Steve Lelievre wrote: > 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
Right on both counts, Steve: - the conventional horizontal+analemmatic pair do indeed both show 'noon' when their N-S axis faces the sun. If it isn't noon they also agree at two other orientations. You cannot distinguish between before- and after-noon. For instance, at 10:45 the dials agree when oriented to 10:45, 12:00 or 1:15. So it must be 10:45 or 1:15. - any two dials with styles that are not parallel can be used. My own favourite uses the Foster-Lambert projection dials that have equiangular circular dials and gnomons inclined to the north at (90 + latitude)/2 and to the south at (90 - latitude)/2. Fun, eh! Chris Lusby Taylor Newbury, England 51.4N 1.3W
