What Is a Moon Phase Watch, and Why Do People Love Them So Much?

A moon phase watch reminds us that we’re all just spinning through space on this pale blue dot, orbited by a glowing rock, mere cogs in a bigger clockwork.

By Jason Heaton

Published July 13, 2023

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Timekeeping was born out of man’s desire to make sense of the movement of the sun, the moon and the constellations. When the early horologists graduated from the sundial to gears and hands, one of the important events they tracked was the lunar cycle. This is because although the sun’s arc dictates days and seasons, observing the moon was the first method of telling the passing of months.

From new moon to full moon and back again is 29 and a half days, providing a regular pattern that has been the life cycle for any number of things over the centuries, from pagan celebrations to paychecks. Its form and movement can also be represented in miniature, right on your wrist.

What, exactly, is a moon phase watch?

A moon phase watch shows the current phase of the moon as you see it above. As our orbiting pet rock passes across the sky, so too does its miniature version in the watch travel around, displayed in an aperture on the dial. This type of watch is perhaps the most elemental in its graphical representation of the passage of time, as opposed to the more precise and sterile hands and numbers that other complications use. It is a more whimsical and artistic complication, one that is fun to own, linking us with our sky-watching ancestors and a lineage of watchmakers and artisans all at once.

What are moon phases?

A “moon phase” is the lighted area of the moon that is visible as it travels around the earth. The movement of the Earth and the moon’s position relative to us means that we are constantly seeing a different view. While astronomers have several names for the appearance of the moon throughout its cycle — “waxing gibbous” to “waning crescent” and others in between — there are really only four phases you need to know:

gold watch laying flat on a table — The moon phase complication is often associated with traditional watches and formal styles. Shown here, a Patek Philippe reference 2438/1 auctioned by Phillips.

A “new moon” occurs when the moon and sun are in alignment on the same side of the Earth and thus there is no moon visible to us. As the moon travels through its monthly orbit, more of its surface becomes illuminated by the sun — known poetically as “waxing.” The second phase, known as the “first quarter,” is when the right-hand side of the moon is in the sun’s light. Next comes the “full moon” when the entire surface is lit up. Then it moves to the “last quarter” when the left-hand side is visible — waning — until it finally goes back to a new moon again.

This entire cycle of phases takes 29 and a half days, which is the average length of our calendar month. Given the moon’s regular movement, it is logical then that our modern calendar is tied to the phases of the moon. Thanks to some ancient Roman political and religious fiddling, days were added to or subtracted from certain months, which is why our 12 months aren’t all 29 and a half days. Besides, can you imagine starting a new month at noon on day 30?

Why are moon phase watches cool?

The moon has always played a major role in man’s movements and daily existence, from the ebb and flow of tides to religious ceremonies, crop harvests and maritime navigation, which is why it has always been historically important to keep track of its orbit. Though the phases of the Moon were tracked thousands of years ago in crude fashion, the moon phase was commonly found in the more complicated pocket watches in the late 19th and early 20th centuries. When the pocket watch made way for the wristwatch, so too did the moon phase migrate to the wrist. The moon phase really represents the watchmaker’s connection to time itself; it is a sort of miniature version of the gear train that is our universe.

closeup of a moon phase watch dial — Moonphase complications are often discrete, but Arnold and Son’s interpretation emphasizes it in captivating form.

How does a moon phase watch work?

Behind the dial of a typical moon phase watch is a disc with two identical moons on it. This disc rotates one complete cycle every twenty-nine and a half days, with the waxing and waning face of the moon accounted for by the curved edges of the dial aperture. The moon disc is driven by a 59-tooth gear that is advanced one notch by a mechanical finger every 24 hours, thus corresponding to one full rotation for the entire lunar cycle — almost.

In reality, the moon isn’t this cooperative. The true lunar cycle lasts 29 days, 12 hours and 44 minutes, or 29.53 days. While this accuracy is good enough for the average person, watchmakers are a precise lot and not content with rounding up or down. Being off by .03 days per month means that the entire moon phase cycle will be off by one full day every two years, seven and a half months. To overcome the discrepancy between the measured lunar cycle and the actual moon phases, a more sophisticated mechanism was developed that incorporates a 135-tooth gear to drive the moon disc. This improvement increases the accuracy of the movement so that the moon phase complication will only be off by a day once every 122 years, meaning that your grandson might need to do some adjustment, hopefully remembering your generosity when he does.

Our lives may no longer be tied as closely to the moon’s orbit, just as the wristwatch has largely become an anachronism in this age of smart phones and smartwatches. Which makes it all the more fitting to own a moon phase watch: its timeless charm and whimsical artistry remind us that we’re all just spinning through space on this pale blue dot, orbited by a glowing rock, mere cogs in a bigger clockwork.