Dear John, This is a good riposte...
> You point out here that the declinations > lines normally used (which I assume are > conic sections) are not really correct... They are indeed conic sections but they ARE correct provided you call them "Constant-Declination Lines" or something equivalent. If you could somehow freeze the declination of the sun at spot-on zero for 12 hours then the shadow of the nodus would run along the equinoctial line seen on many dials. This is, of course, a straight line (on a plane dial) and is, strictly, the line associated with declination zero rather than the day of an equinox. > ... have any dials been made which take this into > account? Well, there isn't a strong case for drawing narrow S shapes because it would be different for each equinox. The S would cross the straight equinoctial line at the instant of the equinox and the hour-angle of that instant at a given place is different for each equinox. That said, you could argue that the "cronometri" that Bianchini incorporated into the meridiana in S. Maria degli Angeli in Rome are there to measure the point where the (undrawn) S shape crosses the (also undrawn) equinoctial line. In short, these instruments enable you to estimate the instant of the equinox by the offset of the point where the centre of the solar image crosses the noon line. To check their performance, Mario Catamo has recorded the crossing on the day of every equinox since 2003. Best wishes Frank --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
