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The moon phases on your wrist

Dernière mise à jour : 13 janv. 2021

A classic complication, the moon phase reproduces the lunar cycle on your watch dial. This function is almost always found as a complementary feature of perpetual calendar watches.

The moon phases

In astronomy, the moon phase refers to the portion of the Moon illuminated by the Sun visible from Earth. The Moon orbits the Earth in approximately 29.5 days. This cycle is made up of eight phases: new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, last quarter and waning crescent.

The moon phase complication

Timepieces with this complication have a small sub-dial, usually located at 6 o'clock, that shows the different cycles of the Moon. A 59-tooth wheel advances a disc one notch every 24 hours.

It makes a full rotation in 29.5 days, although the actual duration of the cycle is 29 days, 12 hours, 44 minutes and 2.8 seconds (that is, 29.53 days). This difference means that the system must be manually adjusted every two years, seven months and 20 days.

The astronomical moon phase

Meanwhile, the “astronomical” moon phase system found in high-end watches has a 135-tooth wheel. As a result, this moon phase has an accuracy of 29 days, 12 hours and 45 minutes and only needs to be adjusted once every 122 years and 46 days.

After consulting a lunar calendar, the mechanism is adjusted using a push-button. By doing so, you can obtain the number of days between the date of adjustment and that of the last new moon.

In addition to providing astronomical information, the moon phase usually features stunning decoration, making the feature an attractive aspect of any timepiece. This complication gives watch lovers and enthusiasts a poetic escape allowing them to keep their eyes on the stars...


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