How to Set a Moonphase Complication

Setting a moonphase complication is straightforward if you follow the correct procedure. It requires knowing the date of the last full moon and pressing the moonphase corrector the appropriate number of times. The LoupeLab Moon Phase Visualizer can tell you the current phase and the date of the last full moon.

Before You Start

There is one critical rule: do not adjust the moonphase (or the date, or any calendar function) when the movement is in the process of changing the date. On most movements, the date change begins between 8 PM and 2 AM. Some calibers begin earlier (6 PM) or end later (4 AM). During this window, the switching levers are partially engaged, and forcing an adjustment can bend or break them.

Set the time to 6 AM or any safe hour outside the danger zone before touching any correctors.

The Corrector

The moonphase corrector is a small recessed pusher on the side of the case, typically between the lugs at 10 o'clock or 8 o'clock. It is recessed to prevent accidental activation and requires a pointed tool (a toothpick, a dedicated corrector tool, or the tip of a ballpoint pen) to press.

Each press of the corrector advances the moonphase display by one day. The disc rotates by 1/29.5 of a full moon cycle with each press.

Some watches have correctors that are accessed through the crown in a specific position (pulled out to an intermediate setting). The procedure depends on the specific caliber. Consult the manufacturer's instructions if there is no visible case-side pusher.

Step-by-Step Procedure

Step 1: Find the date of the last full moon. Use the LoupeLab Moon Phase Visualizer, a lunar calendar, or any astronomical reference. Note the date.

Step 2: Set the time to a safe hour. Pull the crown to the time-setting position and set the time to 6:00 AM or any hour outside the 8 PM to 2 AM danger zone. Push the crown back in.

Step 3: Advance the moonphase to show a full moon. Press the corrector repeatedly until the full moon is centered in the aperture. The full moon position is when the complete circular moon image is centered in the opening, with equal amounts of the night sky visible on both sides.

Step 4: Count the days since the last full moon. If the last full moon was on March 10 and today is March 25, that is 15 days.

Step 5: Press the corrector that number of times. Press 15 times for 15 days elapsed since the full moon. Each press advances the display by one day.

Step 6: Set the correct time. Pull the crown to the time-setting position and set the correct current time. When passing through midnight (the date changes), verify that the date display shows the correct date. If it does not, you may need to advance or adjust the date using the date corrector.

Common Mistakes

Pressing the corrector during the danger zone. If the time is set between 8 PM and 2 AM and you press the moonphase corrector, you risk engaging the corrector against the switching levers that are already in motion. Always verify the time is outside this window before pressing any corrector.

Counting from the wrong reference point. The procedure assumes you set the display to full moon first, then advance by the number of days since the last full moon. If you set the display to new moon, you need to count from the last new moon instead. Full moon is the standard reference because it is the easiest phase to identify visually on the display.

Pressing too hard. The corrector requires a firm but gentle press. If it does not engage, the tool is not seated correctly in the recess. Forcing it with excessive pressure can damage the pusher tube or the underlying mechanism.

Forgetting to set the date. The moonphase and date are independent displays. Setting the moonphase does not correct the date, and vice versa. Both must be set separately.

Ongoing Maintenance

For a standard 59-tooth moonphase mechanism, the display drifts by approximately one day every 2 years and 7 months. When the displayed phase is visibly one day off from the actual moon (you can verify with the LoupeLab Moon Phase Visualizer), press the corrector once to re-synchronize.

For high-accuracy moonphase mechanisms (135 teeth or more), the drift is negligible within a human lifetime. No correction should be necessary.

If the watch stops (because the power reserve depleted or the watch was not worn), the moonphase will be wrong by the number of days the watch was stopped. Reset it using the same full-moon-plus-elapsed-days procedure described above.