A moonphase is a complication in horology that displays the current phase of the moon as seen from Earth. This feature is typically found on the dial of a watch and is designed to track the lunar cycle, which lasts approximately 29.5 days. The moonphase complication is often represented by a small aperture or sub-dial that shows a rotating disc with two moon images, simulating the waxing and waning of the moon. As the disc rotates, it reveals the current phase, such as new moon, first quarter, full moon, or last quarter.

Moonphase watches are appreciated not only for their aesthetic appeal but also for the technical craftsmanship involved in their creation. The mechanism requires precise engineering to ensure the disc advances correctly each day, maintaining accuracy over long periods. Historically, moonphase complications were essential for navigation and agriculture, providing a visual representation of the lunar calendar. Today, they are valued for their elegance and the romantic notion of connecting the wearer to the celestial rhythms. Collectors and enthusiasts often seek moonphase watches for their intricate design and the skill required to integrate this complication into a timepiece, making them a cherished addition to any horological collection.