Chris you are right. Moving a sundial in another place, you can redirect it so that the style becomes polar. Then you can revolve it around its style to get the hour lines in the correct position. To make this immaginary moving there are different theories, for example you can calculate the new inclination and declination with some formula. Anyway the point is that a sundial may be repositioned in an other place although that is, almost always, only a theoretical amusement. It is true for the modern hours but not for the italian ones (or babilonian). A sundial moved and redirected in an other place continues to show the italian hours of the origin latitude. Italian hour lines depend on the daylight arc and change with latitude.
This is more evident considering the sundials with a conic gnomon. The vertex angle of the cone is twice the latitude. If I use a different angle I can show the italian and babilonian hours of another latitude. The italian hour lines are the intersection between the dial and the tangent plane to a cone with the vertex on the hedge of the polar style and with the same axis. If I change the latitude, the vertex angle have to change too, then change also the italian hour lines. ciao Fabio Fabio Savian [email protected] Paderno Dugnano, Milan, Italy 45° 34' 10'' N 9° 10' 9'' E GMT +1 (DST +2) ----- Original Message ----- From: "Chris Lusby Taylor" <[email protected]> To: "Frank Evans" <[email protected]>; "Sundial" <[email protected]> Sent: Wednesday, March 31, 2010 5:24 PM Subject: Re: re how italian dials > Dear Frank > Sorry, but I'm pretty sure you can't use that trick because Italian and > Babylonian time are "local" in a way that modern hours are not. The modern > time at a location ninety degrees away is always a fixed amount before or > after the time you're interested in. For instance, for a wall in England > facing west of south, you could find a spot ninety degrees away in the > South > Atlantic where, say, the modern time is always 2 hours earlier than in > England. > > But, except at the equinoxes, that location's sunrise and sunset will not > be > 2 hours after yours. So, a horizontal dial showing correct Italian and > Babylonian hours for that location will not show the Italian and > Babylonian > hours in England, even if you adjust the numbers by 2 hours. > > This is because the Italian and Babylonian hours relate to the local > horizontal plane. So, whereas a sundial reading modern hours can be moved > to > a new location and positioned so as still to work correctly, perhaps by > using a wedge to cant it, this is not true of an Italian/Babylonian > sundial. > Such a dial can be used only at one latitude. > > Chris Lusby Taylor > 51.4N 1.3W > > > > ----- Original Message ----- > From: "Frank Evans" <[email protected]> > To: "Sundial" <[email protected]> > Sent: Wednesday, March 31, 2010 2:01 PM > Subject: re how italian dials > > >> Greetings, fellow dialists, >> I'm starting to think that for a vertical dial in Italian hours it would >> be simplest to use the old dialist's trick of laying it out as a >> horizontal dial at ninety degrees away. >> Frank >> >> --------------------------------------------------- >> https://lists.uni-koeln.de/mailman/listinfo/sundial >> > > --------------------------------------------------- > https://lists.uni-koeln.de/mailman/listinfo/sundial > > --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
