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Ferdinand Berthoud - The seesaw moon

Ferdinand Berthoud The seesaw moon

On top of the blue moon, the harvest moon and the moon shadow, here comes a saw-tooth moon. However informally, the term aptly defines the inner workings of Chronometer FB 1L.

Usually, which is to say traditionally, the indications of moon phases and moon age rely on gears. Wheels with abundant teeth mesh in order to compute as accurate a moon cycle as possible. Since said cycle is 29 days, 12 hours, 44 minutes and 2.8 seconds long, managing that specific approximation is a tall order. Watchmakers either rely on mechanisms accurate to within a day every 32 months, or every 122 years, at which point one is allowed to call their moon astronomic. Anything more accurate than that is a rarity. 

Since their creation, Chronométrie Ferdinand Berthoud have decided not to go about watchmaking the usual way, although they fully embrace the traditional side of it. It so happens that their tradition is Berthoud's. And the man, once watchmaker to the French Navy under kings Louis XV and Louis XVI, was an inventor first and foremost. So when they set about adding a new complication to their collection of highly finished chronometers with tourbillon, center seconds, constant force with fusée and chain, and pillar architecture, Chronométrie Ferdinand Berthoud took the inventive way. 

The seesaw moon

Chronometer FB 1L.1 © David Chokron/WorldTempus

Chronometer FB 1L 's moon works with the combination of a cam and a feeler. That cam is a wide ring whose circumference isn't smooth. Instead, it's serrated like the edge of a saw. At 9 o'clock, a spindle, coupled with a feeler, rests against that component. Together, they drive a hand, which goes up and down a small steel plate, upon which the numbers 1 to 14 are engraved. When the moon is on the rise, the moon hand goes up, day after day, until the 14th and a half day or so, when she becomes full. Then that same hand goes down until the new moon, so dark it's invisible. As the feeler goes back and forth against the cam, which is spinning while its profile goes up and down, the moon waxes and wanes.

The seesaw moon

The feeler and spindle sub-part being fitted onto the movement © Ferdinand Berthoud

The imminent arrival of the full moon is also made visible through an opening on the dial, at 5 o'clock, where it is symbolized by a polished gray dot. This construction does away with the constraints of gears and their tedious mathematics, and allows a moon phase indication accuracy equal to a one day of error every 577 years. And that is a luminous record. 

The seesaw moon

Chronometer FB 1L.4 © Ferdinand Berthoud


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Chronométrie Ferdinand Berthoud offers a contemporary take on the work of a master watchmaker. Its exclusive timepieces, developed by today’s master watchmakers, are a tribute to the excellence of yesteryear.

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