Moonphase
A moonphase is a complication that displays the current phase of the moon, from a thin crescent to a full disc, through a small aperture on the dial. It is largely decorative and traditional rather than practical, driven by a moon disc with 59 teeth that a finger on the hour wheel advances one step each day. A standard version drifts about a day every two and a half years, whereas a high-precision version stays within a day for more than a century.
At a glance
- Complication type
- Decorative, traditional
- Moon disc
- 59-tooth disc, advanced by a driving finger
- Accuracy
- Standard drifts a day every ~2.5 years, high-precision a day per ~122 years
The appeal of a moonphase lies in tradition more than in function. Through a small aperture on the dial, you watch the current shape of the moon in the sky, its journey from a thin crescent to a full disc. It tells you nothing practical, yet it remains one of the oldest and most poetic details in watchmaking.
How the mechanism works
A classic moonphase is built around a moon disc with 59 teeth beneath the dial. Once a day a driving finger on the hour wheel advances the disc by one tooth, and because the disc carries two painted moons, the roughly 29.5-day lunar cycle is approximated on the dial.
- Moon disc: a 59-tooth disc that a driving finger steps forward one tooth each day
- Accuracy: a standard calibre drifts only about a day every two to three years, and a high-precision calibre only about a day in roughly 122 years
The small error of a standard moonphase needs an occasional manual correction, much as you would reset a date complication after a short month.
Why people choose one
Unlike a measuring chronograph, a moonphase is a purely decorative complication. People wear one for the romance of following the lunar cycle on the wrist, not for any daily reading. For more options, see our guide to the best Japanese watches.
Examples
The Orient Sun and Moon is often taken for a moonphase: its sun-and-moon disc is in fact a day and night indicator, whereas a true moonphase follows the 29.5-day lunar cycle.
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Comparison
A moonphase and a chronograph serve different ends.
| Option A | Option B | Notes |
|---|---|---|
| Moonphase | Chronograph | A moonphase is decorative and traditional and measures nothing, whereas a chronograph is a functional complication that times elapsed seconds. |
Related terms
Watches that show this
Frequently asked questions
Is a moonphase accurate?
No, a standard moonphase is not a precise astronomical instrument. It is largely decorative and traditional. A standard version drifts over time, falling about a day behind every two and a half years, so it needs an occasional manual correction. A high-precision moonphase is far more exact, staying within a day for more than a century.
How does a moonphase complication work?
A classic moonphase is built around a moon disc with 59 teeth under the dial. Once a day a driving finger on the hour wheel advances the disc by one tooth, and because the moon is printed on the disc twice, the roughly 29.5-day lunar cycle is approximated and the current phase shows through the aperture as the days pass.
Is a moonphase useful for daily wear?
No, a moonphase gives no practical daily information, it is a largely decorative and traditional detail. People choose one for the pleasure of following the lunar cycle on the wrist, and it pairs naturally with a calendar complication on a dress watch.