29 February 2020 marks the date again: once every four years, we have one extra day in our calendar. Since 2001, A. Lange & Söhne has launched eight timepieces that consider this. And there’s more: the Saxon manufactory has developed a fundamentally different technical approach to the perpetual calendar mechanism – the A. Lange & Söhne Leap Year. DATOGRAPH PERPETUAL TOURBILLON (calibre L952.2); the hold-down device is being fixed with a screw. Its function is to hold the leap-year disc in its position.
The year 2020 is the beginning of a new decade. However, true watch connoisseurs appreciate it as a leap year, a year that separates perpetual from annual calendars. The anticipation starts on New Year’s Eve when, at midnight, the leap-year indication switches from 3 to 4. From then, it takes 59 days until the perpetual calendar can finally demonstrate what it is capable of. It will correctly reproduce the transition from 28 to 29 February, but subsequently also switches directly to 1 March at midnight of the leap day. This may sound simple, but it is a formidable technical challenge. It requires the development of a mechanical programme that maps the different durations of all 48 months across the entire four-year cycle.
TOURBOGRAPH PERPETUAL “Pour le Mérite” (calibre L133.1); the bearing of the programme wheel is being lubricated with oil. It goes around once in four years and contains the information as to the different lengths of the 48 months in the four-year cycle.
The classic way
The innovative way
LANGE 1 TOURBILLON PERPETUAL CALENDAR (calibre L082.1); assembly of the leap-year indication. The numeral disc is positioned on top of the leap-year arbour.
The month ring is driven via its internal gearing. It rotates around its own axis once a year. The inside of the gear rim features a circumferential contour with wavy recesses. A spring-loaded sampler lever glides along this contour and is deflected by a magnitude that corresponds to the depth of the respective recess. The more it is deflected, the shorter the month. In February, an extender of the sampler lever contacts a cam beneath the leap-year disc. This tells the mechanism whether it is a common year with only 28 days in February or a leap year with 29 days.