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.
My rule of thumb is that the angular resolution of a pinhole is one radian (57 degrees) divided by the diameter of the pinhole in wavelengths of light. Since the wavelength of green light is about 0.0005 mm, the resolution for a 2 mm pinhole would be about 0.0005/2.0 = 1/4000 radian or about 0.8 arcminute or 3 seconds of time. This seems consistent with your observations. To achieve this resolution the pinhole would have to be greater than 4000 * 2 mm = 8 meters from the image. Using a slit perpendicular to the direction of motion of the image would increase the brightness of the image without decreasing the time resolution. A lens, such as one from a pair of reading glasses, would provide still greater brightness. As a rough approximation, the brightness (technically the illuminance) of the sun's image relative to that of a sunlit surface is proportional to the area of the pinhole divided by the area of the sun's image. Since the angular diameter of the sun is about 1/100 of a radian, the brightnesses are about equal when the pinhole diameter is 1/100 of the distance from the pinhole to the image. Gordon At 07:52 PM 5/3/99 , Roger Bailey wrote: >By observing the shape of the image of the sun, the middle and the two >edges of the penumbra were easily determined to a precision better than one >inch over the width of the penumbra (about 2 ft). This gives a precision >equivalent to about 5 seconds per day for this size of shadow. The bisected >hemispherical image would be the appropriate position for the sun as a >point source with no semi-diameter correction required. Fixing the hole and >screen and timing the movement of the shadow would give even more precise >results for these events (but if I had a watch, why would I need a sundial). Gordon Uber [EMAIL PROTECTED] Reynen & Uber Web Design http://www.ubr.com/rey&ubr/ Webmaster: Clocks and Time http://www.ubr.com/clocks/
