What is a Moonphase Watch?

A moonphase watch (sometimes written as moon phase or moon-phase), is a timepiece that displays the illuminated portion of the Moon as we see it from Earth at any given time during the lunar cycle. While there are several types of moon phase displays, the most common one consists of a crescent-shaped cut-out on the dial where we see a depiction of the current phase of the moon, be it new, waxing, waning, full, or anything in between. Join us as we discuss the brief history of the moonphase complication, explain how a moonphase watch works, and outline some of our favorite moonphase timepieces.

Brief History of the Moon Phase Watch

The concept of tracking phases of the Moon dates back as early as Ancient Greece with the Antikythera mechanism, which was used to predict astronomical positions, eclipses, and even the dates of the Olympic Games. More advanced astronomical clocks, which could display the position of the Sun, Moon, constellation, and other planets, appeared throughout other parts of the world over the following centuries.

By the 1300s, mechanical astronomical clocks, which typically displayed the positions of the Sun, Moon, constellations, and sometimes other planets, began appearing in clock towers around Europe. The Prague Astronomical Clock, attached to the Old Town Hall in Prague, Czech Republic, is the oldest functioning astronomical clock in the world, constructed in 1410.

As horology progressed, astronomical indicators such as the moonphase display were fitted into personal clocks, then pocket watches, and finally, wristwatches.

In 1925, Patek Philippe introduced the world’s first perpetual calendar wristwatch in the form of Reference 97975, which was home to, among other indicators, a moon phase display. Other early adopters of wristwatches with moonphase indicators (typically coupled with other complications) included Rolex, LeCoultre (before it became Jaeger-LeCoultre), Audemars Piguet, Vacheron Constantin, and the like.

Rolex’s first watch with a moonphase was Reference 8171, a triple-date watch made from 1949 to 1952 and nicknamed the “Padellone.”

Further reading: Your Go-to Guide to Rolex Nicknames

Today, many high-end watchmakers offer several watches with moon phase indicators — either as a standalone complication or paired with others. The moonphase is often considered the most romantic of all watch complications, mimicking the passage of our Moon as it travels through the sky.

How Does a Moonphase Watch Work?

There are several types of moonphase displays. However, the most common consists of a rotating disk beneath an aperture on a dial, known as the bosom moon phase. There’s also the radial moon phase, which includes a hand to point to the current position of the cycle.

On a bosom moon phase, the disk is decorated with two moons and the edges of the window opening serve to cover or reveal parts of the lunar depictions to mimic the Moon’s current form in the sky. Why are there two moons? Because when one completes its cycle, the second one will appear to begin the new phase.

While the concept may appear simple, executing the phases of the moon mechanically is more complex. Remember, a full lunar cycle takes 29.530588853 days (29 days, 12 hours, and 44 minutes) to complete; not a nice round number.

So, many watchmakers round it down to 29.5 days and rely on a 59-tooth gear (double 29.5) to move the moon phase disk, which will complete a full rotation every two months. This approximation means that a moon phase will have to be adjusted roughly every three years to remain accurate.

However, high-end watchmakers will typically opt for gears with more teeth to reduce the deviation, thus achieving moon phase accuracy for 122 years. In 2015, the IWC Portofino Complete Calendar debuted with a caliber that only requires the moon phase to be adjusted once every 577.5 years!

How To Set a Moonphase

How to set a moonphase watch can vary from watch to watch; as such, it’s always important to refer to the watch’s specific manual. That said, there are some general guidelines that apply to many moonphase watch models.

The first step is to understand what phase of the moon you’re currently in so that you can match your watch to reality. Start with a full moon on your watch, calculate how many days it’s been since the last full moon, and press the moon phase corrector button accordingly. For example, if it’s been 5 days since the last full moon, pressing the corrector five times will get to today.

However, we recommend setting the moon phase to one day before (along with any other calendar indications such as day or date) and then moving the hands to past midnight to advance everything to today. This way, you can accommodate whether it’s currently a.m. or p.m.

As always, whenever you’re adjusting any complication on a watch, make sure you’re not in the “danger zone” (generally between the hours of 9 p.m. and 3 a.m.), which can damage the watch’s movement.

Which Brands Make Moonphase Watches?

As mentioned, most luxury watch brands include moonphase watches in their collections. Some of the best watch brands that make moonphase watches include Patek Philippe, Jaeger-LeCoultre, A. Lange & Söhne, Omega, IWC, Breguet, Glashütte Original, Hermes, Moser & Cie, and Blancpain.

Rolex used to make moonphase watches including vintage references such as the 8171 and 6062 from the 1940s and 1950s, as well as the more contemporary Cellini Moonphase released in 2017. However, Rolex discontinued the Cellini Moonphase in 2023.

Further reading: New and Discontinued Rolex Watches for 2023

While the semi-circular aperture is the most common type of moon phase display, some watchmakers such as De Bethune, MB&F, and Greubel Forsey have more creative approaches with their moonphase watches.

Some of our favorite moonphase watches include:

• Omega Speedmaster Moonphase
• A. Lange & Söhne Lange 1 Moonphase
• Patek Philippe Annual Calendar, Moon Phases 5205G
• IWC Portugieser Perpetual Calendar
• Jaeger-LeCoultre Master Control Calendar
• Breguet Classique Moonphase
• Moser & Cie Endeavour Moon
• MB&F Sarpaneva MoonMachine 2
• De Bethune DB25 Moon Phase

The Allure of a Moonphase Watch

Throughout human history, the allure of the nighttime sky has held a firm grip on our imagination, inspiring countless artistic endeavors and imaginative storytelling. Although we’ve since deciphered the nature of these celestial bodies and even sent people to the lunar surface, the Moon and stars still ignite our sense of wonder. Watchmakers have been especially taken by the waxing and waning of the Moon for centuries, which is embodied in the moon phase complication.

While nothing can beat looking up and seeing the bright white Moon lighting up a dark sky, looking down at one’s wrist to see a realistic representation comes pretty close.

Other Watchmaking Complication Guides:
What is a Grand Complication Watch?
What is a World Time Watch? 
What is an Annual Calendar Watch?
What is a Minute Repeater Watch?
What is a Perpetual Calendar Watch? 
What is a Tourbillon Watch? 
What is a GMT Watch?
What is a Chronograph Watch?