The FB 1L complication shows the age and phase of the Moon. The hand moving back and forth is driven by a rack-and-gear system (called the “feeler-spindle” by Chronométrie Ferdinand Berthoud). This Moon complication accumulates only one day error in 577 years of operation.

Astronomers have found that the lunar phases gradually change over the period of a synodic month that is estimated to be 29.53059 mean solar days in length, i.e. 29 days 12 hours 44 minutes and 2.8-2.9 seconds. The FB 1L approximates the synodic month to 29 days 12 hours 44 minutes and 15 seconds (29.5307291666 days). This is accomplished thanks to gears arranged in such a way that the gear ratio from the hour hand to the Moon indicator is: 56699/1920 = (31^2×59)/(2^7×3×5). Note that the moonphase cam (12-pointed star) is driven by a 236-toothed internal gear (236/12 = 59/3).