Dave, sorry if I hadn't been clear enough! >From where is 'z' measured? Is it 0 degrees when it lies on the ground and >pointed North (in the northern hemisphere)?
It can be defined in several (equivalent) ways, but the easiest is measuring it anticlockwise from the northern semiaxis to the gnomon. Its value must be then between 0 deg and 180 deg (z > 90 deg implies a gnomon leaning southwards) >What is the sign/direction of 'd'? Thanks for this one, Dave! I forgot to say that the gnomon must be contained in the meridian (NS) plane, ie., its gnomonic declination should be 0 deg or 180 deg). So the positive direction of 'd' is towards the South. You can derive as well the equations for a declining gnomon, but then they become a bit messy and there isn't much different with the former case: only that the date scale is crossed along the horizontal plane. (you may consult Savoie's book for more info on this topic: his explanation is not as crisp as that by Bruno Ernst but he gives the full equations). If I've got time, I'll try to draw an sketch. Best regards, Anselmo Perez Serrada [ 41º 39' N 13º 15' E (from the Hierro island) ] -
