[EMAIL PROTECTED] wrote: > > Hello, I'm new to the list and don't know much but am interested. > I tried looking in the archive for this list but was unable to > connect. I hope I am not repeating a question which has been > asked 1000 times, but I probably am. 8-) > > I saw in a shop a perforated ring dial with circumferentially-sliding > perforation. I liked the look of it but now can't remember much of > the technical specifics; I should have bought it or sketched it. I > got to thinking about it and I think that near noon, it should be hard > to discern one time from another unless the ring is very large since > the sun's altitude peaks and recedes gently and this dial seems to be > an altitude sensor. Is this a well-known problem of this type of dial?
Indeed, around noon an altitude sundial isn't accurate because the change in the altitude is ( very) small for a certain period of time. > Also, I tried to draw the dial and I think even with the sliding ring, > the scale for the time, located on the inside of the ring, must be > two dimensional and not linear as I believe I saw on the product for > sale in the shop. Is this true? The second dimension would be for > different seasons. In drawing the dial, I said that the position of > the perforation for a season would be the average solar altitude for > that season. By average, I mean splitting the difference between > the solar altitude at 8 am (or 4 pm) and noon. Is this the right > way to lay out one of these dials? What scheme does one use? > Again you are right. Inside there must also be a scale for the date. Otherwise it is a dial by which the time only roughly may be read. Look in the bulletin of the British Sundial Society about such ringdials. Issue 96.1 and issue 97.2, articles by Colin Thorne. > Thanks very much, > John B > 37.72 N 122.07 W > > PS: I read Waugh's book and that's where I learned the designation, > "perforated ring dial". Is that what this is generally called? > > PPS: I hope you can figure out what I am saying. I am so new to this > that I don't know the correct terminology. -- Fer J. de Vries [EMAIL PROTECTED] http://www.iaehv.nl/users/ferdv/ lat. 51:30 N long. 5:30 E
