The complication people love
Of all the complications, the moonphase generates the most unqualified affection. It is not useful by modern standards — your phone reports the moon's phase more precisely, and the tidal information it historically carried is available everywhere. But it is beautiful in a way that is difficult to argue with: a small hemisphere of gold or blued steel rising and falling against a star-scattered sky, tracking a cycle humans have observed, named, and found meaning in for as long as we have been human. The complication has been in continuous production since the seventeenth century for the simplest possible reason: people like watching it.
How does a moonphase actually work?
A lunation — new moon to new moon — lasts 29 days, 12 hours, 44 minutes and 2.8 seconds: roughly 29.5 days, but not exactly. The standard mechanism approximates it with a 59-tooth wheel advanced one tooth per day: the disc carries two identical moons 180 degrees apart, so each moon takes 29.5 days to cross the aperture and the wheel completes a revolution in 59. The built-in error — the difference between 29.5 and 29.53059 — accumulates to one full day roughly every two years and seven months, at which point the display shows full when the sky is slightly past it.
The precision tier replaces the 59-tooth wheel with cleverer gearing. The widely used 135-tooth astronomical train cuts the error to one day in about 122 years — the standard at A. Lange & Söhne and the better moonphase calibres generally — which means the watch, serviced on schedule, never needs lunar correction in an owner's lifetime. At the extreme, De Bethune's gearing achieves one day of error in 1,112 years, and a few makers have pushed past even that — precision at this level being, charmingly, pure sport. For most owners of standard mechanisms, the small correction every couple of years is a minor ritual performed at full moon with mild ceremony.
Find the date of the most recent full moon (any almanac or search will give it), use the corrector pusher to advance the disc until the moon shows dead full, then advance the date/time forward to today. The disc only moves forward — there is no running backward — and as with all calendar work, avoid corrections in the hours around midnight when the mechanism may be engaged. Many collectors keep the year's full-moon dates in the drawer beside the winder.
The display: what separates good from great
The moonphase is the most design-sensitive complication because it is entirely visual — nothing to feel or hear, only to look at. The disc's craftsmanship determines everything: the depth of the sky (fired enamel, lacquer, or galvanic blue), the moon's material and modelling (applied gold, turned and polished steel, hand-engraved relief), the precision of the stars, the finish of the aperture surround. On the finest examples — Patek Philippe's gold moons on deep blue, Lange's lacquered discs with their precisely cut star fields, F.P. Journe's blued moon on solid gold in the Octa Lune, De Bethune's extraordinary three-dimensional polished-and-blued sphere rotating in the DB25 — the display is miniature decorative art. On lesser versions it is a stamped disc behind a thin mask, and the difference is visible across a room.
The transition is the technical test: the moment one moon slips behind the aperture's edge as the other emerges. On a quality display the disc runs true and the moon slides behind a crisply defined frame; on a poor one, wobble, misalignment, and ragged aperture edges betray themselves at exactly the moment the display is supposed to look effortless. Aperture shape matters too — the classic double-hump frame is not decoration but the geometry that makes the moon appear to wax and wane correctly as it traverses.
Collecting context
The moonphase rides along in some of the most significant watches in collecting history: the Patek Philippe 1518 and 2499 perpetual calendar chronographs; the Patek 3448 (1962–1981, 586 examples), considered by many the most beautifully proportioned complicated watch ever made; the Royal Oak perpetual calendars that carried the complication into the luxury-sports category in the early 1980s. For all such references, the disc itself is a specific condition consideration — discs fade, suffer in careless servicing, and are sometimes replaced with incorrect later versions, each affecting value. Some ageing is prized: the documented drift of certain vintage Patek discs toward grey-green, a "tropical" patina of the sky itself, commands premiums in the right examples. As ever in vintage, the part's originality outweighs its prettiness.
The moonphase has survived into the twenty-first century not because it is useful but because it is true: it tracks something real that happens in the sky, translated faithfully — if slightly imprecisely — into the movement of a disc behind an aperture. A watch that participates in the astronomical rhythm that governed human life for most of history offers something the phone's perfect data does not. The fact that the phone is more accurate is irrelevant to the experience of looking.