Stellar Aspirations

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Among the lesser-known facets of the famous rivalry between 1920s-era industrialists James Ward Packard and Henry Graves Jr.—two avid collectors who each ordered successively more complicated pocket watches from Patek Philippe—is that both men set their sights on a highly personal complication: They each wanted their nec plus ultra timepiece to feature celestial time with a star chart depicting the sky over their respective homes. While astronomical complications may seem arcane to some, enthusiasts recognize (as Packard and Graves once did) that they touch our primal notions of time, which is why they can still command center stage in the most complicated timepieces today.

Even before watches and clocks had minute hands, 17th-century horologists developed gearwork to display moon phases, useful in an agrarian society lacking artificial lighting, yet hardly straightforward since the moon orbits Earth every 29-and-a-half days. Or 29 days, 12 hours, 44 minutes, and 2.9 seconds to be precise—and accuracy is the common cause of astronomy and watchmaking. Horologists have approached this mechanical challenge by increasing the fineness of their gearing. On older watches, 59 teeth were cut into a wheel with two full moons—each covered in turn—an arrangement that lost a day every couple of years. By increasing the number of teeth to 135, watchmakers have improved accuracy significantly. The large classical lunar display on the Arnold & Son HM Perpetual Moon shows the monthly waxing and waning with a lag of just one day every 122 years. That performance is matched by the Chopard L.U.C Lunar Big Date, which presents the lunar cycle on a novel “orbital” display that shows the various moon phases for both the Northern and Southern Hemispheres.

But 122 years is not the mechanical limit. Ludwig Oechslin has developed an innovative new moon-phase module for the Ochs und Junior Moon Phase that deviates by just one day in 3,478.27 years. Oechslin achieves this feat by using epicyclic gearing, in which a train of gears controls the rotation of a disc inside an internally toothed hub. The epicyclic gearing and selection of gears within it allow him to impart an extremely precise ratio of rotation between the hour hand and the lunar display: The disc revolves once every 29.5306122449 days.

For Oechslin, this mathematical feat is child’s play. He may be best remembered for the trio of astronomical watches that brought Ulysse Nardin’s mechanical watchmaking back to prominence in the 1980s and ’90s. Recapitulating the early history of astronomy, the Ulysse Nardin Tellurium Johannes Kepler, for instance, shows the apparent motion of the sun, the Earth, and the moon from a terrestrial perspective. The Moonstruck watch Oechslin conceived with Ulysse Nardin a few years ago takes up where the Tellurium left off. Earth is fixed at the center of the dial, shown from above the North Pole, with disks for the sun and the moon orbiting around it. Since Oechslin’s mechanism alters the moon phase in tandem with the lunar disk’s rotation, the wearer can see how the relative position of the sun and the moon affects the moon phase. More profoundly, the watch tracks global tides, which are caused by the combined gravitational pull of the sun and the moon on the seas. Of course, the world view shown by Oechslin’s watch is make-believe, a geocentric model predating Copernicus. Yet the watch also helps to explain why that world view was so persistent, since the information on the Moonstruck dial corresponds to Earth-based observations of the sky above.


Other astronomical functions reveal how our very concept of time is itself a matter of perspective. The complication known as equation of time is especially unsettling, since it openly challenges the position of the watch’s minute hand by showing the discrepancy between official “civil” time and the time you’d read on a sundial. The 24-hour increments of civil time never vary. However, if you define noon as the time when the sun is highest in the sky—as you would read on a sundial—it wanders by as much as 16 minutes depending on the time of year. The variation is caused by our planet’s elliptical orbit and the 23-degree tilt of its axis; this deviation can be mechanically simulated by the annual rotation of a kidney-shaped or “ellipsoidal” cam that geometrically represents seasonal time variation. The Audemars Piguet Royal Oak Equation of Time elegantly displays the difference by placing a special blued hand on the same axis as the regular hours and minutes, and orienting the equation to your local solar zenith (which depends on your longitude). The cam rotates the blued hand to the exact minute when your sundial would tell you it is noon.

Blancpain’s Equation of Time Marchante takes another approach to displaying this deviation. A special secondary minute hand shows sundial time throughout the day, rather than only indicating solar noon. Such a running equation is very difficult to execute mechanically because it requires the hand to be oriented by the ellipsoidal cam while it is being rotated by the regular gear train. For that purpose, Blancpain developed a unique set of differential gears. Over the course of the year, the solar hand speeds up or slows down to match the path of the sun.

Sidereal time, the measurement that drove the star charts of rival collectors Graves and Packard,

differs from both civil and solar time because it derives from the stars rather than the sun. Astron­omers define a day as one complete rotation of our planet on its own axis, as measured by the daily crossing of a fixed star infinitely far from Earth. Since our standard 24-hour day accounts for both planetary spin and Earth’s rotation around the sun, the sidereal day is ever so slightly shorter: 23 hours, 56 minutes, and 4.0905 seconds. Watchmakers seeking to show the night sky must build a special gear train that runs at this minutely faster rate.

Sidereal time has been built into the Jaeger-LeCoultre Rendez-Vous Celestial, which builds the daily passage of the constellations in the Northern Hemisphere into a romantic women’s timepiece, and the Van Cleef & Arpels Midnight in Paris, which depicts the starry sky specifically over the French capital.

Patek Philippe has also revisited the celestial realm with the Sky Moon, a complication whose origins are rooted in the watches made for Packard and Graves and which is now the hallmark of some of the brand’s most complex and exclusive watches. Displaying a broad panoramic view of the sky, with an overlaid oval indicating what can be seen from a given latitude and longitude (usually Geneva), the Sky Moon is driven by a mechanism that was first released in the Star Caliber and subsequently miniaturized for the well-known Sky Moon Tourbillon. The mechanism features an aperture in the deep blue dial opening to a view of the moon with both the phase and angle accurately shown. Packard and Graves had the privilege of carrying their portions of the night sky in their pockets. Only now, with the Sky Moon, can you wear all of it, revolving with the planet, on your wrist.  

Arnold & Son


Audemars Piguet








Ochs und Junior


Patek Philippe


Ulysse Nardin


Van Cleef & Arpels