How it works
The mechanism is mechanically simple: a wheel with two moon images painted on it sits behind the dial, partially visible through a cutout. A daily-advance finger moves the wheel by exactly one tooth per day. The wheel has 59 teeth, sized so that 59 daily advances equal two full lunar cycles (the moon orbits Earth in 29.5305 days, so 2 × 29.5 = 59). After 59 days, the same moon image is back at the cutout; after 29.5 days, the second moon image is.
Standard vs astronomical accuracy
The 59-tooth standard mechanism is accurate to ~44 minutes per lunar cycle; this accumulates to a 1-day error every 32.5 months, requiring manual correction every 2.5 years or so. Astronomical-grade moonphases use a 135-tooth wheel (or 270-tooth dual-disc design) calibrated to the actual lunar cycle; drift is 1 day per 122 years. Patek 5159, Lange 1815 Moonphase, A. Lange Datograph Perpetual all use astronomical-grade.
Modern variants
3D / spherical moonphase: De Bethune's innovation, which replaces the flat disc with a tiny rotating sphere with palladium and blued-steel hemispheres, showing actual lunar visibility from Earth. Double moonphase: shows the moon in both Northern and Southern hemispheres simultaneously. Dial-position variations: 6 o'clock is most common; some designs put it at 12, integrated with the centre seconds, or as a sub-dial.
What it costs
Entry-tier moonphase: CHF 2,000-5,000 (Frederique Constant, Tissot Le Locle Moonphase, mid-tier Longines). Mid-tier: CHF 5,000-15,000 (Jaeger-LeCoultre Master Calendar, Blancpain Villeret Quantième). Haute-horlogerie: CHF 25,000+ (Patek 5396, Lange 1815 Moonphase, Vacheron Patrimony Moonphase). See /styles/moonphase/ for current moonphase news.