It's maybe worth noting that this could be a problem even for a sundial with a gnomon parallel to the Earth's axis of rotation: sundials based on a stereographic projection have curved hour lines (e.g. http://www.de-zonnewijzerkring.nl/art-arch/eng-home-art-09-06.htm). On these sundials, even if the solar hour angle remains constant, a change in declination will produce a change in apparent time. It's only on sundials based on the gnomonic projection (admittedly, almost all sundials) that hour lines are straight--a consequence of the fact that the gnomonic projection projects great circles to straight lines.
Also, for, say, a very large horizontal dial, the sun's change in declination over the course of a day will make the sun depart perceptibly from a "date" line based on some particular declination. Will Vaughan
--------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
