Expert: John Newman - 1/19/2010

Question
Hi,
I am a jeweller with no clockmaking experience but am wanting to make a clock with a working moon dial...however I am struggling to find anywhere that I can buy the actual moon dial mechanism.  If you have any suggestions they would be great!
Any further info or links you may have about how the moon dial mechanism works would also be greatly appreciated as I am struggling to find out exactly how it works...I am hitting a brick wall with this project & starting to wonder if my designs are possible!
Hope you can help,
Thanks

Answer
Ish, moving moon dials are a part of the clock dial itself.  The way a moon dial works is to depict the phases of the moon which are in a 29-1/2 day cycle, that is from full moon to full moon, or new moon to new moon.  The moon dial is constructed with teeth on the edge so an arm or pin can advance the moon dial by pushing on a tooth.  With the cycle being 29-1/2 days, there cannot be 29-1/2 teeth on a circle.  Therefore, the design is such that the number of teeth is doubled to 59 and there two moons on the dial opposing each other.  The dial is advanced 1/2 revolution for a complete moon phase cycle.  The earlier moon dials were advanced by a tab or pin on the hour tube (the part to which the hour hand is attached).  Of course in a 24 hour period the hour hand makes 2 revolutions.  This would advance the moon dial twice as fast.  To slow it down, the number of teeth (59) was doubled to 118 teeth.  On more modern dials there are 59 teeth and a gear train with a ratio of 1:2 drives it the proper amount once a day.  One more feature is that on the moon dial setting there are two globes on each side of the display.  The moon(s) hide behind the globes.  On the new moon depiction, the moon on the right has just moved behind the globe on the right and is hidden.  The moon on the left is hidden by its globe.  As the moon dial is advanced to the right (clockwise) the first day, a sliver of the moon will be visible.  Each day it will appear a little more until, on the 14-1/4th day it will be a full moon.  The following days the moon will move to the right, being hidden a little more by the globe on the right until it disappears on the 29-1/2th day to be a new moon again.  The moon that originally was hidden on the right has now rotated to the left globe and is ready to peek out on the following day.

As far as your finding a moon dial, Meadows and Passmore in the U.K. might have one.  They can be found on a search engine such as Google.  I will also check with some of my Internet Clocksmith members in the U.K. and see if they have any suggestions.  I have plenty of them in my stock, but let's see if we can find you one closer.  Most clock dials come in about three standard sizes, those for grandfather clocks, grandmother clocks and mantel clocks.  If you are designing a clock, you would need to choose one of these dials and design the clock case around it.

You have taken on an interesting project.  Let me know how it progresses and I'll be glad to help when I can.

John Newman
THE VILLAGE CLOCKSMITH
Old Prattvillage
Prattville, Alabama