[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! [ 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 of the figure is illuminated by the beam through the hole when the sun's declination is minimum at winter solstice and the bottom of the figure is illuminated when the sun's declination is a maximum at the summer solstice. Intermediate points are calculated taking account of the beam-length and the equation of time.(Get sun's meridian passage from a nautical almanac for dates say 5 days apart and make a table. - The vertical is crossed on 20th April and 1st October). Careful allowance must be made for the site time difference from your time-zone origin. The beamlength increases with the cos of the declination but the offset from the vertical is the beamlength * tan of the equation of time offset allowing one degree per four minutes. (In my case, 7.1 cms to the left on the 15th February, 2.7 cms to the right on 15th May etc.) I have made an Excel spread-sheet that tabulates the shape of this figure for walls that are _not_ south-facing, as the maths gets a little complicated. Malcolm McClure
That that is,is.-WS-Twelfth Night. This is Malcolm McClure
