How a moon phase complication works

A lunar cycle — new moon to new moon — lasts about 29.53 days. Watchmakers model it with a rotating disc painted with two moons, seen through a curved aperture on the dial. The aperture’s “humps” hide part of the moon to fake the waxing and waning shapes.

The 59-tooth trick

The disc has 59 teeth and is advanced one tooth per day. Because the disc carries two moons, each moon takes 29.5 days to complete a cycle — very close to the real 29.53. The small difference adds up to about one day of error every two and a half years.

• Standard 59-tooth disc: off by one day roughly every 2.7 years.
• High-precision 135-tooth disc: off by one day roughly every 122 years.

Setting it

1. 1Find the corrector. It is either a recessed pusher on the case side (use a soft wooden or plastic tool) or a crown position.
2. 2Advance the disc until a full moon sits perfectly centered in the aperture.
3. 3Now advance one click for each day since the last full moon. The moon phase card on the home page tells you exactly how many clicks.