Hello Jim,
William Walton's "Pinholes and Shadow Sharpeners" and Gianni
Ferrari's "The Shadow Sharpener," each in Compendium 9-4, December
2002, deal with this question.
107:1 seems to be the ratio between the projection distance from a
pinhole to a screen and the diameter of the Sun's image. That is, the
diameter of the Sun's image will be about 1/107th of the distance
from the projecting pinhole.
Since a pinhole image is very dim, for brighter images, the ratio
between the projection distance and the hole diameter is recommended
as ranging from about 200:1 to 400:1.
Hope this helps.
Mac
Hi John,
I have also considered using this type of arrangement (a "dot" on a
clear pane) for several projects in the past. I wonder just how big
the dot would have to be? I assume penumbral effects would be at
play, and the distance to the shadow receiving surface would
probably be a factor in determining the best size of dot to use?
Perhaps there is a good ratio to use for dot/disc size vs. distance
to the shadow field? Perhaps some of the list members know the
answer to this...
Best,
Jim
We're also discussing what I call "antiaperture" gnomon sundials. These are
neat. Imagine a large clear pane of clear or light colored glass on a wall.
And on this glass is a small very dark dot. Instead of a hole in a wall
casting a beam of light, the dark spot casts a shadow onto the sundial face.
(John Davis, do you have a term for these?)
Does anybody have any opinions on the practical differences of using
aperture gnomons versus antiaperture gnomons?
thanks
John
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- Re: Sundial Carpet Mac Oglesby
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