Re: Sundial Carpet

[Mac Oglesby]

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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