Hi John, This looks like an interesting experiment but I don't have a photometer so I cannot help directly.
In "Sundials Australia", John Ward described a set of photometric experiments to measure the umbra and penumbra shadows. He used a long tube to focus photometer on a limited section of the sky, and a variety of gnomons to cast shadows. He then compared the experimental result to a simple mathematical model, a few points calculated to produce a histogram. Sorry but I do not have a copy of the book. I read a library copy while in Australia. Perhaps he published the results as a BSS paper. The math to generate a continuous function should be interesting. It is much more to it than the umbra/penumbra shadows. First there is the fairly simple area of the circular section vs position relationship as the gnomon crosses the sun. Then there is limb darkening to account for. Finally there are the light scattering and diffusion effects which determine the contrast between the full shadow and sunlit areas. No wonder that John Ward used a simple model! Roger Bailey Walking Shadow Designs N 51 W 115 -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Behalf Of John Carmichael Sent: August 25, 2002 10:29 AM To: Sundial List Subject: Need Photometer for Conference Hello Conference People I was wondering if any of you could bring a good photometer with you to the conference. On tour at Kitt Peak, I'd like to take lumens measurements of the full sun and the penumbral and umbral regions of the sundial's shadow edge. Fred and I are working on neat experiment we'd like to conduct with the group, but we need a photometer. John John L. Carmichael Jr. Sundial Sculptures 925 E. Foothills Dr. Tucson Arizona 85718 USA Tel: 520-696-1709 Email: [EMAIL PROTECTED] Website: <http://www.sundialsculptures.com> - -
