First off, I must make it clear I am talking of portable equatorial dials. Unfortunately, as these usually collapse into a ring shape, they are sometimes called ring dials (even by the best authors), thereby becoming confused with the simple single ring dial. Bryan Loyall asks what the sliding gnomon-hole achieves. Here is an answer.
On a fixed equatorial dial the gnomon is a wire along the N-S polar axis. On any day only one point on the wire casts its shadow on the dial. That point depends on the sun's declination: in the summer it is high up the wire, in the winter it is low. If the wire has, say, a bead at the correct point, the bead's shadow stays exactly on the dial all day. Imagine now what happens if the sundial is turned, so that it is not aligned N-S. The bead's shadow will move off the dial. So, if we pick the dial up and move it, we can always find both south and the time by turning it until the bead's shadow is on the dial. The only slight snag is that we cannot immediately distingush between some time after noon and the same length of time before noon, but if we wait a few minutes we can see if the bead's shadow stays on the dial or moves off. The portable sundials which use this principle usually have but a small hole in the gnomon plate, which can be moved up or down the polar axis according to the sun's declination for the date. If the sundial's suspension point is adjustable for longitude, the dial is truly universal. Hope this helps Chris
