I recently came across a sundial on a swivel mounting, and I don't understand the purpose of the mounting. Can anybody enlighten me?
The location is Chester, Nova Scotia, on the waterfront. It is a commercially produced dial in the form of an armillary sphere. I have seen similar but slightly simpler dials in my local New Age shop. The equatorial band which carries the hours-scale looks as if it can be moved to allow some degree of calibration - presumably so that the time can be set to that of the timezone centre or perhaps adjusted monthly to achieve partial EoT correction. In this instance, the hour scale has been positioned with no offset and then locked in place by means of a screw. That is, it is set to show local solar time. The armature in which the dial is mounted can also be altered, so that the dial's axis can be adjusted for latitude and then locked in place. It has been set at a suitable angle for the location, about 45°N. So far so good, but now for the third control - a swivel mounting which allows the entire dial to be rotated about a vertical line running through the centre of the sphere. This kind of dial is normally fixed with its central axis (the gnomon/style) pointing to the North Pole. The dial as manufactured, is designed to be rigidly fixed to a plinth and includes bolt holes for the purpose. Rather than simply fix it in place, the installers attached the dial's base to a metal plate which is in turn attached to the plinth by means of a single loose-fitting carriage bolt about which the dial can turn. Unlike the other controls, the extra mounting has no facility for locking the dial in position, so it inevitably ends up pointing away from North. The loose attachment seems to be deliberate, rather than from inadequate tightening of this solitary nut and bolt, and it would have been easy to fix the dial directly to its plinth without the need for an extra mounting plate. I suspect the whole thing arises from lack of instructions and lack of understanding on the part of the workers who installed it, but just in case there is some gnomic purpose which I don't know about, I must ask - is there any benefit in allowing the dial to rotate freely about the vertical axis? Specifically, I'm wondering if there is some magic self-Southing procedure which I have yet to learn of. Steve Steve's Site is at www.glinx.com/users/srgl
