Actually, it is the moon phase dial. Early on, a clock was that most expensive item in a home. It took two years of a skilled craftsman’s wages to buy one. And if you could afford a clock, why not get a little more arrogant and buy something that hints that you can get more than ‘just a clock’ and add the moon dial?
Back then moon phase dial has a purpose. It could help you plan:
Your trips. You could travel farther walking or on horseback (if you were lucky and the night
Your field work. You could plan on having someone in the field longer during a full moon.
When you could expect the first hard frost (often during the full moon).
And, it also affects the tide.
There were even lunar societies. Some folks even got their hair cut based on the lunar cycle.
Even now, people who work in law enforcement, hospitals, and social work can tell when there is a full moon. People can do weird things.
Anyway, what do you do with the moon(s) and those numbers? There are actually 2 moons on your dial, and they are 180 degrees from each other. When there is a new moon, the two moons are hidden behind the globes on the sides of your dial. Your moon travels clockwise. The numbers surrounding the moon dial indicate how many days since the last new moon. Since the lunar cycle is 29 1/2 days long (new moon to new moon) the full moon is indicated when the moon is at 15. All you have to do is move the moon to match the number of days since the last new moon. If the moon doesn’t budge when you try and move it, wait a couple of hours. The moon dial can’t be moved when it’s being advanced by the clock.
Actually the moon phase is 27.3 days long, but since the Earth is also moving, it takes 29 days, 12 hours 44 minutes and 3 seconds to return to the same position as seen from the earth. With that said it’s just a little too complicated to get those pesky lunar phases to correspond to your clock dial. But it’s easy to correct. Just make a note to adjust it every 2 years and 8 1/2 months. You’ll have to set it back a day. It’s just the opposite of what you do in February during a leap year.
(My calendar doesn’t go that far…)