I used what is being called a shadow sharpner in a noon mark made by drilling a hole in an aluminum plate located at the bottom of a south facing sky light. To determine the hole size I first tried various holes in cardboard. A starting point is that a pin hole camera would have an "f" number of 100. That is the hole diameter would be 1/100 the focal length (or distance to the dial). If the hole is smaller the image is dimmer, as the hole gets larger the sun's image gets more fuzzy. The image was an ellipse since it was projected on a wood floor. I had a stiff piece of paper with a number of ellipsis and a small hole in the center. Using a watch synchronized to WWV and the beeper turned on, I tracked the sun's image and stopped when the watch sounded. Then just drive in a brass tack.
Have Fun,
Brooke Clarke
Gordon Uber wrote:
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]
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