How to set a moonphase watch in three steps

Zach Blass

While I certainly believe I am schooled up on a fair many things pertaining to the watch world, the smartest watch enthusiasts understand that there is always more to be discovered and learned within this space – and that is in part what makes watch enthusiasm so fun. To be honest, setting a moonphase complication on a wristwatch was never really a concern of mine until I acquired my Breguet Classique Perpetual Calendar ref. 3057. I had some idea of what was required, but when the time came to set my own, I wanted to be 100% sure. Depending on the model you have and the level of complication it boasts – because you could have a moonphase, a triple calendar, an annual calendar, or a perpetual calendar – some watches will have the complication set via a pusher and others through strictly the crown. Furthermore, depending on the movement, some of these indications will be linked or independent. Here is my best advice on how to set a moonphase watch.

Step 1: read the manual (or watch the video)

I’m not saying this to be patronising, but in the modern era, not only will brands include written guides to setting their moonphase watches in the included manual, they may even have videos to walk owners through the process too. The reason this step is crucial is to ensure you are fully across the nuances of setting your particular moonphase complication, for example, the particular danger zone for a given watch where you should not adjust it. While many moonphase watches will have a similar setting process, it is best to be sure before you find yourself staring at a hefty service bill. The safest thing you can do is follow a brand’s instructions within a manual, and you can always enlist a watchmaker at a boutique or authorised dealer to assist you should you like.

Step 2: find a clear reference guide that displays the current phase of the moon

Because they are our colleagues in the horological trenches, not our enemy, I have no issue revealing that I have the Hodinkee app downloaded on my phone. I often open the app to use the clock feature nestled in the top left corner of the screen. Here, you not only have a clock to synchronise your watch with, but also a visual of the current phase of the moon. There are also websites that offer such visual guides, but this approach is not the most accurate way to set a typical moonphase watch.

The more precise and proper method to set your moonphase is more mathematical than simply aligning imagery by eye. What the Hodinkee app does not display is an indicator of how many days until the next full moon. I personally use the timeanddate.com lunar calendar so that I can see the precise dates where the Moon will be in its various phases – and this is displayed in your local region as well. With this information on hand, as Vacheron Constantin suggests in the video above, you would then increment the Moon phase to a stage in which both moons are perfectly hidden beneath the aperture. Then, using a lunar calendar, determine how many days have passed since the last new moon, and increment the indication forward accordingly.

For example, were you setting your Moon phase on July 11th, 2024, the last new Moon would have occurred on July 5th. Therefore, you would increment the moonphase disc six steps forward. You could also use the full Moon as a starting point, working off of centering the moon in the aperture, and then adjusting forward based on how many days it has been since the last full moon.

Step 3: know the accuracy of your moonphase watch

Not all moonphase indications are created equal. The basic design of a Moon phase-indicating mechanism will use a gear with 59 teeth that result in each of the two moons on the moonphase disc making a full pass through the aperture every 29.5 days. But, being exact, a lunar cycle is not technically 29.5 days – this is a rounded number. Therefore, a basic moonphase complication with 59 teeth will be behind by a full day after around 19.5 months (a deviation of .03 days per day). Understandably, most are not too fussed with their moonphase indicating the exact phase of the moon, but should you want to keep things precise you will need to make periodic adjustments.

This is why, in an horological arms race of sorts, high-end watch brands have worked to develop more accurate moonphase indications with more precise and finer gear trains. The record holder is currently IWC’s Portugieser Eternal Calendar with a double Moon display that, once set, remains accurate for 45 million years.

How to set your moonphase watch recap

Putting all of the steps together, you can normally easily set a moonphase complication by yourself.

1. First, ascertain the nuances of setting your particular watch in order to properly and safely set your moonphase via the brand’s manual.
2. Then, pull up a lunar calendar that can inform you of the phases of the Moon in your region.
3. Lastly, once in the safe zone, increment your moonphase to the proper starting point and increment the disc forward the number of days that have passed since that phase – whether a full or new Moon. Depending on the accuracy of the complication and your personal preference, you may decide to periodically readjust your moonphase.