There's something I don't understand about a compound portable dials. As I understand it, a conventional horizontal dial can share its mounting plate with an analemmic dial to form a self-southing dial. The user simply turns the whole contraption until both dials read the same time. In this position the dials are aligned on their meridian, and showing the correct time.
At first, I thought this was simple. The two gnomons are oriented differently because the dials are of different fundamental types, so their shadows are cast in different directions. There would only be one way to align the device so that both shadows simultaneously point to the same time on their respective dial faces. Easy! But what about the case where the dial's own "NS" axis is facing the sun? Won't both shadows be pointing to the dial's North and thus both indicate noon - irrespective of the actual time of day. How do I distinguish this false noon from a real one? I'll hazard a guess, but would appreciate confirmation. If the dials can be made to agree on a time other than noon, it is that time. If they only agree on noon, it is noon. Secondly, I assume the underlying principle works because the two dials use different "fundamental" measures of solar position. One uses azimuth, one uses angle of rotation about the polar axis (Right Ascension?). Presumably, you could do the same trick using other combinations. In particular, any two different selections from: An altitude dial An azimuth dial An RA dial A dial using Hectemoros angle Is this so, and do such dials exist? Thanks, Steve
