A moon phase watch shows the Moon as it looks from Earth, from new moon to full moon, over a cycle of about 29.5 days. A disc carrying two moons advances one step a day behind a small window; on most watches it drifts by about one day every two and a half to three years. To set it, you start from the date of the last full moon, outside the hours when the date change is engaged.
What a moon phase display shows
The aperture, often a half-circle with two rounded humps, reveals part of a disc decorated with two moons. Day by day the moon appears on one side, grows until it fills the window (full moon), then disappears on the other side (new moon).
It is one of the oldest complications in watchmaking, originally useful to sailors, farmers and night travellers. Today it is loved mostly for its poetry, and because it shows that a mechanical watch can follow an astronomical cycle with just a few wheels.
Some watches show the moon as seen from the northern hemisphere, others from the southern hemisphere, where it waxes the other way. A few models show both.
Reading the four main phases
Between two new moons, the disc passes four landmarks, which also let you check your setting against the night sky:
| New moon | day 0 · empty window, moon not visible |
| First quarter | around day 7 · waxing half-moon |
| Full moon | around day 15 · whole moon centred in the window |
| Last quarter | around day 22 · waning half-moon |
The full moon is the easiest landmark to recognise, on the dial and in the sky. That is why it is used for setting.
How does a moon phase complication work?
The cycle of phases, called the synodic month, lasts on average 29 days, 12 hours and 44 minutes, or about 29.53 days. It is the time between two new moons.
The classic watchmaking solution is elegant: a disc with 59 teeth carries two moons. A finger, driven by the gear train once a day, pushes it forward by one tooth. A full turn of the disc therefore takes 59 days, which is 29.5 days for each moon shown.
| Actual synodic month | about 29.53 days |
| Standard 59-tooth disc | 29.5 days per lunation |
| Difference | about 44 minutes per lunar month |
| Drift | about 1 day every 2.5 to 3 years |
How accurate is a moon phase watch?
Those 44 minutes per lunation add up. After a little over two and a half years, the display is one day behind the sky, and a single press of the corrector brings it back.
Some manufactures offer high-precision or astronomical moon phases with more elaborate gearing. Their drift is far slower, in the order of one day in more than a century for the common designs in this category, and longer still for some. For everyday wear, the standard display is more than accurate enough.
Either way, that accuracy assumes the watch never stops. A moon phase that has been stopped for a week shows a week's delay, however refined its gearing.
How to set a moon phase watch
The principle is always the same: start from a known full moon, then advance the disc one step for each day that has passed. The controls vary from watch to watch, so your manual remains the reference.
- Find the date of the last full moon. Any lunar calendar or almanac gives it.
- Move the hands out of the danger zone. Roughly between 9 pm and 3 am, the date-change mechanism, which the moon phase is often linked to, is engaged. Set the time to around 6:30 am first, not the evening: here is why.
- Bring the full moon to the centre of the window, using the corrector (a small pusher recessed in the case side) or the crown, depending on your model. Each press or click advances the disc by one day.
- Then advance one step per day since the full moon. If it was nine days ago, press nine times.
- Set the date, then the time, checking am and pm: the date should change around midnight, not at noon.
For recessed correctors, use the tool supplied with the watch or a wooden or plastic stylus. A metal point can scratch the case. If a corrector resists, do not force it.
Automatic moon phase watches: what changes
Moon phases exist on quartz, hand-wound and automatic watches. On an automatic, the moon depends on the power reserve like everything else: left off the wrist for more than a few days, the watch stops, and the moon stops with it.
Resetting the time takes seconds. Resetting the date and the moon means finding the last full moon, counting corrections and respecting the date-change hours. On a watch that combines a moon phase with a full or perpetual calendar, it becomes a lengthy job.
Why a watch winder keeps the moon on time
A watch winder keeps the mainspring wound while you are not wearing the watch. It never stops, so the moon phase, the date and the time all stay correct, and the watch is ready to wear.
That is why watches with calendar complications benefit most from a watch winder. We cover this in more detail, including what not to do, in our article on the perpetual calendar watch winder. For rotation direction and turns per day, see our watch winder settings by calibre.
FAQ
Why are there two moons on the disc?
With two moons, a 59-tooth disc shows two cycles per turn, 29.5 days each. It allows a smaller disc and a simple gear train.
Can you set a moon phase at any time of day?
No. Avoid the hours when the date change is engaged, usually between 9 pm and 3 am. Check your manual: some calibres have a different window.
How do you set a moon phase without a corrector?
On some models the moon is adjusted from the crown, in a dedicated position or by turning the opposite way to the date. Otherwise you advance the hands 24 hours for each day to catch up, which is slow: your manual will give the right method.
My moon phase is a day out. Is it faulty?
No, that is the normal drift of a standard disc after a few years. One press of the corrector puts it right.
Can a watch winder harm a moon phase watch?
No. The winder only turns the watch, as your wrist would, and the slipping bridle prevents overwinding. The moon advances through the movement exactly as when the watch is worn. Simply set the winder to the right direction and turns per day for the calibre.