Tony, Mac's advice is best. Got to http://194.230.53.250 , click on 'Search', select Sundial(English) from the list of lists, and search for 'shadow sharpener' 'a peculiar sharpener', 'penumbral head swelling' (yes, really) and 'sundial with a second hand'. If that fails, I do have copies of the most relevant messages. As I recall, there were at least two distinct ideas presented. One was from Charles ([EMAIL PROTECTED]):
"The shadow sharpener is simply a stiff sheet of opaque material with a clean edged round hole in the middle. I made one from the thin cardboard backing of a pad of paper, with a hole about half a barleycorn in diameter. To use, hold the sharpener a short distance (1 to 3 feet) away from the fuzzy zone, with one side facing the sun. An image of the sun will appear on the surface the shadow is on. Move the sharpener around until the image of the sun is bisected by the edge of the roof of the building, (or whatever comprises the gnomon). I believe then that the image of the gnomon will be in the exact place it would be if the sun was a point source." This, then, is a hand-held device, though some messages discussed how to make a fixed version with a diagonal slit. The other was from John Carmichael ([EMAIL PROTECTED]): "The design which worked the best was a 1/8 inch spherical bead, suspended by thin brass crosswires, in the exact center of a 1/4 inch round hole. (The style was about 24 inches from the analemma)." This second idea might be very much up Mark's street, if he is to achieve the accuracy of reading needed to do justice to the accuracy of marking of his Meridiana dial. I presume the same technique could be applied both to an aperture and to the mirror used by a reflected spot dial. Regards to you all Chris Lusby Taylor 51.3N 1.4W Tony Moss wrote: > Fellow Shadow Watchers, > One presentation at the recent BSS Conference > referred to a series of planned dials using sunrays passing through > circular holes onto walls/ceilings. This brought to mind recent postings > which mentioned placing a spherical 'bead' in the aperture so that the > sunlight was passing through an annulus to yield a light spot with a > sharp central shadow - if I remember it correctly. > > 1. Does anyone have these postings conveniently saved or can recall > their approximate date? > > 2. I also seem to remember noting that all postings to this mailing list > are archived somewhere so given the date and archive address my > correspondent could look them up for himself. > > Thanks in anticipation > > Tony Moss. Content-Type: text/x-vcard; charset=us-ascii; name="clusby.vcf" Content-Transfer-Encoding: 7bit Content-Description: Card for Chris Lusby Taylor Content-Disposition: attachment; filename="clusby.vcf" Attachment converted: Macintosh HD:clusby.vcf (TEXT/ttxt) (000123BA)
