This is a complication that has been introduced in horology long before. This enables the users to view the sunlit portion of the moon that is visible from Earth. In a watch’s dial, there is a half-circle aperture that consists of two moons on a rotating disc. This is the usual version of the moon phase. The shown aperture symbolizes the portion of the moon thus allowing the appropriate phase of the lunar cycle to show.
It takes 29 days, 12 hours, 44 minutes and 2.8 seconds for the moon to complete a lunar cycle. There is reducing gear train being formed in the center wheel that creates a series of gear ratios that is important for the transition of the hours to a lunar cycle.
A huge part of the motion works that controls the hour and minute hands rotation is the hour wheel. The hour wheel rotates once in 12 hours at the same time driving the moon disc driving wheel to achieve a ratio of 1:2. The moon disc is operated by its finger that is a single gear mounted co-axially. There are 59 peripheral teeth that consists a moon disc. This keeps the moon disc one day ahead. If you divide 5 by two, you will have 29.5 explaining the two moons in a moon disc.
To assure that the moon discs is not unintentionally driven by its driving wheel finger; there is a jumper spring that holds it in place. The jumper spring will ascertain that the moon disc is properly driven only if the driving wheel is operating.
Approximately, a lunar cycle consists of 29.5 days. It is important to manually adjust the moon phase every 3 years to secure synchronization. Although given our advanced technology today, watch manufacturers already found a method to synchronize the moon phase complication that will go on for a thousand years. This is a far more complex version that the traditional one.