Most professionals are familiar with the moving moon dial. A Hermle dial is shown on the right. The moving moon portion first appears prior to the year 1700. The point was to alert travelers to when there would be enough moonlight to travel at night. Many farmers also planted by the phase of the moon and some still do. The lunar calendar is 29 1/2 days. In other words, a new moon occurs in that cycle. The numbers indicate the “age of the moon”, or the number of days since the new moon.
In looking at the dial, what one sees in terms of the moon is what one should see in the sky if the dial if properly set. To set the dial, find the date of the last new moon. Determine the number of Days since that time. Draw an imaginary line through the nose of the man in the moon or the diameter of the disk and set it on that number. Note that at new moon that moon should be hidden on the side and at full moon it should be at the 15.
The moon dial is usually driven from the minute shaft via a moon gear shown in photo on the left. Note that in replacing a gear, it must match both the movement and the dial in terms of diameter and number of teeth. Also, the bore of the gear must be the same as the diameter of the minute shaft.
PROBLEMS
- The first one is that they are often installed backwards causing jamming. The gear must be out away towards the dial and the set screw in towards the movement.
- Second, is that it needs to be checked when installing the dial that the teeth on the moon gear mesh properly with those of the dial. Sometimes the dial is pushed on the the gear on the dial on top of the moon gear. The fact that the moon can be rotated does not mean the gears are meshing. Manually advance the dial by turning the hands to insure that the click spring seats properly inside a tooth and not on top of one, etc. Also make certain that the pin advancing the dial is not bent or missing.
- Third, over time it is possible for the dial to sag due to its weight and change the depthing on the gears. Any of these issues will usually cause the clock top stop.
Finally, I received a call from a repair person who stated that the movement ran fine on a test stand without the dial but stopped in the clock when the dial was attached, so there must be a problem with the movement. When a situation like this arises, the problem is ALWAYS associated with how the dial was attached to the movement. Usually that the moon gear is not meshing properly.
He also took pains in telling me he had been doing this for 25 years. Needless to say, we will not be shipping to him in the future.