On Thu, 13 Jun 2002, John Carmichael wrote: > This is embarrassing, but I've just repeated the pinhole experiment with the > setting sun which is about 15 degrees high and I must conclude that the > results of my earlier test that I did at noon were flawed. At noon I had > tried to hold the pinhole and projection screen at odd angles to simulate a > setting sun while holding the whole thing in the penumbra (Not to easy a > thing to do). I must have held them wrong and drawn the wrong conclusion. > Besides, Mac is one of those sundial people who I most respect and I just > couldn't believe he could make a mistake. The solution to convert an > elliptical image into a circle by holding the pinhole parallel to the ground > does not work. > > This time, an hour before sunset, my projection screen was horizontal and I > held my 2mm pinhole 1 meter away. I used full sunlight, no penumbra. Holding > it perpendicular to the sun produced an ellipse, and tilting it slowly so > that it is parallel to the ground causes a dimming of the image, but the > images remains the same shape and does not become a circle. > > Maybe for these low sun markings, I could devise some sort of tilted little > screen that is positioned above the timeline that will be perpendicular to > the sun rays so that it would have a circular image. I'll have to think > about that. > > John
A tilted card *above* the mark probably won't work, as I'm sure you've decided by now. You could use a tilted card, whose edge is in contact with the ground. Move it until the image of the Sun and shadow edge are split over the grounded edge, 50% on the card, 50% (elongated) on the pavement. At least you get the good imaging surface, and can tilt it to normal to the Sun/shadow line. Does that make sense, without a drawing? Dave 37.29N 121.97W -
