Hi Sara and others, A useful classification scheme should not only accommodate historical dials, but also novel (i.e. 20th century) designs: bifilar, shadow plane, conic, digital, polarizing, holographic, etc.
I don't have such a scheme available ;-), but would like to propose the following basic approach. Start with the definition of a sundial: a device that tells the time from the position of the sun in the sky. This position is shown for my own latitude (53 N) in the attached graph (20 KB, is that allowed?) The hour lines appear curved in this projection, but are in fact the intersections of the hour planes with the celestial sphere. Here the usual 24 equal hours are plotted. Babylonean/Italian hours, temporal hours and unequal hours also have their hour planes and thus hour lines in such a graph, but I have not worked that out yet. Four main sundial types could then be defined by four basic ways to "read" this "map". 1) nodal dials, which read the position of the sun directly, usually through a point projection of the map on whatever surface. Scaphes and meridian lines are among these. As a bonus, the date is also obtained (with the ambiguity that two dates have the same declination). The node can be a fixed, material point, such as the tip of a rod, or a moving, virtual point, such as the intersection of two shadow lines in the bifilar dial. 2) hour plane dials, which read the hour line on which the sun appears. For the usual 24-hour system, the gnomon is parallel to the earth's axis: a pole style. The dial face can have any shape and orientation: plane (horizontal, vertical, equatorial, polar, etc.) or wobbly (Bob Terwilliger's now defunct jacuzzi dial). In a more general case, each hour may have its own pole-style/hour line pair, as in the shadow plane dial. The pole-style dail is thus a special case of the shadow plane dial family, in which all pole-styles coincide. The cone dial invented by Bores is also a hour plane dial, indicating Babylonean and Italian hours. 3) altitude dials, which read the altitude of the sun (the concentric circles in the graph). This requires a date-dependent readout. The shepherd's dial is among these. 4) azimuth dials, which read the direction of the sun (the radial lines). This also requires a date-dependent readout. Analemmatic dials are among these. Multiple dials combine several dials of the same or different types. The pole-style in type (2) may be provided by a node or index in order to read also the date. Another example is the self-orienting analemmatic + horizontal pole-style couple. The digital dial of Scharstein is a 'super-multiple' pole-style dial, which generates a readout coded as numerals (or whatever symbols). ---------------------------------- Now, applying this to what Sara wrote: "Now, to apply your logic to universal ring dials. The gnomon slides along the polar axis, and the hour scale is on an equatorial (equinoctial) ring, which makes the dial an hour-angle dial.", I don't think that's right: you don't have a dial yet, you are just preparing the instrument to function as an altitude dial, i.e. to translate the measured altitude of the sun into the hour. Regards, Frans Attachment converted: Macintosh HD:sun-pos.gif (GIFf/JVWR) (0006AB79)
