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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. 

Hermès Arceau L'heure de la lune