A moon phase watch does just that: shows you the phase of the moon as it appears in the night sky. It’s displayed using an aperture on the dial that tracks the movement of our actual lunar pal, rotating around the Earth every 29.5 days. These displays are often ornately decorated, a this complication has more aesthetic than utilitarian appeal to us now. When a new moon is turning into a full moon, it’s waxing; as it reverts back to a new moon (perfectly aligned with the sun on the same side of Earth, such that we can’t see the moon in the night sky), it’s waning. It’s a beautiful complication, and while it has little utility, neither do wristwatches; it’s a great way to feel connected to our less developed ancestors, whose lives more completely revolved around lunar cycles.