In responding to W. Wedemey's inquiry about "ring dials, C. L. Taylor and W. Sullivan are in disagreement about their accuracy. In fact, both are right, but they are talking about two different types of sundial. The confusion arises from the term "ring dial."
Typically, "ring dial" refers to a simple type of altitude dial in which a suspended, wide ring of brass contains a pierced slider (embedded within the ring) that allows a spot of light to fall on a graduated hour scale inside the ring. The slider is adjustable for solar declination and the inner hour scale is calibrated for the seasons. The crudest versions dispense with the slider and substitute a single pin-hole in the ring. In this case, the interior of the ring has two hour scales (one for winter, and one for summer). There are other variations, which I won't go into here. Although large, complex versions are known to me, they are rare. Typically, ring dials were crudely made in sizes small enough (3-5 cm diameter) to be carried in one's pocket, or "poke," and so were sometimes called pokes. Their accuracy was extremely limited, generally dividing time into half- or one-hour intervals. They were particular dials--i.e., made for a fixed latitude. Common folks in the 17th and 18th centuries were very fond of these dials, which were produced in many parts of Europe. I presume this is the type of dial to which Mr. Taylor and Mr. Wedemey referred. I suspect that Woody on the other hand was referring to what is conventionally called a "universal ring dial" or a "universal equinoctial ring dial." Whereas the ring dial is strictly an altitude dial, the universal ring dial is a combination altitude and directional dial. It is also equatorial and universal. This dial consists of a meridian ring suspended from a sliding shackle, which is set for the user's latitude. Nested inside the meridian ring and fixed at the zero position of the latitude scale, there is a pivoting hour-ring, which is set at right angles to the meridian and parallel to the equator, when the sundial is used. The gnomon is a perforated slider set on an axial bridge, which is inscribed with a solar declination scale. A spot of sunlight passing through the slider hole falls on the hour scale and marks the time. The universal ring dial is self-orientating. When not in use, these dials fold flat. They were made in large and small sizes, beginning in the late 17th century. Many had an hour scale divided into two-minute intervals and so offered the prospect of precision. I hope this clears up the confusion. There is much more I can say about these two types of sundials and their history, but will not take up people's time here. ** This information is excerpted from the forthcoming catalogue of ** sundials at the Adler Planetarium and Astronomy Museum, Chicago, ** and should not be quoted without my permission. I am the ** principal investigator of the two volumes on time-finding ** instruments, as well as the Editor of the catalogue series. (It may interest readers to learn that the first two volumes--on eastern and western astrolabes and related instruments--will be published by the Adler Planetarium by next spring.) Sara Schechner Genuth Secretary, North American Sundial Society Committee on the History and Philosophy of Science Department of History phone: (301) 593-7144 Francis Scott Key 2115 fax: (301) 314-9399 University of Maryland email: [EMAIL PROTECTED] College Park, MD 20742-7315
