"Frank King" <[EMAIL PROTECTED]> wrote on Thursday, March 16, 2006 7:49 AM Subject: Rods versus Knife-Edges
> > > It seems to me it would work, but I can't see any > > > advantages over a cylindrical gnomon. > > > > I just tried the shadows of a knife edge and of a > > 3/4" diameter rod and they look about equally sharp. > > I should do a more careful experiment, but to first > > order, it appears to be true. > > Indeed so, at equal distances you get equal fuzz, but > this misses the main point... > > With a rod you can readily estimate the centre of > the shadow ignoring the (approximately equal) fuzz > on either side. With a knife edge you have to > estimate where in the fuzz is the true centre-line > of the shadow of the edge. This is error prone > and, to some extent, subjective. > > I say `spare the rod and spoil the sundial'. > > Frank King > Cambridge, U.K. > My original point: that the shadows of the edges of a polygon and of a cylinder are equally usable. Frank makes the different point that better than any edge is two edges close together: either the shadow of a narrow slit or of a thin gnomon. The human eye is much better at judging its centre than of judging a single edge. A shadow sharpener might help even things up, but it's an awkward solution. But narrow slits and thin gnomons have physical problems. There will certainly be places where only a fat gnomon can be used. And whether its edges are rounded or sharp makes no real difference. One might imagine it easier to make an accurate square section than a cylinder, but I doubt it. There may be aesthetic or design reasons why you want a square section, but I don't see a functional one. Chris Lusby Taylor Newbury, U.K. 51.4N 1.3W --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
