MOONPHASE
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This complication displays the phase in which the moon is located.
Once used in agriculture as a unit of measurement, the moon phase is displayed in the form of a disc, half of which appears through an aperture on the dial.
A lunar cycle lasts on average 29 days, 12 hours, 44 minutes, and 2.8 seconds. Two main mechanisms are used to incorporate this moon phase into a watch movement with varying levels of accuracy.
One system assumes that a lunar cycle lasts 29 days and 12 hours. It has a disc with two moons and a 59-tooth wheel, representing 29.5 days twice, which receives an impulse once every 24 hours. This calculation requires the mechanism to be corrected by one day every 2 years and 233 days.
Another system uses a disc with two moons mounted on a 135-tooth wheel. This mechanism also receives an impulse every 24 hours. This more precise calculation only requires a correction every 122 years and 158 days.
The impulse to the moon is generally given around midday to avoid adding impulse at midnight, which might already count other impulses, such as those for the date, day, and month.
Some watchmakers go as far as to adapt their moon phase displays to the northern or southern hemisphere. Although the dates of the full moon and new moon are the same in both hemispheres, the moon’s phases appear reversed between these two days. Thus, some watches display two moons, one for the northern hemisphere and one for the southern hemisphere.
Occasionally, watchmakers integrate a depiction of the existing night sky into their moon phase discs, and some mechanisms even respect the moon’s inclination in the sky according to the designated hemisphere.