Hi John Short answers to some of your questions.
Refraction does not change the azimuth, it significantly changes the altitude. Time lap between visible and astronomical sunrise (sunset) depends both on latitude (heavily, see what is going on beyond polar circles) and on season. If you want to be really accurate then for the given day you calculate time of astronomical sunset (altitude 0, center of solar disk is on astronomical horizon). Then you have to know your height of eye above the horizon to calculate dip of the horizon (pretty tricky when not at sea) and know Sun's radius on the given day. Knowing those you may calculate exactly the time when the Sun is below the visible horizon and its upper limb just touches it. Slawek At 06:40 PM 1/19/99 -0700, you wrote: >Hello dialists: > >Hoping that I might get your help with one last question that needs >answering before the new edition of my Sundial Owner's Manual goes to print. >In the chapter on setting a (horizontal) sundial, I recomend that a dial >should be oriented north not by a compass and not by polaris but by the >"time method". I say that this method is the most accurate but will only >work on those dials that are properly designed, constructed and leveled. >Correcting for the Equation of Time and longitude, using a reliable time >source, the sundial should automatically point north if it reads the right time. > >Then I say that you can set a horizontal dial with greater precision in the >early morning or late afternoon because the space between the minute lines >is larger at those times > (This is when the shadow points due east or due west). It is easier to >read a dial at those times because the lines aren't as bunched up as they >are at apparent noon. > >Here's my question: Is a sundial more accurate when the sun is setting or >rising? Is the benefit of larger spacings between the minute hands wiped >out by atmospheric effects (refraction and the mirage effect.)? By the way, >does anybody know by how many minutes an actual sunset precedes the apparent >sunset?Does this amount vary with latitude and seasons? > >Sorry, one question turned into four! > >Thanks for your help > >John Carmichael Slawek Grzechnik 32 57.4'N 117 08.8'W http://home.san.rr.com/slawek
