Many mechanical watches with a calendar function include a moon phase function. While beautifully done, it is obvious to ask what is the point of this function? Why does anyone need to know what phase the moon is at? The results of a recent scientific research project may just give us a new way to use them.

A “moon phase function” is where the dial contains a display showing the phase of the moon as it will appear in the night sky. The most common form of this function, or complication, is a small drawing of the moon and some small stars to indicate the night sky shown through a window on the dial.

Jaeger LeCoultre Moonphase

What is a Moonphase?

Firstly, it is worth remembering what moon phases are. The moon circles the earth every 27.3 days. The earth circles the sun every 365.25 days. The moon moves in and out of the sun’s shadow relative to our viewpoint on Earth. Sometimes we can see the moon reflecting the Sun’s light towards us. When it does this 100% we call it a full moon, and at other times we can see a mix of sunlight and shadow on the moon’s surface, we call these Crescent or Gibbous moon, depending on the point in time. When there is a new moon we cannot see the moon at all, it is not reflecting any sunlight at all towards earth. The phases of the moon take 29.45 days to go through a full cycle.

Source: Wikipedia

Moon phase watches have been created for centuries. The very first moon phase functions were included in astronomical clocks found in public town squares, especially in Europe. As clocks were made smaller and became used in homes, fancy clocks often retained this moon phase function. This was for exactly the same reason as we see moon phase functions in watches today. The watchmaker wants to demonstrate they are able to make a more complex and interesting movement and a more interesting dial. It is a function included for design and to make a dial more interesting, rather than to be used.

The Orloj Clock in Prague, Czechia, a public astronomical clock dating from the 14th century. The tiny silver ball shows the rotation and phases of the moon.

Smart Watches To Track Your Sleep

Recent trends in watch wearing have seen a boom in smart watches and fitness watches. One of the reasons for this is that people like to track their health, and to do this many people like to track their quality of sleep. Many devices and smart watches have sleep tracking functions, as this review site highlights. A fascinating knew research project by some scientists in Argentina has suggested that people’s sleep may be impacted by the phase of the moon. This is the first time any scientists have shown that these effects with a carefully designed test. The summary findings were that people sleep less during the period right before the new moon, so when the moon would be brightest in the sky in the evening, and sleep more during the period of the New Moon, when the moon can provide no illumination. Crucially, the test showed that this occurs even for people using artificial light.

Smart Watch photo from UnsplashWhile it is too early to say while this fascinating piece of research could be repeated, it does finally suggest a use for a mechanical moon phase. If you’re lucky enough to have a moon phase watch, and it is set correctly to your location, then the moon phase will tell you whether or not to expect a longer or shorter period of sleep. This could be useful if you had to schedule something important early one morning, or wanted to know when you might get a bit more rest.

Hermès Arceau L’Heure de la Lune

There are many different forms of moon phase display and two of the most interesting, distinct, and unusual displays in recent years have come from the luxury French fashion company Hermes, and the British watch company Christopher Ward. Both watches use a huge display of the moon and rotate the discs around the whole of the dial. Both watches have proven very popular, with Christopher Ward saying it is their best ever selling watch.

Christopher Ward Moonglow

For those who have further interest, find the link to the scientific paper explaining the study here. An easy to read article from the Guardian newspaper explaining the findings is here.

We at The Watch Collectors’ Club have always enjoyed the moon phase as an interesting design feature and symbol of good mechanical watch making. We do not believe there is a best moon phase watch, and their inclusion brings charm to a dial in many different ways. We are very glad this ancient function might have a fascinating modern day use.

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