RE: the care and feeding of analemmas

[Larry Bohlayer]

Dave,   Assuming that the projection is from a fixed nodus, a flat surface (plane) as the destination target of the projected sun image will necessitate the beam having an incidence angle that varies over the period from solstice to solstice. The greater the angle the larger the spread of the sun’s image. The greater the spread, the less intense is the light from each point in the spot image; hence less contrast with the shaded area on the rest of the dial face adding to the difficulty in reading the sundial. A spherical bowl shaped section could be utilized to provide a constant distance of the dial face from the nodus. Keep in mind that unless you have a nodus that has articulation to allow the user to point the nodus (assumed to be a circular hole) at the sun (nodus plane aligned perpendicular to the sun) the projected image will have an elliptical cross section.   See http://www.celestialproducts.com/images/aboutsunvial.htm  for an example of the sun’s image projected onto a cylindrical wall.   I am sure others on this list will be happy to point you to examples both planar dial faces and bowl dial faces used in various sundial designs.   Larry Bohlayer 39N 78W     -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Aten Heliochronometers Sent: Tuesday, December 07, 2004 8:53 PM To: sundial@rrz.uni-koeln.de Subject: the care and feeding of analemmas   Hello group, I am new here, but have been reading the posts for a few days. I have a question for the group, I am designing a new heliochronometer (afterwards to be referred as a "helio" for brevity) and I am striving for a bit more accuracy.  Roughly, I intend to try and cast a point of light on a surface that contains an analemma. Should I create the analemma on a flat or bowl shaped surface?  In other words would it be more accurate to project the analemma onto a plane or a section of a concave sphere? I am mainly concerned with the 47deg. motion of the sun over the course of the year.   TIA for any thoughts Dave G.   PS sorry if this is a double post having trouble getting the list to work