Hi all, I have read with a lot of interest the numerous messages dealing with the azimuthal sundials and I have noticed that sometime there is a little confusion on the names that we use for the different types of sundials. I write you my opinion.
In the sundials the time is pointed out from the shadow or of a point (nodus) or of a curve (generally a rectilinear style). The calculation of the position of this shadow depends - except rare cases - from the two coordinates that determine the position of the Sun in the sky : hour angle and declination of the Sun (equatorial coordinates) or Azimuth and Height (local coordinates). Moreover obviously the Latitude and Longitude of the place. If we want that the position of the shadow depends only on one of the coordinates we have to consider as element that casts the shadow a rectilinear style whose shadow intersects a set of date lines that we can trace as we want on the dial . In an azimuthal sundial the only coordinate that we want employ is the azimuth of the Sun. Since the azimuth is the angle that the vertical plane containing the Sun makes with the meridian plane (and therefore it is the angle between planes all passing through the vertical line of the place) the only style that is possible to use for having a REAL azimuthal sundial is a vertical style. The Azimuth is given by the angle that the shadow of this vertical style forms with the line N-S If the plane is horizontal we have the several sundials that Fer de Vries has shown with concentric circles as date line ; if the plane is vertical it is opportune (but not necessary) to take the date lines horizontal; etc. A really azimuthal sundial can be obviously built also on a tilted plane (the style always vertical ! ) The well known Analemmatic Sundial is an Azimuthal S. In the same way if we want that the position of the shadow depends only on the hour angle we have to take a polar style, that is a style parallel to the straight line through which all the hour planes pass : all the sundials with Apparent Time with polar style are so. In similar way we can build sundials in which the position of the shadow depends only on the "Meridian angle" or on the "Vertical angle" (see the Ptolemaic Coordinates in "The Compendium", March 1999). In this cases the style would have to be horizontal in direction E-W and in direction N-S. In all the other cases (for ex. Horizontal plane with tilted style, vertical plane with horizontal style, etc.) the position of the shadow doesn't depends only on one coordinate (Azimuth) but also from the Height of the Sun : for this reason they cannot be called strictly azimuthal. Since I could not call azimuthal these solar clocks (because they don't have this characteristic even if their shape look like azimuthal) I have called them (in my program with which them can be calculated) "Monofilar" since the shadow is made only from one style or " thread " (filus). This in analogy with the well known " bifilar " Sundials in which the time is indicated by the point where the shadows of two different straight lines or " threads " cross each other. Best regards Gianni Ferrari
