The development of a perpetual calendar for the LANGE 1 challenged Lange’s engineers and watchmakers in many respects. To preserve the characteristic design with the asymmetric arrangement of the time indications, the calendar displays had to be repositioned. The result was a totally new display concept with a peripheral month ring as its most salient design element.
A totally new mechanism was needed to execute the switching process for which the month ring has to traverse a relatively long distance because of its unusual position on the periphery. Thus, contrary to conventional mechanisms, the durations of the months are not coded on a separate programme wheel: they are sampled directly at the month ring. It features a contour with twelve recesses on its bottom side. A lever glides along this contour and “reads” the depths of the recesses. The deeper the recess, the shorter the month, and the earlier the switching process is triggered for the next month jump. In February, the extender of the sampler lever concurrently makes contact with a cam beneath the leap-year display. If it comes to rest against the larger radius of the cam, this means that the mechanism has to switch after 29 days (leap year). Otherwise, if it senses the smaller radius, the switching action in the month of February takes place after 28 days.