>The hour markers on an analammatic dial are not lines from the centre of the >ellipse they are positions along the periphery of the ellipse and are >associated with the point of the shadow arising from the top of the vertical >gnomon. So I'm afraid you_will_need to specify the height of the person.
If I'm not mistaken, a traditional analemmatic dial only requires a minimum height of a gnomon. While the hour markers are positioned along an ellipse, these are not necessarily associated with the top of the gnomon, but with the portion of the shadow of the gnomon that passes "through" the ellipse. For example, consider where the shadow of the head of a typical person would lie at high noon--not so far away. However, near sunset the shadow would be extremely long--the end would be much further away than the distance to that hour marker on the ellipse (it would be "infinite" at sunset), so the shadow passes through the ellipse. The minimum height of the gnomon is simply that which will produce a shadow reaching the ellipse at at all times, and I _think_ that would be defined by the shadow at noon. The ellipse is used for the hour markers only because it allows you to use the same hour markers for any of the various positions you stand on for the various days of the year. For any such position, though, the end of the shadow will trace a hyperbola tending toward infinity at the extremes, and in general the shadow will pass through the ellipse (more so as the hours get further from noon) and it is the intersection that gives the time. However, I am open to other opinions on this. Ron Doerfler
