SIHH 2019 is more than a year in our past, and the models debuting there now have a clear public perception, including some going on to be darlings at the 2019 Grand Prix d’Horlogerie de Geneve.

One of my favorites from the year, and the one not coincidentally winning the Calendar and Astronomy prize at the 2019 GPHG, was the Hermès Arceau L’Heure de la Lune. Not only is it a stunning double moon phase watch with wandering dials, but also features an incredible module designed by Jean-François Mojon and his team at Chronode.

It was a clear winner from the beginning with many journalists and fans calling it a show favorite, and out of all the watches seen during there it stood out for its ingenuity and terrific design. Given my proclivity to adoring moon phase watches, it was no surprise that I instantly fell in love with it.

Sadly at that time I wasn’t able to dive into its awesomeness as our intrepid editor-in-chief had put her inimitable talents to covering the watch during the show.

So now I present my deep dive into the technical details, exposing the inner world of the wandering moon phase in the Arceau L’Heure de la Lune, which is definitely deserving of a closer look. Thanks to an in-depth discussion with Thomas Houlon of Hermès and Jean-François Mojon of Chronode, combined with a scale mockup of the mechanism, I was able to wrap my head around the mechanics.

Let’s begin with the basics and then dive into the nitty gritty of what makes the Arceau L’Heure de la Lune so amazing.

Hermès L’Heure de la Lune

L’Heure de la Lune is based on a static base dial that displays two moons: one at 12 o’clock for the southern hemisphere and one at 6 o’clock for the northern hemisphere. This might seem counterintuitive, but it is based on the direction that the area of the moon in shadow passes across the surface during the phases.

The moon phases are created by two wandering subdials that make one full rotation around the dial clockwise once every 59 days, allowing each subdial to create two full displays of moon phases.

This motion sweeps across the moons, mimicking the shadow. For the northern hemisphere at 6 o’clock, that means the shadow (and dial) travels from right to left (as people would see it while facing southward), while the opposite is true for the southern hemisphere where people are oriented northward while viewing, reversing the apparent direction of the phase change.

The subdials are not only home to the moon phases but also house mechanics for a date mechanism in one and the display of hours and minutes in the other. That is where the real fun begins.

The twin subdial carriage contains half of the entire module’s mechanism, squeezed into two disks connected by a central pivot and supported by a ring that encapsulates everything.

This layout resulted in two realities: first, no screws, pins, or assembly components are visible from the dial, making for an extremely clean presentation; and second, miniaturizing the proposed mechanism to fit within those small disks and central pivot was the first headscratcher for development.

Mojon and his team at Chronode needed to find a way to achieve all of the mechanical properties without compromising the look and operation of the display, resulting in at least one new patent and many unique solutions.

The visual requirements added a lot of difficulties to the mechanics as the main limiting factor was to restrain the inputs to the central pivot coming out of the base movement.

The base manufacture Vaucher H1837 movement could have been heavily modified if needed (especially as Hermès is part owner of Vaucher), but restricting the interface to the regular outputs of the movement – hours, minutes, and the date – focused the design of the Chronode module by providing just three points of engagement that allow all of the motion of the moon phase mechanics to be driven reliably.

Module mechanics

The moonphase module supports a gear train that translates the hours into a 24-hour wheel. Attached to that wheel is a sprung finger that once a day advances the carriage via the 59-tooth moon phase wheel.

Opposite this is a star wheel that meshes with the moon phase wheel and 31-tooth date wheel. As the moon phase advances, it drives the star wheel forward, which in turn drives the date wheel at a different rate due to the number of teeth.

This is also how the date-setting function works, though slightly in reverse. As both the date and moon phase utilize the same star wheel to advance simultaneously, the date mechanism drives a central wheel connected to the date indication and the date wheel.

As it drives that, the teeth of the star wheel interact with both wheels, driving the moon phase and the date display, though engagement is in slightly different amounts.

The date wheel engages deeper and advances faster than the moon phase since it has 59 teeth to the date wheel’s 31. And since the wheels needed to be similarly sized to facilitate driving both with one star wheel, the teeth are very different in size to accommodate different depths of engagement.

Further complicating the mechanism is the quick-adjust pusher on the left side of the case, which only advances the moon phase while keeping the date from advancing (because you need the ability to set them separately).

