A watch with moon phase complication, how does it work?

The stories of astronomy and astrology are closely linked to watchmaking, the first men were able to guess the seasons according to the celestial vault and it is most certainly for this kind of reasons that Stonehenge was built. Sailors also used the stars to navigate and in particular to read the time with various tools.

Today we will look at an elegant and poetic complication: the Moon phase.

History and origin

The first moon dials date back to the first century BC. A moon dial from a Roman ship that sank in the year -60 years was found off the coast of Greece. This mechanism is still being studied and is said to have been used to read the lunar phases. As for modern watchmaking, the first “moon phases” appeared at the same time as astronomical clocks (such as the one in Prague, for example, dating from the early 15th century). In France, you can admire the clock in the Cathedral of Notre Dame in Strasbourg. It is difficult to date this complication on pocket watches and wristwatches. However, we assume that they appeared at the same time as calendar complications.

Usefulness of the Moon Phase

Today we do not benefit from knowing the phase of the Moon, it is mainly an accessory and a complication of great elegance. This complication would be useful for an astronomer for example, but a more technical watch or more precise tools would be preferable. For a “civilian” use it therefore represents a moderate technical utility.

The other utility of these watches lies in the meaning that can be transmitted through these phases of the Moon, a poetic complication which, due to its elegance and mysteries, makes people talk about it.

Of course, some people have superstitions or beliefs about the position of the moon and its age. We can hear that the full moon helps to whiten our clothes, or that it is more difficult to sleep on days of a full moon.

The variants

There are various ways of showing the lunar phases, the best known being a crescent-shaped window rotating a lunar disk at the rate of one notch per day.

More rarely, we can also find a needle indicating the phase of the Moon, a raised moon covered by a cover or even a two-tone circle rotating on itself to communicate the phase of the Moon.

The mechanisms

To understand the mechanism we must first understand the lunar cycle. A lunation lasts approximately 29.53 days, or 29 days 12 hours 44 minutes and 2.9 seconds.

When watchmakers became interested in this complication, they noticed that it was easier to make 59 pinions (29.5 x2) on a wheel than 29.53 pinions. The difference of 0.3 creates a shift of one day every 2 and a half years (2 years 7 months and 20 days).

Each pinion represents a phase of the Moon, each time a day passes, the lunar disk turns one notch and therefore one day in the lunar cycle.

So the lunar disk has two moons, one is visible while the other is hidden.

As usual, watchmakers like to complicate things and more precise moon phases have emerged such as:

• The Astronomical Moon with 1 day deviation every 122 years

• The de Bethune DB28 Digitale with 1 day difference every 1,112 years

• The Christiaan van der Klaauw Real Moon with a 1 day difference every 11,000 years.

You will never notice this discrepancy, but if a solution to our ephemeral life comes to light, we will have watches that are always accurate to the moon!

Conclusion

Moon Phase watches are captivating because of the originality and beauty of this complication. Although they are partially obsolete and outdated, they remain one of the most popular watch complications. They are relatively accessible and will not leave anyone indifferent, whether they are an amateur or an initiated watchmaker.

SOURCES:

• Astronomical clock of the Notre-Dame Cathedral of Strasbourg : https://fr.m.wikipedia.org/wiki/Fichier:Cathedrale_de_Strasbourg_-_Horloge_Astronomique.jpg

• Girard Perregaux watch: https://www.girard-perregaux.com/row_fr/99290-53-653-ba6a.html