Frank Evans wrote: > > Greetings, fellow dialists, > > A kind BSS member has given me a sundial. It is made of cardboard and > is about 15 cm high. He writes: I am enclosing a dial sun-compass which > may be new to you. The principle was described in an article by Fred > Sawyer (Bulletin, British Sundial Society, 91, 3). The device should be > assembled, using the card wedges to hold the two semicircles at 55 deg. > (my latitude) with June-July at the upper edge. The bottom of the V > should coincide with the date. Each arm casts a shadow to tell solar > time on the inner and outer scales. When the dial is placed on a > horizontal surface so that the two times are the same the plane of the > vertical card coincides with the meridian. Invented in 1664 according > to Sawyer. > > I assembled it and it works! The dial plate is at 55 deg. to the > horizontal, two gnomon edges cut out of a single card are at right > angles to each other and each is at 45 deg. to the dial plate. The > whole thing has the feel of a tilted analemmatic dial with two oblique > gnomons. The dial plate and upper gnomon are said to equate to Gordon > Taylor's large Royal Observatory dial now in Cambridge. I only sort of > see how it works despite Fred Sawyer's benign instruction. Can anyone > provide a few simple words to help me further? > > Thanks in advance, Frank. > > -- > Frank Evans
Hello Frank, You have an horizontal sundial for latitude 0 degrees, but placed at your latitude. Therefore the plane is tilted 55 degrees. It is a Foster/Lambert sundial. The 2 gnomons have angle (45 + 0.5 lat) and (45 - 0.5 lat) to the plane. For lat is 0 degrees both answers are 45 degrees, but one gnomon is to the north and one is to the south. The shadow of one gnomon runs clockwise, the other shadow runs anti clockwise. In modern terms these dials are named projection dials. See my pages on the WEB with an article about projection dials, written by J.A.F. de Rijk. (Adress below) The gnomons are just between a vertical line and a polestyle. At lat 0 degrees a polestyle has an angle 0 degrees to the horizon and so the gnomons are between 90 and 0 degrees = 45 degrees. In this way the projection of a circle in an equatorial plane stays a circle, hence the hourpoints lie on a circle in your dial. For any latitude these dials can be directly constructed as an horizontal dial, but the diameters of the 2 hourcircles then will be different. Also the analemmatic sundial is a member of the "family" of projection dials. Usualy the projection is from the zenit onto the horizontal plane, but on any plane an analemmatic dial can be constructed. I can't explain these dials in a simplier way but perhaps these notes will help you further. Best wishes, Fer. -- Fer J. de Vries [EMAIL PROTECTED] http://www.iaehv.nl/users/ferdv/ lat. 51:30 N long. 5:30 E
