What does “Moon Phase” mean?

A moon phase complication shows the current phase of the moon, usually through a disc with one or two moons moving behind a curved opening in the dial. A standard moon phase is based on a lunar cycle of about 29.5 days and drifts by roughly a day every two and a half to three years; more precise versions are far more accurate and need correcting only after many decades.

Moon phases are often paired with calendars and give a dial a classic look. When buying, check that the display advances correctly and that the moon disc is clean and undamaged. Setting is usually done with a small pusher or through the crown, and it helps to know the current moon phase to set it accurately. In listings, say whether it is a standard or a precision moon phase, since some buyers look specifically for the more accurate type.

Example: 'Moon phase set and running, disc in excellent condition.'