As far as I am aware, this uses a lever to engage the moon phase wheel while keeping the date indication from changing thanks to a small feature on the lever that blocks the date wheel from advancing.

Moving higher

During regular operation, the date and moon phase advance at midnight as the sprung finger on the 24-hour wheel slowly charges and then pushes the mechanism past the spring, keeping the mechanism in position. As the carriage rotates, the dials, which are mounted to it, must slowly rotate counterclockwise to maintain a perfectly vertical orientation.

This is accomplished thanks to an internal ring gear that meshes with a wheel with a small pinion on the outside edge of each side of the carriage. As the carriage moves forward, this pinion drives the dial disk, which rides in a small channel and is retained by four tiny fixed feet and a set of sprung feet that allow screwless installation from the front.

That helps keep the time and date upright during the 59-day journey around the entire dial, but how is the time display driven?

The minute wheel drives the time display using a patented planetary gearing system that also drives the hours as there was no space for a more typical arrangement. This system sees the minute wheel carrying two small pinions that pivot on tiny axes on the face of the wheel.

These mesh with a set of teeth attached to the minute wheel and the hour wheel pinion that rides just above. As the minute wheel progresses, the difference in the number of teeth between those affixed to the minute wheel and those of the hour pinion cause a 1/12 speed progression of the hour hand.

This allows one single input to drive everything on that dial and reduce the interactions that need to happen from a small central axis.

That small central axis also causes concerns about rigidity, so the carriage has a ring around the entire structure that matches the dial diameter. This ring rides just below the internal ring gear and above a track that restricts its ability to flex or twist during a hard shock.

During normal operation it floats just between the two, but in the event of a bump it bounces off the structures around it (it only moves once a day so there wouldn’t be any friction concerns).

This is a simple way to design a very large carriage supported with a small central diameter with less components.

Lunar module assembly

Probably the most impressive aspect about the entire module is that from the front it has no visible screws, pins, gears, or anything to indicate how it functions; instead it allows you to simply appreciate the movement and let it seem a bit more magical.

This is accomplished with a very specific assembly order starting from the base movement.

First the module base plate is attached, then the plate is populated with all the components hidden behind the main dial. Then the aventurine or meteorite dial is attached, leaving only the central-axis components exposed.

Next, the support ring for the carriage is attached, then the carriage, and then the internal ring gear above it. Finally, a cover is applied to hide the ring gear, leaving just the central mechanics visible.

Now the gear trains of the carriage are assembled on each side before the rotating subdials are put in place to cover those mechanics. Finally, the hands are installed and a cap is placed over the central axis to finish the presentation.

Of course, all movements require very specific assembly orders, but this one takes the cake in that a lot of effort was put in to ensure that no assembly method was obvious for the wearer.

I think that says a lot about how Hermès values the story and experience alongside the mechanics. Many brands would be proud to show off a Mojon creation, but that was not the sole purpose of creating this watch.

Instead, the purpose was the overriding Hermès theme of 2019, “It starts with a dream,” which inspired and directed development of all the new creative projects across the entire brand (a staple of Hermès creativity).

With that, the ingenuity of Mojon and Chronode was matched with the creativity of Hermès and the result was one of the best moon phase watches of the decade (and that is saying something).

I truly appreciated the result, but even more that the brand produced a mockup to demonstrate the incredible mechanics at a scale of 20x so we didn’t have to rely on second-hand dictation about functionality.

This is a reason why Hermès is continually one of my favorite small watchmakers: it has a long and developed ideology around creativity that matches perfectly with what I want to learn when new pieces come out.

Hopefully this technical look helped you understand just how interesting the mechanics of the Hermès Arceau L’Heure de la Lune are, providing a glimpse into the thought processes that go into creating something of this magnitude.

It inspires more wonder and passion for these machines when you can fully understand what is hidden beneath the beauty!

For more information, please visit www.hermes.com.

Quick Facts Hermès Arceau L’Heure de la Lune
Case: 43 x 13.27 mm, white gold
Movement: automatic Hermès Caliber H1837 with Chronode module; 4 Hz/28,800 vph frequency
Functions: hours, minutes; date, double moon phase as seen from both the northern and southern hemispheres
Limitation: 100 pieces of each version
Price: CHF 26,000

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