Moonphase Complication Explained
A moonphase complication displays the current phase of the moon through a small window on the dial, cycling from new moon to full and back over roughly 29.5 days. Behind the aperture sits a rotating disc printed with two moons; as it turns, a shaped opening reveals more or less of a moon to mirror what is happening in the sky. It is one of the oldest complications in watchmaking and, for most owners today, one of the least practical and most loved.
- What it shows: the moon's phase, new through full through new again.
- The cycle: a lunar month of about 29.5 days.
- Accuracy: a standard moonphase drifts about a day every 2.7 years; an astronomical one, far less.
How the disc works
The classic mechanism is elegantly simple. A disc carrying two identical moons sits under the dial, and the movement advances it one step every 24 hours through a small toothed wheel. A curved opening in the dial, often with two arcs, frames the disc so that a moon appears to grow and shrink as the disc rotates. Two moons are printed because the display shows one full lunar cycle, then the second moon comes round for the next, keeping the aperture filled throughout.
Most standard moonphase wheels have 59 teeth. That covers two lunar months of 29.5 days each (2 × 29.5 = 59), advanced one tooth per day. It is a clever approximation, but the real lunar cycle is 29.53 days, not exactly 29.5, and that tiny shortfall is where the drift comes from.
Why cheaper ones drift
Because the 59-tooth wheel treats the month as 29.5 days when it is really 29.53, a standard moonphase loses about one full day of accuracy every 2.7 years. In practice you nudge it back into line every couple of years and think nothing of it. High-end astronomical moonphase movements use a much larger gear, sometimes 135 teeth, to track the cycle so precisely that they stay accurate for over a century before needing correction. That precision is what you pay for at the top end; the display itself looks much the same.
How to set a moonphase
Setting one is straightforward, if a little fiddly:
- Find the date of the most recent full moon from a calendar or almanac.
- Using the moonphase corrector (a recessed pusher on the case, or a crown position), advance the disc until the moon sits dead centre in the aperture, representing a full moon.
- From that full-moon reference, advance the disc one step for each day that has passed since. Many movements let you do this by cycling the date forward past midnight repeatedly, which nudges the moon along with it.
- Then set the actual time and date normally.
A word of caution common to calendar complications: avoid using the corrector or changing the date late at night, when the movement's change-over gears are engaged, as forcing them can cause damage. Stick to mid-morning or midday for adjustments.
Is it worth having?
Practically, no. Almost nobody needs to read the lunar phase off their wrist, and a glance at the sky or a phone does the same job. Aesthetically and emotionally, it is one of the most appealing things a dial can do, a small piece of the heavens tracked mechanically on your arm. That is the honest case for it. Buy a moonphase because you find it beautiful, not because it is useful, and you will not be disappointed. Where it sits among the other functions is covered in our complications overview, and if you are drawn to it on a formal watch, the dress watch collection is the natural home for it.
For a first serious purchase, weigh it against simpler options in our first luxury watch guide, and browse the full range in the shop.
Common questions
How accurate is a normal moonphase?
A standard 59-tooth moonphase drifts about one day every 2.7 years, so a quick correction every couple of years keeps it honest. Astronomical versions stay accurate for a century or more.
Why are there two moons on the disc?
The display covers one lunar cycle at a time, so a second moon is printed to fill the aperture for the following cycle as the disc rotates, keeping the window continuous.
When should I not adjust it?
Avoid correcting the moonphase or date between roughly 9pm and 3am, when the calendar gears are engaged. Forcing the mechanism then can damage it. Adjust around midday instead.