The moon orbits the earth once a month, which in turn orbits the sun. The moon is always at a different angle to the earth and the sun. As the moon phase, which lasts 29 days, therefore, one designates the changing light shapes of the moon.
To always know which lunar epoch we are in, some watches have the additional function of a lunar phase display, which requires mechanical and complex constructions.
Here, the power reserve not only ensures the exact time, but also for the correct moon phase display. This is usually done by the hour hand.
The moon phases are represented by a disk with two opposite moons, this is located on a gear. Depending on the accuracy, there are different models in which the number of gears varies.