John, If the edges of the gnomon / style are straight and parallel to the earth's axis, the shadow of the gnomon will not depend on the sun's declination, so it doesn't matter what day you mark the hour lines, however irregular the ground might be. Drawing an hour line could take several minutes, during which time the sun has moved on. I would suggest that you mark just one point exactly on the hour (corrected for the Equation of Time), then get an assistant to stand on the sun's side of the gnomon in a position where that point is just visible beyond the gnomon. They can direct the line drawing operation to ensure that, from where their eye is, the entire line is just visible beyond the gnomon.
Hope this helps Chris Lusby Taylor Newbury, England. 51.3N 1.4W John Carmichael wrote: > Hello dialists: > > Let's say you want to build a large sundial using the ground as the dial > face. The ground is somewhat irregular and not quite horizontal. You > decide to draw the hour lines, not by calculation, but by using the > technique of building the gnomon (style) first and then marking the position > of the shadow on the ground at selected time intervals using clock time and > correcting for EOT and longitiude. > > You draw the hour lines from the edge of the dial face to the dial center, > tracing the shadow. Since the ground is irregular and not flat, you notice > that the shadow line is not straight, but irregular also, depending on the > terrain. This produces hour lines that are not straight. > > Because the sun's declination changes during the year, changing the angle > that the sun strikes the style, will this technique produce the same shape > hour lines if it is done at any time of the year or is the declination > irrelevant? Will this technique produce the same shape hour lines at any > time of the year? I think it should. but I'm not sure. > > John Carmichael > Tucson, Arizona
