Bob, Steve, et. al., I have no knowledge of the origin of the practice of using zero to mark noon, but there is as much logic as for twelve, since it is the beginning, or "zero," for P. M. hours, and so can be expected to precede hours 1, 2, .... etc.. by an hour, especially with the sun's meridian passage as specific reference in "P.M. (post meridiem.)"
Sciagraphically, Bill > I have recently seen a couple of dials which use a nought (a digit > zero) > in place of XII,... I've never seen this done on a clock,...
