On Sat, 6 Sep 2003, John Carmichael wrote: > 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? > > John
My goodness! A timepiece that uses a shadow as an indicator... What will they think of next? Seriously, it should work fine. For yet another variant, see E. Roebroeck's projection sundial in the Netherland's, Dial 8 in the Stained Glass Sundials photopage, linked from http://tinyurl.com/frf4 Very similar physics sets limitations on the size and resolution of an anti-aperture as on a true aperture. If I remember correctly, the ideal diameter of a hole is on the order of 1/107th the distance to the target. One advantage might be the relative simplicity of using one of the shadow sharpening ring apertures discussed last year. The "spot" placed on the clear window pane could easily consist of a dark ring, or even a series of rings, of decreasing width as the radius increases. Think of a Fresnel lens, implemented as alternating dark and clear rings. Dave -
