Moonphase

The moonphase (German Mondphase) is the astronomical complication that shows the current phase of the moon on the dial — new, waxing, full, waning. It classically appears in a crescent-shaped aperture at 6 o'clock, often beneath the maker's signature or above a date segment.

How it works

Beneath the dial a disc rotates carrying two symmetrically placed moons and a star-field between them. The disc is advanced one notch per day by a switching wheel. Each moon appears, waxes, becomes full, wanes and disappears over 29.5 days — the synodic lunar month, the time between two full moons as seen from Earth.

The standard switching wheel has 59 teeth, corresponding to 29.5 days because the disc carries two moons. This standard arrangement gains or loses about one day every 2.5 years against the actual lunar motion. Astronomically more precise solutions use 132 or 135 teeth and reduce the error to one day in 122 years or more. The Patek Philippe Ref. 5396 with annual calendar and moonphase uses such a higher-tooth version.

Aesthetic role

The moonphase is traditionally the most decorative spot on a dial. Hand-painted stars, struck or engraved moon reliefs, aventurine or mother-of-pearl backgrounds, high-gloss lacquer — the moon disc is the maker's display surface for craft. At Patek Philippe the disc is traditionally gilt brass with punched stars; A. Lange & Söhne uses a coating embedded in glass with over 1,100 stars.

Which watches carry it

The classical moonphase placement is at 6 o'clock, frequently combined with a date sub-dial, an annual calendar or a perpetual calendar. The complication is closely associated with high-end dress watches:

• Patek Philippe Calatrava 5396, Calatrava 6119R, Nautilus 5712, Ref. 5236P In-Line Perpetual Calendar.
• A. Lange & Söhne Lange 1 Moon Phase, Saxonia Moon Phase, 1815 Annual Calendar.
• Jaeger-LeCoultre Master Ultra Thin Moon, Reverso Tribute Calendar.
• Blancpain Villeret Quantième Complet — historically the model that brought the moonphase back into the mainstream in the 1980s.

At Audemars Piguet and Vacheron Constantin the moonphase appears predominantly inside perpetual calendars, rarely as a standalone complication.

Setting and service

The moonphase is set via a recessed corrector on the case — typically a small pusher hole operated with a stylus. Incorrect adjustment in conjunction with the midnight switching cycle can damage the switching pawl, which is why most manufacturers define a protection window of about two hours around midnight in which correctors must not be operated.

At our atelier in Munich we check three points on intake of a moonphase watch: correct daily advance, centring of the moon disc in the window at full moon, freedom of play in the switching wheel. A moon disc that has not been adjusted for years — common on dress watches that are seldom worn — often has hardened oil on the switching wheel, causing the indication to skip during running operation.