Arthur Carlson wrote: > Chris Lusby Taylor <[EMAIL PROTECTED]> writes: > > > Another approach would be to have a ring for each value of the > > EoT. That turns the figure-of-eight hour lines into a > > figure-of-infinity date line. Having found today's date one follows > > the ring it's on to read the mean time directly from the shadow. > > This should be easy to use. The hour lines could be almost straight. > > This would work with an axial gnomon, where the direction of the > shadow does not depend directly on the declination. I haven't done > the math(s) in detail, but I don't think it would work with a vertical > gnomon because the azimuth of the sun depends not only on the hour > angle (Equation of Time) but also on the declination. In other words, > the direction of the shadow at a particular time depends on the date > for two reasons (EoT and declination) with different dependences on > the date. Consequently, it would be a great exception if two dates > could share the same time markers. >
You're right, Art. I wasn't thinking of the vertical gnomon with which, as I now recall, we started. It only works, as you say, with an axial (polar) gnomon. Chris Lusby Taylor, Newbury, England 51.3N 1.4W
