How does a moon phase watch work?

Quality watches are known to have complications, that is, features other than those that indicate the hour, minute, and second. Among the popular complications, the moon phase is one of the most mysterious, but also one of the most elegant.

The 29 and a half day lunar cycle

The moon phase watch can have different looks, including the way the moon phase is presented on the dial. It may be a fixed lunar disk more or less obscured depending on the time of the lunar cycle, or a moon which advances throughout the cycle on the dial. Whatever presentation the watchmaker chooses, the moon phase watch generally works on the same principle. Thus, each moon phase watch has a disc which carries two moons which are displayed alternately on the dial. The disc has 59 teeth and advances one tooth every 24 hours.

As the disk 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 29 days, 12 hours, 44 minutes and 2.9 seconds exactly how long the lunar cycle lasts.

How to adjust a moon phase watch?

On a quartz watch, the moon phase is programmed automatically. On a mechanical watch, the setting is different.

You must use the setting crown and rely on the lunar calendar to set the date of the last full moon or new moon and advance the time until you reach the current moon phase and time.

For example, if the last moon was on March 20, you would set the calendar to March 20 and indicate the full moon. Then simply advance the time to the current date and time to obtain a precise setting.