[EMAIL PROTECTED] (Mr. D. Hunt) writes: > In relation to the recent question/replies, regarding detecting/correcting > 'errors' in the setting of sundials - is there any feasible way of varying > the layout of an "Analemmatic" dial, to cope with it being on a GRADIENT ? > > My own thinking is that this is just NOT possible, if the dial has to tell > 'correct' time (disregarding EOT effects) at all times of DAY, plus at all > seasons of the YEAR - but will welcome any comments/confirmation, on this.
The gnomon, whether vertical or not, together with the direction to the sun, defines a plane. The intersection of this shadow plane with the ground plane, whether horizontal or not, defines a line. If you think of the celestial sphere as being a finite size and centered on the base of the gnomon, then the position of the sun projected along the direction of the gnomon onto the ground plane will lie on the shadow line. The orbit of the sun during the course of a day is a circle, generally not centered on the base of the gnomon. The projection of the orbit on any day will be an ellipse, though the center of the ellipse will move from day to day. An analemmic sundial is designed by rescaling all the ellipses to the same size, then translating them to lie on top of each other, which implies that the gnomon must also be translated to a particular position for the projection to be accurate on that date. The upshot is, an analemmic sundial properly designed for sloping ground will be just as accurate as one on the level.