Watch Education The Moonphase Explained

Often described as the least practical and most romantic of complications, the modern luxury watch moonphase indicator encompasses a broad range of styles, complexity, and artistic approaches.

Consider the process of creating a moonphase watch in the year 2016. Product planners first select the principal methods of conveying a given phase of the moon; escalating levels of engineering effort yield commensurate leaps in the precision of individual moonphase systems. Finally, artistic elements can be marshaled to bridge the gap between cold cosmology and the emotional pull of a lunar display.


Before examining a selection of moonphase complications, it is sensible to discuss semantics. By convention, the Swiss watch industry refers to a lunar phase complication as a “moonphase”; this nomenclature is fairly consistent from Patek Philippe at the high end to Frédérique Constant at the entry level. Scientists, astronomers, and even astrologers will refer to the actual stages of lunar visibility as “lunar phases” or “phase of the moon.”

Styles of Moonphase

Most of the luxury watch moonphase complications feature either a “bosom” moonphase (so named for the shape of its aperture) or a radial moonphase with indicator hand. The former is well-represented by the Frédérique Constant Classic Moonphase; when the last crescent sliver of the waning moon disappears from view, the first sliver of the waxing moon is preparing to emerge from the opposite side of the window. Among watches featuring a moonphase display, the bosom style is most common.


“Radial” moonphase displays feature a conventional hand that traces the roughly 29.5 (clarity to follow regarding that figure) full lunar cycle. Due to the inverse relationship of moon age in the northern and southern hemispheres, the radial moonphase easily can be configured with a double-sized hand that indicates the phase and age of the moon in both hemispheres; the 2005 version of the Jaeger-LeCoultre Master Grand Reveil featured such a display.

Technology Behind the Complication

Technology derived from astronomical science allows moonphase complications to achieve a broad spectrum of precision ranging from several years to millions of years. True lunar cycles consist of roughly 29.53 days, and even that figure assumes rounding to the nearest hundredth. This fact lies at the root of moonphase display error and the recent arms race among watchmakers to build the most accurate mechanical watch lunar display.

Regardless of how (e.g., “bosom,” “radial,” or other) a moonphase display is depicted, the most common moonphase display system is based on a 59-tooth driving wheel. This system requires a manual correction roughly once every two-and-a-half years.


Traditionally, the “deluxe” moonphase display – and its 135-tooth driving wheel – has been reserved for perpetual calendars and dedicated celestial complications. This system, which can been seen in action on the new Vacheron Constantin Patrimony Moonphase Retrograde Date, requires a manual correction to the display only once every 122 years. IWC’s Portugieser Grand Complication ups the ante; its moonphase is accurate for a period exceeding 500 years.

Extreme Engineering

Naturally, an industry that breeds engineering extremes offers moonphase solutions to match. Lucerne-based ochs und junior, the current venture of former Ulysse Nardin complication maestro Ludwig Ochslin, offers a moonphase display with legs: 3,417.27 years between corrections. But independent watchmaker and AHCI stalwart Andreas Strehler’s “Lune Exacte” effectively ended the arms race (for now) with a nuclear strike. The ultimate moonphase, the Lune Exacte is designed to operate for two million years between corrections.

Moonphase as an Art

Beyond the display type and underlying technology, moonphase complications offer a slow-moving canvas for practitioners of the watch industry’s rare arts. Efforts in this vein occupy a spectrum of sensibility that ranges from stately luxury to satire.


Given the primarily cosmetic role of a moonphase complication, material choice matters. Vacheron Constantin and A. Lange & Söhne offer moonphase models with solid gold discs. Patek Philippe offers the same type of bosom aperture display while employing sapphire as the disc material and gold as a vapor-deposited surface layer.


IWC, DeWitt, and Omega have offered moonphase discs with aventurine glass (“goldstone”) that creates a background luster and gleam to match the true night sky. The current Omega Speedmaster Professional Aventurine with full aventurine moonphase and dial is the ultimate expression of this practice.


Photo-realistic moonphases have begun to play an increasingly large role in the luxury watch space. The very notion of a photo-realistic rendition of a purely poetic mechanism may seem like a contradiction in terms; consider this take on the moonphase as the first postmodern complication.


Girard-Perregaux, Linde Werdelin, and Omega have jumped into this literalist micro-niche with gusto, but even Jaeger-LeCoultre has tested these waters with a version of its Duomètre Quantième Lunaire. Omega, in particular, has taken this concept to the extreme with its 2016 Speedmaster Moonphase Co-Axial Master Chronometer Chronograph. While the name may exasperate and amuse, this watch features a moon display to match; Omega claims individual Apollo-era footprints can be viewed under extreme magnification.

Perhaps even the photographic moonphase retains a foot in the stolid sobriety of the past; Stepan Sarpaneva, in comparison, casts all conventions aside. The Finish independent graces his moonphase complications with a unique “mug-in-the-moon”; a caricature of his own face.

While moonphase displays are as old as clocks, interest in this beguiling appurtenance has reached an all-time high over the last two model years. Regardless of appetite and budget for display type, craft arts, and underlying technology, the luxury watch marketplace has evolved to offer an option to suit all tastes. In this case, the rising tide – and its lunar locomotive – truly does lift all boats.


About Tim Mosso
Tim Mosso is a lifelong watch enthusiast and historian. He is an active member of the online watch community and his passion for watches is second to none. As a self-proclaimed watch nut, he loves to bring his knowledge of horology across all different platforms.

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