In this days I am just reading the book De Analemmate by Claudius Ptolomaeus. While describing references for celestial coordinates he calls gnomon what we mean nowadays for the local vertical in a celestial sphere. Such term in such meaning can be found all over the book. In the first pages Ptolomaeus says (latin version of the XIII century translated from Greek) "... communis sectio meridiani, et verticalis gnomon...." (common section axis between the meridian plane and the vertical plane is called "gnomon"). For vertical plane in the book is intended the prime vertical, oriented by East-West. So I think that, but it's only my opinion, for gnomon should be (and were by ancient astronomers) called only vertical objects (such as obelisks) casting the shadow on a horizontal surface. So the gnomon is perpendicular to the surface where hour lines are designed. Because vertical sundials were surely created after horizontal ones, I think that gnomon, by extension, was called also the horizontal rod employed in early vertical sundials.
Strictly speaking (by an historical point of view) every rod casting shadows that is neither vertical nor horizontal could not be called gnomon. Regards F.Martinelli Viareggio - Italy
