On the 3399 perpetual calendar clone, the moonphase is a 59-tooth disc driven off the calendar train, and it moves one step per day. You set it with a recessed corrector pusher, one push for each day of the current moon age, and you never press that pusher while the calendar wheels are engaged, roughly 8 p.m. to 3 a.m. Two presses a day apart will not fix a wrong moon; only counting from a known moon age does.
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Prerequisites
- The current moon age, meaning days elapsed since the last new moon, from 0 to 29. A lunar calendar or almanac gives this in seconds.
- A plastic toothpick or the corrector tool that came with the watch. Skip paperclips, needles, and screwdrivers; they scratch the caseback and chew the pusher tube.
- Good light and a loupe or a phone macro shot, so you can read the aperture edge.
- A wound, running watch, with the crown pushed fully home before you start.
How to set the moonphase
- Look up the moon age for today and write the number down. Guessing produces a disc that is off by a week.
- Pull the crown to the time-setting position and move the hands to 6:00, then push the crown home. Hands pointing down put the calendar train out of engagement and make the correctors safe to use.
- Find the moonphase corrector. On most 3399-family clones it sits on the caseback, in line with the aperture at 6 o'clock. Confirm the mapping before you press anything, since the four to six correctors look identical.
- Press the corrector once for each day of moon age, pausing a full second between presses so the jumper settles. Moon age 12 means 12 presses.
- If you overshoot, keep pressing forward through the remaining days of the 29-day cycle. Never push a moonphase backward; the jumper and the disc teeth are not built for it.
- Set the day, date, month, and leap-year indications with their own correctors, working in that order.
- Pull the crown, set the correct time, and push it home.
- Check the result at the next midnight. The disc should jump once and land with a distinct click.
Telling a real moonphase from a day/night disc
- A true moonphase jumps once per 24 hours, close to midnight, and the two moon faces sit 180 degrees apart on the disc.
- A day/night fake either sweeps continuously or doubles as a 24-hour hand. Watch it for two minutes; motion without a jump means it is not a lunar disc.
- Count the aperture travel. A 59-tooth disc advances about 6 degrees per day, so the moon clears the window in roughly 29 days and reappears on the other side.
- Expect drift. A 59-tooth disc tracks 29.5 days against a true synodic month of 29.53059 days, so it gains about one day every two and a half years and needs a single correcting press.
Common faults
- Disc will not advance. The corrector is stuck or the jumper spring is mislocated. Stop pressing and have the movement opened.
- Disc advances two steps. Worn jumper or a bent tooth. This needs a replacement disc.
- Moon sits half out of the aperture. Set at the wrong moon age. Count again from a new moon.
- Calendar and moon drift together. Normal on a clone whose moonphase shares the date train; reset the full calendar, then the moon.
Keeping it correct
Reset the moonphase after any period on the winder, after a battery or movement service, and whenever the watch stops for more than a day. Store it with the crown home and the correctors untouched, and the disc will hold its position for years.