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This is one of a series of articles written for "Clocks" magazine by the late Noel Ta'bois, and reproduced with permission here as a memorial to him.
This article originally appeared in Clocks in June 1987
Last month I mentioned the ambiguity of two names for the same thing. The study of gnomonics is fraught with similar perplexities. This month's title embraces no fewer than three quite different types of ring sundials but I know of no books which make distinctions between any two of them, let alone three! Is it any wonder the tyro get bewildered?
The three types are:
The first group function by the sun's altitude irrespectively of its azimuth (direction). From their size and shape they are sometimes also referred to as bracelet dials and, if small, as 'poke dials'! Figure 1 shows a simple altitude ring dial. It is suspended by the eyelet at the top and rotated until the sun shines through the hole, seen top left, on to the diagonal hour lines engraved on the opposite inside surface. The light spot is lined up with the centres of these lines at the equinoxes, with the bottom ends at the summer solstice and the top ends at the winter solstice. No knowledge of direction is needed.
Figure 2 and 3 show an example of the second group. They are known as equinoctial, equatorial, astronomical, or universal ring dials. Four names for the same thing! They usually have an eyelet from which they are suspended but are occasionally fixed to a solid base.
In use, the suspension point is et against the latitude scale, and the sliding cursor on the diametral arm is adjusted for the sun's declination. One side of the arm is graduated in degrees and the other shows the date. Either scale may be used. The cursor has a small hole which allows a spot of light from the sun to fall on the inside edge of the equatorial hour ring which is marked with equiangular hour lines.
The dial is suspended from the eyelet and rotated until the light spot falls on the line engraved along the centre of the inside edge of the hour ring. In this position the spot shows the time, the central arm is parallel to the earth's axis, the hour ring is parallel to the earth's equator, and the vertical ring lies in the plane of the meridian. For precise readings the central arm is turned on its pivots to face the sun and so produce an undistorted light spot.
At the equator, as the dial is rotated the light spot will cross the hour ring at right angles, but as the latitude increases the angle becomes progressively more oblique and the setting of the dial progressively more imprecise. At the pole the spot moves parallel to the equatorial ring making it impossible to set the dial by this method. However, being an equinoctial dial, if it is correctly oriented, its accuracy of time measurement is unaffected by latitude; it is only the setting of the dial which becomes more inaccurate as the latitude increases.
The altitude ring dial has to be reoriented every time a reading is taken. After initial setting the equinoctial ring dial would, theoretically, continue to function for the rest of the day, although the cursor would have to be rotated to follow the sun. The fact that these two types of dials are set in a similar way but function differently can give rise to muddled thinking.
For example, in a museum a display of equinoctial ring (and other) dials was headed 'Universal Altitude Dials'. This used to puzzle me but my difficulties ended when it dawned on me that they are in fact equinoctial dials and not altitude dials. I protested to the curator and I am glad to say that the display is now headed merely 'Universal Dials'!
The third group are usually horizontal dials. I will discuss them next month.
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