Hi, I think Dave Bell is right. To avoid problems with shadows that are to short, give this instruction: "Stand on the right date point, go exactly in the direction of your shadow until your shadow meets the ellips with hour points."
Willy Leenders Dave Bell wrote: > Hi, John! > > > I think I'll just use hour, 1/2 hr and 15 min lines. > > > > Also, in order to be able to tell time with short shadows around noon in the > > summer and to accomodate short people, it will be very important to mark the > > hour lines as close as possible to the gnomon foot (where the people stand). > > Correct? > > I may be wrong, and it would be wise to verify this with Fer or someone > else far more knowledgeable about analemmatic dials, but I see a problem > with your vision of the dial: > > Analemmatic dials, as we have been discussing them, do not have hour > *lines*. They have hour *points* only, on the periphery of the ellipse. It > is the "responsibility" of the human gnomon to be tall enough to cast a > shadow all the way to the points. This sets the scale of the dial > somewhat, as a truly monumental dial would require Paul Bunyon (or at > least Wilt Chamberlain) as a gnomon! You can't beat it by - > > * Drawing hour lines, from the hour points to the gnomon foot, because the > foot moves with the time of year. > > * Drawing several "nested" dials at different scales (major axis length), > because the entire dial, including the date line/scale, is proportional to > the major axis. While you could draw lines connecting the corresponding > hour points, the central date lines would be of different lengths, so the > "gnomon" wouldn't have a unique point on which to stand. > > This also messes up your thought of painting footprints for different > height users, because all users, regardless of height, stand on the same > (date) point. Some just have to extend their arms over their heads, to get > a long enough shadow. > > To some extent, the dial design corrects for the "shortcomings" of the > users. Note that the date points for Summer are much closer to the North > rim of the dial, and the hour points for Noon are much closer to the date > line than 0600 and 1800. These automatically correct for the varying > shadow lengths per time of day and time of year... > > Does this make sense? > > Dave Bell > N37.29W121.97 Content-Type: text/x-vcard; charset=us-ascii; name="willy.leenders.vcf" Content-Transfer-Encoding: 7bit Content-Description: Card for Willy Leenders Content-Disposition: attachment; filename="willy.leenders.vcf" Attachment converted: Macintosh HD:willy.leenders.vcf (TEXT/ttxt) (0001930B)
