Hello Jim Refraction depends on the state of atmosphere and land/sea below it. Temperature, pressure, humidity, density, clarity, temperature of land/sea et.c. Generally the only way to know current refraction is to measure it for given direction and for given altutde. Refraction for average conditions is tabulated in almanachs. For sundialing purposes it is not that important because it is measured in minutes of arc, not degrees so the fuzzines of the shade is much bigger than error resulting from not taking refraction into account. John's question was about exact time of visible sunset/sunrise and then refraction is significant.
Slawek P.S. I like your explanation of "automatic" cited in signature. As an automatic control engineer by education I remember another one. If someone's mother was a bitch then the guy was automatically son of a bitch. At 09:56 AM 1/20/99 -0700, you wrote: >"Slawomir K. Grzechnik" <[EMAIL PROTECTED]> wrote: >> Hi John >> >> Short answers to some of your questions. >> >> Refraction does not change the azimuth, it significantly changes the >> altitude. > >[...] > >> Slawek > >Yes, the azimuth will be preserved, but the change in altitude will >change the position of the shadow cast by the gnomon, will it not? I >believe this to be John's concern, and him to be correct in his >concern. However, I don't know how to evaluate the magnitude of the >error, which is the heart of his question. > >Jim > ------------------- ---------------------- -------------------- >| Jim Cobb | 540 Arapeen Dr. #100 | [EMAIL PROTECTED] | >| Parametric | Salt Lake City, UT | (801)-588-4632 | >| Technology Corp. | 84108-1202 | Fax (801)-588-4650 | > ------------------- ---------------------- -------------------- >"Automatic" simply means that you can't repair it yourself. -- Mary H. Waldrip > > Slawek Grzechnik 32 57.4'N 117 08.8'W http://home.san.rr.com/slawek
