Question about how my moon dial works

tomb993

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Apr 13, 2020
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Good news, I finally got my Colonial grandfather clock repaired! Everything seems to be working, but I'm curious about how the moon dial is supposed to work. The repairman said it moves two clicks per day, one at noon and the other at midnight. Looking up moon dials on the internet I see "two clicks per day" a lot. This seems to imply two discreet, instantaneous clicks. But based on my observations, this doesn't seem to be the case. It's more like it moves slowly for a few hours, does nothing for a few hours, then moves again.

If you look at the close up of the movement, you can see the eccentric lobe (or cam) pointing to about 12 o'clock and the hour hand pointing to about 2:30. You can see the peg for the wire in the bottom right and the hook that connects to the moon dial in the upper right. If you look closely at the side view, you can see the hook attached to the moon dial.

I'm thinking the cam pulls the wire down, moving the moon dial from say the 10 o'clock position to the 2 o'clock position (this is the position of the cam lobe, not where the hour hand points), does nothing from 2 to 4, repositions the wire on the next tooth on the moon dial from 4 to 8, does nothing from 8 to 10, then repeats the process. This happens twice a day.

Is that somewhat correct or am I clueless? I probably have the positions wrong, I just used those numbers as an example. Can you tell by looking at the cam what positions would move the dial?

Note the original wire was broken (looked like metal fatigue to me) so they had to make a replacement.

IMG_2929.jpg IMG_3094.jpg IMG_3102.jpg Moon Dial Mechanism.jpg
 

J. A. Olson

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Dec 21, 2006
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Good job on having your clock fixed. As you've guessed, the moon dial's clicks do not occur instantaneously.
 

Danwatts2005

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Apr 7, 2022
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Mine do. Once at day, and once at night. The clock makes a particular noise when this happens. The noise is the brass slipping into the next slot.
 

novicetimekeeper

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Jul 26, 2015
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the dial is driven from the hour, so it advances in the same way.
 

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