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 towards the equinoxes because the angle of the shadow would be too shallow. 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
