Moon phase

A complication and display of the different phases of the moon. One complete lunar cycle is divided into four phases: first quarter, full moon, last quarter and new moon.

The display is governed by a disc with 59 teeth (2 x 29.5), which advances the moon phase once per day. The moon phase displayed thus corresponds to a lunar cycle of 29.5 days, while the exact duration of a lunar cycle is 29 days, 12 hours, 44 minutes and 2.8 seconds. The mechanical display thus has an offset that reaches one complete day every 2 years, 7 months and approximately 20 days.

So-called "astronomical" moon phase complications are also available. These use a disc with 135 teeth, which allows a precision of 29 days, 12 hours and 45 minutes for the lunar cycle, meaning that the discrepancy is reduced to one complete day every 122 years.