Hi Sarah (and Ken): you wrote:, >Does anyone have to hand the approximate distance over which a pinhole >gnomon is useful, please?
My pinhole experiments showed that this distance should not excede 24 inches (70 cm.), roughly. >so one must consider whether the thickness of the material that the >pinhole is contained in will obstruct the rays of light intended to pass >through it(especially when the sun is at a small angle from the wall), >requiring material to be removed from both faces around the hole. Yes, Tony's drawing shows this well. If the material is thick, then beveling the sunside edges of the hole would be indicated. It is probably easier to use a thin piece of metal sheeting instead to avoid this problem. As other have pointed out, when the sun is at low angles to the pinhole, the light passing through the hole produces an elipse. At very low angles, the light could completely disappear! I think pinholes work best for sundials like the equatorial heliochronometer described in Mayall that have a pinhole situated on a moveable alidade. This arrangement keeps the pinhole parallel to the light receiving face at all times. One option for Ken's dial might be to make his pinhole sheet rotateable on it's support rod. This would make the dial interactive. The user would have to rotate manually the pinhole sheet when taking a reading in order to keep the sheet perpendicular to the sun's rays. Personally, for the type of dial Ken is considering, I think that a more traditional spherical nodus would be more appropriate. Best John Carmichael Tucson Arizona >
