moon phase

description by ludwig oechslin

task / idea

a wristwatch incorporating a moon phase indication that reflects the lunar cycle as accurately as possible.

solution / implementation

the sun is represented by a large static dot on the dial at 12 o’clock. the moon appears in a crescent-shaped aperture, which describes an exactly computed arc around the centre of the dial. the widest point of the crescent-shaped aperture is exactly opposite the sun at the 6 o’clock position. rotating beneath the dial is the lunar disc bearing a full moon and, opposite it, a new moon. as the disc rotates counterclockwise, the crescent shape of the aperture in the dial makes the full and new moons appear to wane as they approach the sun and wax as they move away from it; they are at their most visible at the 6 o’clock position. one complete rotation of the lunar disc therefore represents one lunar cycle with all of the moon’s phases. the lunar disc is driven by an epicyclic or planetary gear mechanism. the epicyclic wheels on the underside of the lunar disc are driven from the centre by a finger bonded directly to the hour pipe, their circumferences meshing with a recessed ring gear machined into the underside of the dial. this turns the lunar disk beneath the dial anticlockwise. the central finger engages with a wheel bearing 12 teeth, whose pinion with 14 teeth meshes with a wheel bearing 18 teeth, whose pinion with 14 teeth meshes with the fixed recessed ring gear with its 109 teeth machined into the underside of the dial. this results in a single rotation of the lunar disc in 29.5306122449 days. this is equivalent to a deviation of one day in 3478.27 years.