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