Tony Moss wrote: > > Fer de Vries wrote... > > >This happens in the morning as well in the afternoon. > >To read an analemmatic sundial than gives problems...... > >This is a nice aspect to experiment with. > >Make such an analemmatic sundial and place it at your latitude, but > >tilted of course. > >Then you may see the retrogration of the shadow. > > > >Fer de Vries > > I'm puzzled as to why the dial should be tilted? > > According to my method of calculation for dials parallel to the ground:- > > 1. an analemmatic dial at the poles would be a circle and the 'calendar line' > a single point (a simple equatorial dial). > > 2 At the equator the minor axis of the ellipse is reduced to zero so the > major axis is effectively just a straight line with e.g. 5pm and 7pm hour > points overlapping. The major axis lies along the equator with the calendar > line symmetrically in the northern and southern hemispheres. e.g. for a > major axis of 10 metres the June 21st point and December 21st point would be > 2168mm displaced from it. A dial on the equator would work well in the > months near the solstices but would be useless toward > > 3. For a dial located at either Tropic the calendar line would be equal to > the Minor Axis. > > 4. For dials between a Tropic and the Equator the calendar line projects > outside the ellipse e.g. for a 10 metre major axis at 13.41° the minor axis > would be 2319mm with December/June 21st points 2109mm from the Major Axis > (949mm outside the ellipse) At those dates the 'gnomon/observer' would then > be casting a shadow from outside the ellipse but the hour points remain valid. > > I've never actually been to the latitudes in question so I've never had an > opportunity to try out these ideas in practice. Have I got this all wrong? > > Tony Moss
This message I already answered to Tony Moss, but I didn'post it on the list. The mail to Tony is erased, but I I will repeat what I meant with "tilt the dial". Calculate a horizontal analemmatic dial for let's say a latitude of 18 degrees. If your latitude is let's say 48 degrees, tilt the dial 30 degrees at the north side. Then you can see how the shadow acts at a latitude of 18 degrees and the retrogradation can be studied. The problem is the reading of the dial if the gnomon is outside the ellipse. The shadow of the gnomon intersects the ellipse at 2 points and which one I will need to read the dial?..... Fer.
