> > I recently received, as a gift, a necklace-sized equatorial sundial. > However, the instuctions are in French (my French is not very good) and in > badly-translated English, so I can't figure out how to use it. > > There are two rings: an outer sliding ring that can be set to the proper > latitude (I live in Denver, Colorado, USA -- about 40 degrees north, I > believe), and an inside ring which rotates to be perpendicular to the outer > ring. The outer ring is marked with latitudes and decorations. The inner > ring is marked with roman numerals. There is a needle down the center, > fixed to the outer ring, with a small sphere in its center. > > How do I use this sundial? Particularly: > -- do I need to point it exactly north, or just roughly towards the north? > -- should I be looking for the shadow of the needle or of the small sphere? > -- the numerals go all the way around the inner ring -- why? will the > shadow ever fall on the southern side of the ring? > > Please respond directly to [EMAIL PROTECTED], as I am not a subsciber to the > list. > > Thank you for your help! > > Cara Hart > > ------------------------------------------------------ > Cara Hart > [EMAIL PROTECTED] > Systems Administrator > ARITEK Systems, Inc. >
Dear Cara, What you have is a modified form of universal ring dial (aka universal equinoctial ring dial). These dials first appeared in the early 17th century. In the original form, the axial wire had a slider that was adjusted up and down the axis for the sun's declination, which changes daily. Usually there was a calendar scale on the axis for setting the slider. With an adjustable slider the sundial was self-orienting--i.e. it would only give a time reading when it was aligned with the north-south meridian. No compass was required. Hence these dials were often favored by travelers and mariners. Is the bead on your wire adjustable or fixed? I assume it is fixed. In this case your sundial is not self-orienting and operates like a standard equatorial dial. To use it, set the suspension ring to your latitude on the outer ring. This ring represents the celestial meridian. Align the ring along your local n-s meridian using a magnetic compass. The shadow of the bead will be cast on the hour ring (that's the ring at right angles to the meridian ring). You've found the time! At your latitude, the shadow will not sweep around the entire hour scale, but will travel from the time of sunrise to sunset, roughly 4 am to 8 pm. For more on universal ring dials and related instruments, please see my forthcoming catalog of sundials and timefinding instruments at the Adler Planetarium in Chicago. The catalog will not only describe 400+ sundials, but will have interpretive essays that offer perspectives on the history, science, and social context of these instruments. Sara Schechner Genuth Gnomon Research Customized Curatorial Services 1142 Loxford Terrace Silver Spring, MD 20901 [EMAIL PROTECTED] Tel/Fax (301) 593-2626
