Technical: Moon Phase
The movements of the celestial bodies above us have been a source of intense interest to the human race ever since we first turned our gaze skyward. As we studied the Sun, stars and Moon, we began to realise that how they appeared in the sky helped us to understand the world around us—the length of a day, the right direction to travel, and even the ebbing of the tides.
Just as Earth orbits around the Sun, giving us days and seasons, the Moon is constantly rotating around our planet. This orbit affects how much of the Moon is illuminated to us at different points during its cycle. In its first phase, a ‘new moon’, the lunar surface is completely in the shadow of the Sun. The visibility of the surface will increase all the way through to a ‘full moon’, which appears as a completely illuminated disc, and then gradually back down to a new moon once again.
This cycle takes just over 29 and a half days. If that number seems familiar, it’s because it just so happens to also be the same amount of time as an average calendar month—making the lunar cycle a historically handy gauge of the passage of a year.
A moon phase complication takes the form of a display on a watch’s dial that indicates the current stage of the Moon cycle. The most common type features a disc that depicts two identical moons, with one usually concealed under an aperture. This disc is turned by a wheel encircled with 59 teeth. With every 24 hours that pass, the wheel will turn by one notch; as this happens, the moon depicted on the disc will move slowly across the display, ultimately mimicking the cycle of one lunation.
Or as closely as possible, anyway; this process results in a cycle of 29.5 days, where an actual lunar cycle lasts 29.53 days. It might seem an insignificant discrepancy, but mechanical watchmaking is dedicated to the pursuit of precision. Over time, the difference builds up, meaning that the moon phase will be a day out of synch every two years, seven months and 20 days, and will accordingly require readjusting.
To compensate for this, even more sophisticated moon phase indicators employ a gear driven by 135 teeth. This more precise variant means that the difference between the actual cycle of the Moon and the period measured by the complication is just one day every 122 years.
Compared to other popular complications, like the perpetual calendar or power reserve indicator, the moon phase might not seem like the most useful function for the modern-day wearer—but that’s not really why contemporary watches feature them. As well as being a beautiful example of watchmaking skill, the moon phase also connects us to our past, our planet, and how the orbit of our nearest heavenly neighbour helped society to develop the way that it did.
