A moonphase watch has a small window, usually a half-circle near 6 o'clock, that shows the current phase of the moon as it appears in the sky. A disc printed with two moons rotates slowly behind the window, so you see a sliver, a half, a full moon, and back again over a lunar month of about 29 and a half days. It is one of the oldest complications in watchmaking, and one of the least useful, which is exactly why people love it.
How it works
The moon disc is driven off the date mechanism. A gear advances the disc one small step once a day, and because the disc carries two moons, one full rotation covers two lunar cycles. Most affordable moonphase watches use a 59-tooth wheel, which treats the lunar month as exactly 29.5 days. The true cycle is slightly longer, so a simple moonphase drifts by about a day every two and a half years or so. Higher-end movements use more complex gearing to hold accuracy for far longer, but on an affordable watch you will simply correct it now and then.
Quartz moonphase watches exist too, and they work the same way from the wearer's side: a disc, a window, a corrector.
How to set it
Setting a moonphase is simple once you know the date of the last full moon. Any lunar calendar online will tell you.
- Find the corrector. On most affordable moonphase watches it is a small recessed pusher on the side of the case, pressed with a pin or the tool that came with the watch. On some, the moon is set through a crown position instead. Check the manual.
- Set the moon to full. Press the corrector until the moon sits dead center in the window. That is full moon.
- Count forward. Work out how many days have passed since the last full moon, then press the corrector that many times. Each press is one day.
- Set the time and date as normal. Do the moon first, then the date, then the time.
Avoid pressing the corrector during the hours around midnight when the date is changing. The mechanism is engaged and forcing it can damage the gears.
Why it matters
It does not, in any practical sense. Nobody plans their week around a moonphase window. What it does is give a dial a focal point and a bit of romance. On a dress watch it signals that the watch is about something more than telling time. That is the whole pitch, and it is enough.
Where you see it
Moonphase is a dress-watch feature first. It appears on classic three-hand dress pieces, on calendar watches that also show day and month, and on a lot of affordable Chinese-made automatics that use it as a headline feature. In the budget range it is nearly always paired with a date, since the two share gearing.
Trade-offs
The extra mechanism adds thickness and another thing to set when the watch stops. The window can crowd a small dial. On cheap watches the moon disc may be a plain printed disc that looks flat up close. And on some inexpensive models the "moonphase" is purely decorative and does not move at all. Read the description carefully.
What to look for when buying
- A moon that actually advances. If the listing does not mention setting it, ask.
- A corrector you can reach. A recessed pusher is normal. A setting tool should be included.
- A readable disc. Contrast between the moon and the sky matters more than the artwork.
- Thickness. Moonphase adds height. Check the spec if you want it under a cuff.
If you want one, start in our dress collection, where moonphase models tend to live.