Hello Andrew,
Thank you very much for your reply. However, I'm bothered by that little "etc" in your sentence "The reason Italian, Babylonian hours etc do not work..." At the risk of exposing my own ignorance, I have to admit that when I look at the dozens of dial types listed in John Davis' draft glossary, it is very hard for me to determine which may be relocated, and which may not. Hence my request for a list.
Does anyone else have this difficulty? Mac Oglesby Vermont USA Andrew James wrote:
As long as there is an equivalence between the hours at the two points I believe it will work. The reason Italian, Babylonian hours etc do not work is that you cannot find a constant hour difference (normally, depending on the longitude), as the time to sunset/sunrise depends on the latitude, too. If you consider the dial as being moved over the surface of the Earth always keeping parallel to its original setting (with respect to the Earth) you will see that throughout its journey it retains exactly the same ability to show time at the original position, as the distance it has moved (size of the Earth) may be regarded as extremely small by comparison with the distance to the Sun. If therefore you can relate the time you use at its new location to time at its old, you can obtain local time from what it shows. This is the case with the normal 24 hour division of the day - just take the longitude difference and divide by 15. There is obviously another proviso - it may become partially useless corresponding to daylight hours at one spot being night at another, or to the Sun being "behind" the dial. Andrew James 51d 04'N 01d 18'W
