THE BRAIN of a Perpetual Calendar

Achieving a mechanical perpetual calendar construction is no less than a triumph of micromechanics. The watchmaking challenge is the conception of a mechanical method to account for the variations in the length of months throughout every four year period. The secret to all implementations is what is known as a “program wheel”. For Blancpain’s caliber 5954.4 there are in fact two program wheels mounted coaxially. The first turns once every year and carries the program for each of the 30 and 31 day months and a February of 28 days. The second turns once every eight years and programs the two February leap years that would occur in that eight year period.

Before turning to the program wheels and how they both account for the varying lengths of the months and accomplish the change in the month indication, an initial examination of the mechanism should start with the daily change which is commanded by a wheel that turns once every 24 hours (A). That 24 hour wheel carries a cam (B) which engages a lever (C) bearing two “beaks”. For a 31 day month, one of the beaks (C1) advances a 31 tooth date wheel (E) by one tooth every day late at night.

What then occurs for a 30 day month? This is where both the yearly program wheel (D) and second beak (C2) come into play. There is a roller (F), on the long lever (C), which rides along the outside of the yearly program wheel. For a 31 day month, the roller is pushed into a position placing the second beak (C2) out of contact with the 31 tooth date wheel (E). However, for a 30 day month, there is a trough on the yearly program wheel (D) which allows the second beak (C2) to contact a cam (G) underneath the 31 tooth date wheel (E) pushing it ahead on the 30th to the 1st of the following month. The trough is deeper still for ordinary 28 day Februarys. This enables the second beak (C2) to contact the cam (G) on the 28th to accomplish the longer advance to the 1st.

This brings us to the two leap years associated with the eight year wheel (H). This wheel features two protrusions, one for each of the two leap years. These change the position of the first beak, allowing it to add the extra leap year day.

Of course there is one more element central to the perpetual calendar: the change of the month indication to accompany the change in date to the 1st. Mounted atop the 31 tooth date wheel is a cam (I) bearing a beak which engages with a 12 tooth month (of course you knew that) wheel (J) on the 1st.

Despite the complexity of the mechanism (developed and produced entirely by Blancpain), the display of information is clearly distributed across the dial, providing good legibility and aesthetic balance.

MONTH AND YEAR (INCLUDING LEAP YEAR) INDICATION
Conveniently both the month and the leap year cycle are shown on the same display.

MOON PHASE INDICATION
In addition to being the face of Blancpain, the moon phase was the symbol of mechanical watchmaking’s renaissance during the quartz crisis.

AUTOMATICALLY ADJUSTED DATE INDICATION
The very essence of a perpetual calendar is the date display.

ONE OF THE FOUR UNDER LUG CORRECTORS
Each calendar indication can easily be adjusted with a simple press of the finger thanks to the patented under lug correctors.