Moonphase
A complication that displays the current phase of the moon on the dial, through a crescent-shaped window with rotating moon disc.
A moonphase is a complication showing the current phase of the moon (new, waxing, full, waning) on the watch dial through a small aperture.
How it works
A disc with two moons and intervening stars rotates beneath the dial, visible through a crescent-shaped window. The disc advances one notch per day via a 59-tooth wheel. Each moon appears, waxes, becomes full, wanes, and disappears over 29.5 days, the synodic lunar month.
The standard 59-tooth wheel is accurate to about one day every 2.5 years. More precise moonphases (132-tooth or 122-tooth) reduce the error to one day in 122 years or longer.
Aesthetic role
Moonphase displays are typically the most decorative part of a dial. Hand-painted stars, sun-and-moon engravings, mother-of-pearl backgrounds. They are a craftsmanship showcase as much as a function.
On dress watches
Classical moonphase placement is at 6 o'clock, often combined with a date sub-dial or a day-date display. The complication is associated with high-end dress watches: Patek Philippe Calatrava 5396, A. Lange & Söhne Lange 1 Moon Phase, Blancpain Villeret, Jaeger-LeCoultre Master Ultra Thin.
Astronomical variants
The basic moonphase shows phase only. Astronomical complications extend this:
- Lunar age: a separate indicator showing days since new moon.
- Equation of time: difference between solar time and mean time.
- Annual calendar: date that correctly handles 30-day and 31-day months automatically.
- Perpetual calendar: full calendar including leap years.
When a moonphase appears alongside a perpetual calendar, the watch is one of the most complicated pieces in the dress category.
Related: perpetual-calendar, tourbillon, patek-philippe.