Malcolm McClure responded to M. Jacquelin Hardy's questions about building an anlemmatic dial:
>I am still thinking about this one, but hope the following suggestions will >help. (Corrections to logic gratefully received) > I have seen a photo of a wall mounted analemmatic sundial at Greenwich. It >has a fixed perpendicular rod about a meter long at the top, with a >sun-shaped cutout piece of metal welded on the end. This disc has a central >hole about a centimeter in diameter, which acts as the style. >The analemma is painted on the south-facing wall beneath, such that the top The logic is fine, as far as I could tell, Malcolm, but I think you're describing a different type of dial. The vertical dial you outlined seems to be equivalent to a "conventional" dial, with the hour lines drawn as analemmas (..ae?), to correct for declination and EoTime. The one Hardy is asking about is quite unconventional, compared to a horizontal dial, or it's vertical cousins. This type of dial, if you look down on it from above, is layed out as roughly half of an ellipse, with the major axis running East-West, and the half-minor axis pointing North (in the N hemisphere!). The gnomon is not parallel to the Earth's axis, but is a vertical pole placed on the N-S center line, and moved N and S a small amount each day. The "pointer" is the shadow of the top of the gnomon, and falls on the elliptical perimeter to mark the hours. In large garden or monumental versions, the gnomon is often designed to be the user, who stands on date markers. I don't have any references here at work, but I believe Waugh did a good workup on this design. I'll see what I can find, this evening. Dave [EMAIL PROTECTED] >[EMAIL PROTECTED] (M. Jacquelin Hardy) wrote >I am looking for info. on how to build an analemmatic sundial ( we call them >"cadran analemmatique" in french). There are a few of these in Europe, >(near the church of Brou in Bourg-en-Bresse, France) is one of them. > >The dial is horizontal and the gnomon is vertical and moved along the >North-South axis during the year. This movement is my problem: I read in a >book >that d( movement of the gnomon) is equal to tg (Decl. of sun)/cos (lat.) > >For example, on June 21st in lat. 45N , the gnomom should be moved >tg 23.45/cos 45 , ie. approx. .42/.7 or .6 times diam./2 along then N-S axis >to give the time correctly. It doesn't seem to work right according to my >calculations. > >Please help!
