>Discussions here and experiments of my own have established that "shadow >sharpener" techniques allow a shadow position to be read with accuracy on >the order of one second of time. This led me to look for a configuration >that allows a continuous readout with this type of accuracy, not just the >determination of one point in time (e.g., noon). Furthermore, in the >"hands-off" spirit of sundials, I wanted to read the time by just looking, >without having to fiddle with any instruments. Given a gnomon, ideally >subtending an angle a bit less than that of the sun, a properly placed >pinhole allows a very accurate determination of the time when the center of >the shadow passes over the pinhole. Obviously, many such pinholes could be >used, say one for each second of each minute. More elegant is to place this >series of pinholes so close together that they overlap, resulting in a slit. >The slit projects the sun onto a line of well-defined width. The shadow of >the gnomon falls on a short section of this slit and blocks the sunlight. >The result is a sharp-edged band of light intersected obliquely by a >sharp-edged shadow. The position of the intersection moves along the band of >light at a speed which allows, with proper set-up, resolution on the order >of 1 second of time. With an appropriate scale, this can function as an >indicator of seconds. To keep the dial compact, after a suitable period of >time, e.g., 5 minutes, the shadow could pass onto a parallel slit that >starts the process over again. This dial would be complex to build, and >adjusting for the equation of time to 1 second accuracy would be an ordeal, >but I think it must work and would be an intriguing project. I experimented >a little with the principle using my clipboard on my window sill, but it is >now too late in the afternoon for the sun to shine through my window. Looks >like I'll have to go back to work. > >--Art Carlson
Art I have been following the resent discussion with regard to the desire to make a sundial that can read in seconds. I would like to try to tie together the past three threads: Location of analemmas, precision and accuracy, and shadow sharpening techniques to read to within seconds. It may be that one can design a dial to be read to a precision of seconds, but the accuracy issue has still to be resolved. To recapitulate my earlier perspective. Analemic time lines for sundials may be thought of as projections of the analemmas that one may draw on the surface of the earth. The analemma may be thought of as a continuous curve through a set of points. Those points are determined by the geographical position of the sun at a given time of day for every day of the year. A curve through these 365 points is then projected onto a surface to get analemic time lines, a horizontal surface for a horizontal dial, a vertical surface for a vertical dial, etc. The point that I want to make is that the 365 geographical positions of the sun are not the same 365 points from year to year and they do not line on the same continuous curves from year to year. Thus the projected time line is not the same from year to year. What one does in practice is to take some sort of average to determine a useful time line for a usable sundial. If one is building a highly precise and accurate dial then the time line needs to be adjusted in some way to correct for the variation from year to year of the location of the set of 365 points that define the time line. Any goal of reading a sundial with accuracy of seconds has to allow for this adjustment. Without the adjustment one can perhaps achieve the desired precision but not the desired accuracy. Hope I have not cast a shadow upon desired projects. If I have addressed an issue that did not need addressing then I apologize. Dan Wenger Daniel Lee Wenger Santa Cruz, CA [EMAIL PROTECTED] http://wengersundial.com http://wengersundial.com/wengerfamily
