Moonphase Watches Explained: How They Work and What to Look For

A moonphase watch shows the current phase of the moon in a small aperture on the dial. It is one of the oldest and most romantic complications, and it is popular because it adds life to a dial without making a watch complicated to use.

The mechanism is simple. A small disc carrying two moon images turns by one tooth each day, driven by the movement. A full revolution of the disc takes about 59 days, which is two lunar months, and each moon passes through the aperture once per cycle. The real lunar month is about 29.53 days, but the mechanism cannot rotate by a fraction of a tooth, so a typical moonphase uses a 59-tooth disc that approximates the cycle.

That approximation matters. A standard moonphase drifts from the real moon by about a day every couple of years, so after two or three years it needs a manual correction. More refined versions use gearing that approximates the cycle far more precisely. Some high-end watches are accurate enough to need correcting only once in many decades or even centuries.

Setting it is usually done with a pusher on the case side or by turning the crown in a particular position. A common rule: do not change the date or the moonphase when the time is between about 9 pm and 3 am, because the date and calendar gears are engaged and forcing them can cause damage. Always check the owner's manual for your model.

What should you look for when choosing one? Consider legibility, since a moonphase aperture is small and some dials make it hard to read. Check whether it is paired with a date or a calendar, which some people find more useful. Finally, decide how much you value accuracy. A simple moonphase is a charming detail, while a precision moonphase is a technical statement.