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
