Complications · Lesson 10 of 10
The Moonphase
The most romantic complication: a tiny moon on your wrist tracking the real one in the sky.
The moonphase does nothing practical whatsoever — sailors and farmers stopped needing one generations ago — and that's precisely why collectors adore it. It exists purely to put a slice of the night sky on your wrist. It is watchmaking as poetry.
The disc carries two moons and turns once every 59 days — each moon takes one 29.5-day lunation to cross the aperture, and as one sets on the right, the other is ready to rise on the left.
The mechanism
Behind the aperture sits a disc painted with two moons and a field of stars, driven by a wheel with 59 teeth that clicks forward once per day. Two moons across 59 teeth ≈ 29.5 days per moon — a beautiful mechanical approximation of the real 29.53-day lunar cycle. The aperture's curved edges do the shading: as a moon slides past them, they carve it into crescents.
The half-day problem
The real cycle is 29.53 days, not 29.5. That missing 44 minutes per month means a standard moonphase drifts one full day from the sky every ~2 years and 8 months — the owner nudges a corrector and moves on. High-end “astronomical” moonphases add gearing (122 or 135 teeth) to push one day of error out beyond a century. Entirely unnecessary. Completely wonderful.
Setting one
Look up the date of the last new or full moon, set the disc to it (full moon = fully centred in the aperture), then advance one click per day since. And like the date, moonphase correctors have their own midnight danger zone — check the manual before pushing buttons late in the evening.
Knowledge check
3 quick questions to lock in what you just learned.