drawing hour lines by shadow tracing. Chris > Lusby Taylor pointed out that it would be difficult to trace the whole hour > line because the shadow is constantly moving; so he suggested drawing a > point on the hour line first and then visually guestimating the entire hour > line's position on the ground. This seems like it might cause small errors > in the hour lines because it is based on a guesstimate and not an actual > shadow tracing. > > I have an idea, though, that might facilitate drawing an hour line from the > face edge to the center: How about first marking only the hour points as > Chris suggests. Then, at night, fasten a high power, tightly focused > flashlight to a ladder to simulate the sun. Position the light so that the > style's shadow falls on the hour point that was marked earlier using the > sun. The wiggly hour line could then be traced at one's convenience using > an unmoving "sun". > > Whadaya think? > > John
Re: shadow velocity & shadow tracing
Hi Chris: You're right! I realized as soon as I sent my message that a small diameter floodlight with a wide beam would be better than a tight beam, because it must illuminate the entire style and not just a portion of it. Thanks for confirming, John >Good idea John. But when you say "tightly focused" I take it you mean a >flashlight with a wide beam emanating from a small aperture. This is what gives >a >sharp edge to the shadow. Conversely, a flashlight with a narrow beam >from a large reflector would be poor. >The poor man's laser trigon, indeed. > >Chris > >John Carmichael wrote: > >> Hi once more: >> >> Just read all your e-mails on drawing hour lines by shadow tracing. Chris >> Lusby Taylor pointed out that it would be difficult to trace the whole hour >> line because the shadow is constantly moving; so he suggested drawing a >> point on the hour line first and then visually guestimating the entire hour >> line's position on the ground. This seems like it might cause small errors >> in the hour lines because it is based on a guesstimate and not an actual >> shadow tracing. >> >> I have an idea, though, that might facilitate drawing an hour line from the >> face edge to the center: How about first marking only the hour points as >> Chris suggests. Then, at night, fasten a high power, tightly focused >> flashlight to a ladder to simulate the sun. Position the light so that the >> style's shadow falls on the hour point that was marked earlier using the >> sun. The wiggly hour line could then be traced at one's convenience using >> an unmoving "sun". >> >> Whadaya think? >> >> John > >
Re: shadow velocity & shadow tracing
John Carmichael wrote (replying to Chris Lusby Taylor): >You're right! I realized as soon as I sent my message that a small diameter >floodlight with a wide beam would be better than a tight beam, because it >must illuminate the entire style and not just a portion of it. > A drawback with the "floodlight" scheme is that a very tall ladder will be needed unless you are only going to make use of the small bit of the gnomon near the origin. I know to my cost that big gnomons rarely have the small-scale accuracy required! Since what you are actually trying to do is to reproduce the method of a trigon (as Chris pointed out), why not use a real laser trigon. Bob Terwilliger has one on his webpages at http://www.shadow.net/~bobt/trigon/trigon.htm (based on a transit mechanism), and I published a cheap-and-cheerful small version in the BSS Bulletin October 99. Once the trigon is fixed to the style, and the hour angle set using the incorporated auxilliary dial, all you have to do is swing the laser on its declination axis and the spot will follow the humps and bumps on the ground precisely. If done at night, the laser spot is easy to see. If done in bright sunshine, you might need a red specular-reflector target and some red sunglasses to see the laser spot clearly. It's a shame you are so far away, as I built a "big" version of my trigon for the New Hall Oxford dial, but it didn't get fully used and I'm looking for another application!. By indexing the auxillary dial with 0.25 degree intervals, it gives 1 minute of time resolution. Regards, John Dr J R Davis Flowton, UK 52.08N, 1.043E email: [EMAIL PROTECTED]
Re: shadow velocity & shadow tracing
On Thu, 10 Feb 2000, John Carmichael wrote: > Hi Chris: > > You're right! I realized as soon as I sent my message that a small diameter > floodlight with a wide beam would be better than a tight beam, because it > must illuminate the entire style and not just a portion of it. > Also, the smaller and more point-like the source, the better. Try a Mini-MagLite flashlight, with the focussing reflector head entirely unscrewed. This leaves you with a tiny bare lamp, almost painful to look at directly. The total light output is fairly low, but should work on a dark night! Dave
Re: shadow velocity & shadow tracing
On Thu, 10 Feb 2000, John Davis wrote: > Since what you are actually trying to do is to reproduce the method > of a trigon (as Chris pointed out), why not use a real laser trigon. > Bob Terwilliger has one on his webpages at > http://www.shadow.net/~bobt/trigon/trigon.htm (based on a transit > mechanism), and I published a cheap-and-cheerful small version in the > BSS Bulletin October 99. John: Are you at liberty to re-print that design for us, say to put up on the archive sites? Dave Bell
