Moonphase Complication
The synodic lunar cycle averages 29 days, 12 hours, 44 minutes and 2.8 seconds. A standard moonphase mechanism approximates this with a 59-tooth wheel that advances once every 24 hours, producing a cycle length of 29.5 days - accurate to within roughly half a day per lunar month, which accumulates to a one-day error after about two and a half years.
Standard vs astronomical accuracy
A 'standard' moonphase requires manual correction roughly every 32 months. An 'astronomical' or 'precision' moonphase uses a 135-tooth wheel and additional gearing to extend the correction interval. Patek Philippe's secular moonphase used in the Ref. 5102 and the Star Caliber 2000 advances at a rate accurate to one day in 122 years.
Visual conventions
Moonphase apertures typically depict two moons against a starry blue background, with each moon completing a half-cycle over 29.5 days. Some independent makers (Lange, Arnold & Son) instead use a hand-pointer or a fully exposed 3D moon rendered in solid gold or meteorite.
Key facts
- ▸Synodic lunar cycle: 29.530589 days.
- ▸Standard 59-tooth moonphase drifts ~1 day every ~2.7 years.
- ▸Patek's secular moonphase: ~1 day deviation per 122 years.