Moonphase

A moonphase is a watch complication that displays the phase of the moon on the dial, tracking the roughly 29.5-day lunar cycle: a painted or metal moon on a disc advances through an aperture as the days pass, waxing, full, and waning in miniature. It is among the oldest and most poetic complications — and, once a watch stops, one of the most tedious to set right again.

How it works, and why it drifts

The moon disc carries two moons and is driven from the calendar gearing, advancing one step each day (or two half-steps, depending on the movement). The true lunar cycle is 29 days, 12 hours, 44 minutes and change — not a tidy 29.5 — so a standard moonphase movement gains a day of error roughly every two and a half years. High-precision moonphases, geared far more finely, stretch that to a century or more, but the principle is the same: the display is a slow clock within the clock, and it only stays right while the watch keeps running.

That is the catch. Setting a moonphase means knowing the current phase from a lunar calendar, then advancing the disc — often with a recessed pusher, a loupe, and patience — to the matching day, usually after first setting the date and time correctly because the mechanisms interlock. Many moonphases also cannot be corrected during the hours around midnight, when their gears are engaged. Owners of these watches learn quickly that letting one stop is a chore deferred, not avoided.

What it means for your winder

Along with the perpetual calendar, the moonphase is the classic reason owners keep a watch on a winder even when they rotate several pieces: as long as the movement runs, the moon stays true, and the watch is ready to wear with everything — time, date, moon — correct at a glance. The settings are ordinary: find the movement's turns per day and direction in our TPD database. Moonphase movements wind exactly like their base calibres; it is the consequence of stopping, not the winding, that is special.

Frequently asked questions

How accurate is a moonphase?

A standard one drifts a day in about two and a half years — invisibly in daily use, and corrected in moments at a service or with the pusher. Precision moonphases extend that to 122 years or more, but no owner wears a watch uninterrupted that long.

Can I set the moonphase any time of day?

Follow the maker's manual: many movements forbid correction while the calendar is changing, typically late evening to the small hours, because the gearing is mid-jump and forcing it can damage the mechanism.