Thank you to Bill Gottesman for the shadow sharpener diagram for an annulus. I have been comparing a standard nodus (pinhole aperture) with an annulus on my SunVial(tm) sundial for some time now and find that the sun's image projection with annulus is much more readable on the translucent dial face. This means better results of time and date reading from the dial face both on marks and when interpolating between marks. (See http://celestialproducts.com/images/aboutsunvial.htm for a low res image of the annulus nodus in operation.)
I have been producing the annulus using microscope slide glass slip covers. These are extremely thin small squares of glass used to cover specimens/liquid samples. The thinness reduces the amount of refraction of light and require less machining to mount in the sundial. A small opaque dot is made on the glass with paint or ink. In production, I hope to be able to use a metallising process to create the opaque dot and outer area beyond the annulus ring. This will eliminate the apparent offset of the dot within the annulus ring currently caused by the nodus edge thickness and quite evident in the photo on the above reference web page. In my current sundial version, the sun is always at an inclined angle to the nodus and the wall of the cylinder is perpendicular to the nodus plane resulting in projected image that is usually some form of flattened (horizontally) or elongated (vertically) ellipse. Using the annulus for the nodus improves the human eye/brain's ability to select the center of symmetry for this shape. I am experimenting with other shapes in the annulus formation that may further enhance the ability of observers to determine this center of symmetry. e.g., it could be that a ring of wedge shapes might yield even better readability. Experimentation is ongoing with diameter/width of annulus ring and central dot to attain optimum size, contrast, and brightness. A further experiment for my particular sundial is to mount the nodus in a gimbaled housing to allow users to change the orientation to be either perpendicular to the sun or at such an angle to produce a close to circular projected image on the dial face. Larry Bohlayer All MAIL: Celestial Products, Inc. P.O. Box 801 Middleburg, VA 20118-0801 All shipping: Celestial Products, Inc. 20659 St. Louis Rd. Philomont, Virginia 20131 U.S.A. USA (Eastern Time Zone or -5 UT) EMAIL: [EMAIL PROTECTED] VOICE: 540-338-4040 FAX: 540-338-4042 Order Line: 800-235-3783 On-line Catalog: http://www.celestialproducts.com
