How does a moon phase watch work?
Quality watches are known to have complications, that is, features other than those that tell the hour, minute and second. Among the most popular complications, the moon phase is one of the most mysterious, but also one of the most elegant.
Following the 29 and a half day lunar cycle
The moon phase watch can have various aspects, including the way the moon phase is presented on the dial. It can be a fixed lunar disk that is more or less obscured depending on the time of the lunar cycle, or it can be a moon that advances throughout the cycle on the dial. Whatever the presentation chosen by the watchmaker, the moon phase watch generally works on the same principle. Thus, each moon phase watch has a disc that carries two moons that are displayed alternately on the dial. The disc has 59 teeth and advances by one tooth every 24 hours.
Since the disc has two moons, it is able to reproduce the lunar cycle by dividing its number of teeth by two to obtain 29.5 days per cycle, which is very close to the 29 days, 12 hours, 44 minutes and 2.9 seconds that the lunar cycle lasts exactly.
How to set a moon phase watch?
On a quartz watch, the moon phase is automatically programmed. On a mechanical watch, the setting can be more laborious.
You must use the setting crown and rely on either the lunar calendar to set the date of the last full moon or new moon and advance the time until you reach the moon phase and the current time.
For example, if the last moon was March 20, you should set the calendar to March 20 and put the full moon. Then simply advance the time to the current date and time to get an accurate setting.