Gordon, >Roger, thank you for your post. The Shadow Sharpener being a pinhole camera, >why not replace the gnomon with a pinhole? One then could center a circle on >the image and determine the time from its position.
Some years ago, when thinking about heliochronometers, I realized that the sun- sighting element of such a device could be a pinhole compound telescope. (I was avoiding the use of refractive and/or reflective "glass" elements, as part of the game.) This would be placed in an equatorial mounting with graduated hour-circle and declination sector. The symmetry of this sighting system removes a number of possible sources of error. The whole rig could be much more compact than the "giant" concept that triggered this thread. I'll attach a GIF image of such a scope. <pin-htel.gif> --- 1627 bytes. In the sketch: O, the objective, is a plate with central pinhole sized to diametrically subtend a couple of arc minutes at distance Lo, the primary "focal length." T is a translucent target plate, (frosted glass, even a taut sheet of fairly thin white paper,) with concentric circles drawn on it that span the approximately 1/4 degree radius of the sun's disk image, Is. With the limb viewed against the drawn rings, eccentricity of its position would be manifested by width differences in the narrow gaps between limb and circles. The eyepiece, or ocular, at e.p. is also a pinhole. This permits viewing Is at a distance Lep, that is much shorter than normal accomodation for most eyes. Even with my 7th-decade presbyopia, this can be 2 to 3 inches for an aperture of a little less than a mm. A builder had best experiment by viewing a printed page through various diameter holes, to find the optimal aperture to suit him. The 'scope could be modified to use the primary image cast on an opaque white target. This would be viewed from an eyepiece looking into the tube at an angle from in front of the target. E.g. a slit objective-aperture projected image on a 22.5° tilted target could be viewed from 45° off the primary tube's axis. The target would be a series of parallel lines with separations to cover a range +/- the 0.5 degrees of image width. The plane of the slit, target-lines and tilts would all be in a common radius-plane of the sun's arc. The sun's image from the slit would be a bright bar. (These modifications are what I would have tried first, had I gotten around to making a prototype.) Since the whole thing would be enclosed in a tube, light dilution would have little effect. A 60 inch long tube with 2 to 3 inch eyepiece length would be equivalent to a style-to-dial distance of 100 to 150 feet. (A primary image about 0.5 inch across.) The use of the pinholes/slit and diffuse image plane would minimize the danger in viewing the too bright sun. Even a somewhat fuzzy image can, if symmetrical, be placed very precisely on a symmetrical target. If anyone should try this, I hope that they'll let me know how it works out for them. Sciametrically, Bill Maddux
