Dear Membership, Polar Cycloid Book Dial.
In the NASS Compendium of December 1998 Page 24, which is part of Fred Sawyer's article on the Cycloid Polar Dial, it mentions making a vertical dial facing east or west with the noon line now occurring when the surface is fully exposed to the sun. The idea of both an east and west cycloid polar dial mounted together with the flat surface mounted between them occurred to me. The resulting shapes reminded me of a number of dials shaped like books that I'd seen when a child. If the curves were then taken back from the (now) 6 O'Clock edges to form the two leaves of a book, how neat it would be. I then thought how easy it would be to let each cycloid's shadow fall on the other, and how easy it would be to project the new hour lines on to each surface, ie. removing the flat sheet. (The hour lines are no longer equal spaced and parallel of course). An easy to design polar cycloid book dial emerges! This may well not be a first thought of such a design, but it seems quite a neat idea for places of learning. Tilted at the proper polar angle it would keep quite good Local Solar Time. If it were arranged to rotate around the polar axis it could be set to show standard mean time if set whenever the equation of time, longitude correction and daylight savings time required it. The two pages themselves could include whatever text would be appropriate. What do you think? Edley McKnight
