At a slight tangent to the sundial setting correspondence, I have a related question which I am sure someone will be able to answer.
If one has a (declining) vertical dial which is accurately constructed in itself and with a correct gnomon, but which is mounted on a wall not exactly at the designed declination, how can one establish the error in orientation of the dial from observing the error in time (which varies with time of day and solar declination, as well as depending on the declination of the dial)? More generally one might also consider an error in latitude, but declination only would be a start. The formula might lead to an easy (?) way of determining a wall's declination simply by reading a portable dial placed against it temporarily (with the vertical line correct) and knowing the local solar time. There would be good and bad times at which to do this according to how rapidly the error in time changed with the error in declination. Is this a practical method? Several portable dials might be useful or required. I realise that one could measure the angle between the wall and a vertical dial (or an edge of a horizontal dial) oriented to show the correct time, but this would need the measurement or marking of an angle, which simply reading the time would avoid. Andrew James
