Quoted from a Google search of the encyclopedia Wikipedia:
Plekhnatons
The ancient Greeks used a type of sundial called a plekhnaton. The gnomon was a rod or pole upright in a horizontal face or half-spherical face. The shadow of the tip of the rod sweeps out hyperbolic curves on a flat face, or great-circles on a spherical face. The advantage of a plekhnaton is that it can be marked to tell the exact time for all times of year.
A fun version of the plekhnaton is to lay out the hour lines on concrete, and then let the user stand in a square marked with the month. The month squares are arranged to correct the sundial for the time of year. The user's head then forms the gnomon of the plekhnaton. If the sundial is molded into the concrete, it is almost perfectly immune to vandalism, as well as truly fun and reasonably accurate.
Quite a few mistakes in there, now, aren't there? For starters, lines cast in concrete can only be exact, in the sense of accounting for the equation of time, for half the year. Second, there is confusion between using a nodus (a point shadow-caster) and using a vertical gnomon (a line shadow-caster). Only the latter can/needs to be corrected for the equation of time by moving the gnomon (analemmic dial). Finally, using someone's head as a nodus is a bad idea since we are all different heights. With an analemmic dial it doesn't matter because it's not the tip of the shadow but the direction of the shadow that matters.
--Art Carlson
