Grandfather clocks often feature a moon phase dial located above the main clock face. This dial tracks the lunar cycle as it progresses through its various stages. Setting this feature correctly ensures the clock accurately displays the current appearance of the moon in the night sky.
Understanding the Moon Dial Mechanism
The mechanism driving the moon dial is engineered to approximate the synodic month, which is the time it takes for the moon to complete one cycle of phases. The dial is typically geared to complete one full rotation every 59 days, representing two full lunar cycles.
Before adjusting the clock, determine the moon’s current phase and the date of the last full moon. Use a reliable online lunar calendar or astronomy application for the most accurate information.
The dial displays the four main phases: the New Moon, the First Quarter, the Full Moon, and the Last Quarter. Knowing how many days have passed since the last Full Moon is the basis for the setting procedure.
Step-by-Step Setting Procedure
Setting the moon dial requires careful attention to avoid damaging the internal gearing. Always move the moon dial in a clockwise direction only. Moving the dial counter-clockwise can cause stress or breakage within the clock’s movement.
Open the access door, usually located on the clock’s waist, to reach the dial mechanism. Locate the moon dial and gently apply pressure to its edge to rotate it. Avoid touching or moving the minute and hour hands during this adjustment.
The first step is to rotate the dial clockwise until the image of the full moon is perfectly centered. This full moon image should align directly under the “15” mark or the highest point on the dial face, establishing the zero point for the calculation.
Next, use the information gathered about the current lunar cycle to determine the number of days that have passed since the last full moon. For example, if the last full moon occurred five days ago, the dial needs to be advanced by five days from the centered full moon position.
Advance the dial one day for every day counted since the last full moon, rotating it slowly and deliberately. Each day’s advancement is represented by a small movement of the dial, often corresponding to a visible mark or click on the mechanism. Once the dial is set to the correct number of days past the full moon, the access door can be securely closed.
Long-Term Care and Troubleshooting
Once the moon dial is accurately set, it is designed to advance automatically with the clock’s movement. However, the 59-day approximation is not perfectly aligned with the actual 29.53-day lunar cycle. This slight discrepancy means the dial will gradually fall out of sync over time.
The mechanism typically drifts by approximately eight hours every three years. Therefore, the dial only requires manual adjustment every few years or after the clock has stopped for an extended period, such as during a power outage or maintenance.
If the dial becomes stiff or difficult to turn, it may indicate an accumulation of dust or dirt within the mechanism. A stiff dial often requires professional cleaning and lubrication to restore smooth operation. If the dial stops advancing entirely, it suggests a mechanical issue with the gearing that connects it to the main clock movement, necessitating inspection by a qualified clockmaker.