A moonphase watch is a type of mechanical watch that displays the different phases of the moon as it orbits around the Earth. The moonphase complication has been around for centuries and is considered one of the classic complications in horology.
The main component that makes a moonphase watch work is a disc printed with two moons—one waxing and one waning—that rotates beneath an aperture on the watch face. This disc makes one revolution every 29.5 days, which is the length of a full lunar cycle. As the disc rotates, it exposes either the waxing or waning moon to visually demonstrate the current phase.
Most moonphase discs are graduated into 59 steps, since the lunar cycle is 29.5 days. The disc makes one revolution every 59 steps, so each step represents around 12 hours of time. This allows the moonphase display to stay relatively accurate compared to the actual moon phases in the sky. However, over time the displayed moonphase will drift out of sync with the true phase. So moonphase watches need to be adjusted every couple of years to resynchronize the display.
In addition to the moonphase indicator, many of these watches also have an aperture that shows the moon's age. This is a number from 1 to 29.5 indicating what part of its cycle the moon is currently in. So when the number is low, like 1-5, a thin sliver moon is displayed. And when the number is high, like 25-29.5, the full moon is shown.
The moonphase complication first appeared in clocks in the 1400s and was included in some of the earliest pocket watches. Today, it remains a popular feature on high-end mechanical timepieces from brands like Patek Philippe, Vacheron Constantin, and A. Lange & Söhne. The craftsmanship that goes into making the delicate moonphase disc is a point of pride for many watchmakers.
In addition to the traditional moonphase display, some contemporary watchmakers have created more advanced astronomical complications. For example, some watches can track the lunar perigee and apogee (when the moon is closest or farthest from Earth) or feature a sky chart mapping the night sky on a particular date.
The appeal of moonphase watches is that they combine aesthetic beauty with astronomical functionality. Watch admirers enjoy the artistry and mechanics behind the rotating moonphase disc. The display also imparts a sense of the majesty of the celestial body that has captivated humanity since ancient times.
For many watch collectors, owning a timepiece with a moonphase complication holds special significance. The moon does not actually influence the function of the watch—it simply operates like a miniature orrery, mechanically modeling the lunar cycle around the dial. However, the moonphase can be seen as a symbol of time’s cyclical and inexorable nature, as it steadily changes phases but always returns to full. It represents the interplay between the fixed stars and the ever-changing moon, between constancy and change.
