A moon phase complication shows how much of the moon is currently illuminated, using a rotating disc behind a shaped aperture in the dial. It is the least practical complication in watchmaking and one of the most enduring, because it is the only one that connects the watch to something outside the wearer's own day.
How it works
Behind the dial sits a disc carrying two identical moons on a dark, usually star-flecked background. The disc advances one step every twenty-four hours, driven by the same wheel that moves the date.
A shaped aperture in the dial hides most of the disc. As it turns, the visible portion of one moon grows and shrinks, then the second moon comes round and the cycle repeats. Two moons on one disc is what allows a single rotation to cover two lunar months.
Why two moons and 59 teeth
The synodic month, one full new moon to new moon, is about 29.53 days. A traditional moon phase disc carries 59 teeth, which is two rounded lunar months of 29.5 days each.
That rounding is where the error comes from. The mechanism runs at 29.5 days against a real 29.53, losing roughly 0.03 days per month. Accumulated, a standard moon phase drifts about one full day every two and a half years, so it needs correcting occasionally even if the watch is never allowed to stop.
More elaborate high-precision moon phases use a gear train that reduces the drift to a day over a century or more. That is a different and considerably more expensive piece of engineering.
How to set it
Wind the watch, then find the date of the most recent full moon. Advance the moon phase, usually through a recessed corrector on the case side or by advancing the date repeatedly through midnight, until the moon sits exactly centred in the aperture. Then count forward, advancing one day at a time, until you reach today's date.
Two cautions. Use the corrector tool supplied rather than anything metal and pointed, and avoid adjusting the date or moon phase between roughly nine in the evening and three in the morning, when the date change mechanism is engaged and can be damaged.
Is it worth having?
It tells you nothing you need to know. Nobody plans their week around the lunar cycle any more.
What it does is give the dial a small piece of theatre that changes without you touching it, and it is genuinely satisfying to glance down and find the moon in the aperture matching the one out the window. A moon phase is bought for pleasure rather than utility, and there is nothing wrong with saying so plainly.
Claude Bernard places its moon phases in Slim Line, where the thin dress case suits the complication.
What to look at when choosing one
The aperture shape. Some are a plain arc, others are shaped with points or a decorated surround. It sets the character of the dial.
The disc finish. Whether the moon is a flat print or has surface detail, and whether the night sky is plain, textured or star-flecked.
Placement. Six o'clock is traditional and balances a dial well. Twelve o'clock puts the moon under the brand name and reads quite differently.
Common questions
How accurate is a standard moon phase?
It drifts roughly one day every two and a half years, because the mechanism approximates the 29.53-day lunar month as 29.5 days.
What happens if the watch stops?
The moon phase stops with it and will be wrong when you restart. Reset it using the full-moon method above.
Can I adjust the moon phase at any time?
No. Avoid the hours around midnight, roughly nine in the evening to three in the morning, when the calendar mechanism is engaged.
Does a moon phase need more servicing?
No. The normal four to six year interval applies, as described in our servicing article.