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.

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Watch this other video [    • Ferdinand Berthoud FB 1L: Blender mod...   ] which presents an
animated model of the Ferdinand Berthoud FB 1L moonphase complication.
It shows an animated 3D model created in Blender from this CAD assembly
made with ANSYS SpaceClaim.

Check out and download this 3D model on Sketchfab [ https://skfb.ly/6YIKI ]