I agree that a nocturnal, using the circumpolar stars, is indeed superior on a clear night, if you know how to use it and have one about your person. In fact, as it can be universal and needs no correction for the EoT, it could well be argued that a nocturnal is superior to a sundial.
I'm a little confused about what practical use a moon dial would have, since the moon is much more transient than the sun. A superior nocturnal dial would be one that user would line up some of the circumpolar constellations/stars after setting the month and date. Of course you'd need moon light to read it, unless you install indiglow.
Anyway, I'm curious how in principle does it work?
But a moon dial is, perhaps, easier for the untrained, can be fixed and doesn't need such clear conditions, though there's only a 50% chance of the moon being above the horizon. (Why do so many people believe the moon rises at night?)
The principle of the moon dial is that the sun and moon are closely enough in the same plane that you can estimate the sun's longitude from the moon's longitude and phase. Use the moon's shadow to read its longitude, and gauge its position relative to the sun from its phase. But a small error in judging the phase leads to a great error in timekeeping. A smaller error, I think, arises from the fact that the sun and moon are not in the same plane. It might be possible to correct for this but I don't know if any moon dials do so.
Some moon dials, such as the famous one at Queen's College, Cambridge expect the user to add or subtract the time corresponding to the moon's phase. Portable ones often use concentric disks (volvelles) to do it for you - you turn the top disk until a hole in it shows the correct moon phase.
Chris Lusby Taylor
Newbury, England
51.4N, 1.3W
Note: When I say a nocturnal 'can be' universal, I refer to one
I have made myself with no extra parts, but incorporating a map in the
handle. You first set the pointer to pass through your position on the
map. This adjusts for your longitude. I don't know if any other nocturnals
incorporate such a map.
