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Complications: The Moon Phase Watch Explained

While the universe isn’t static, it does take a very, very long time for it to change in any noticeable way. Especially for creatures like us who live for only a tiny fraction of the smallest cosmic moments – relatively speaking. While that makes scientific studies, such as cosmology, a bit more difficult to manage, it can make everyday life a bit easier.

For instance, in ancient times, it was very easy to measure the length of a day by the apparent arc of the sun across the sky. But, measuring longer spans of time was much harder. And that’s where the moon comes in – the Earth’s sole natural satellite, the moon moves slower around the Earth than the rotation of the planet itself. So, when observed from a static point, the light reflected from the sun off of the moon’s surface appears to change over time – causing the phases of the moon. And those phases can be used to measure time across a longer period.

So it makes sense that this ancient form of timekeeping would eventually be incorporated into human devices – namely, clocks and watches. But, a moon phase complication is a little bit more (for lack of a better term) complex than that. As such, we’ve done the legwork for this new addition to our series on watch complications: an explanation of the origins, purpose, and functionality of moon phase watches.

History of the Moon Phase

Ancient Origins

In the modern era, we utilize a calendar system that was developed in the time of the Roman Empire. Originally proposed by and named after Roman emperor, Julius Caesar, this calendar – aptly called the Julian calendar – went into effect on January 1, 45 BC. And, in that time, it hasn’t changed at all.The moon phase complication – as a stand-alone device – actually predates the origins of the clock as we know it by more than 1700 years. Long before that, as far back as 34,000 years ago, we used a different method to tell the passing of long stretches of time: the phases of the moon.

As most folks probably know, the moon changes appearance over time from full to new and back again over the course of roughly 30 days. In fact, that’s the origin of what would later become the Julian months. And since the difference between a day and a year is a ratio of roughly 365 to 1, the month turned out to be a good chunk to break up the difference. As such, it’s been one of the core time-telling units for about as long as mankind has been civilized – and probably longer.

Believe it or not, the moon phase complication – as a stand-alone device – actually predates the origins of the clock as we know it by more than 1700 years. Estimated to have been created around the 2nd Century BC, the Antikythera mechanism (discovered in a shipwreck off the coast of an island of the same name) is an analog computing device developed in ancient Greece to predict astronomical events, such as eclipses and – that’s right – the changing phases of the moon.

It wouldn’t, however, be incorporated into the functionality of actual clocks until much later – originally within the astronomical clocks built into churches and cathedrals, and later into standalone examples. The first instances of moon phase complications being incorporated into standalone clock functionality can be traced to Germany and England in the 16th century, where they were built into grandfather clocks – a luxury reserved for the incredibly wealthy. For a price, people can get their hands on moon phase watches that are accurate for literally thousands of years.The complication was then miniaturized and incorporated into pocket watches and then, not long after, what we now know as the wristwatch.

Now, moon phase watches are so common that watchmakers and brands have developed proprietary versions – their own personal signature on the classic time-telling mechanism. More impressively, perhaps, is the continuing goal to create the most accurate moon phase watch ever made. You see, most standard moon phase complications are rendered inaccurate after approximately two to three years of functioning. However, during the 19th and 20th century, some tinkerers created ones that would last for about 122 years. Now, for a price, people can get their hands on moon phase watches that are accurate for literally thousands of years. The current record holder, however, has a complication that will remain accurate for over 2 million years.

Phases of the Moon

The Synodic Period

To understand how a moon phase watch works, it’s actually quite important to understand how the moon itself works. There are several defined periods for a lunar month, including synodic, sidereal, tropical, anomalistic, and draconic. For our purposes, the only one that matters is the synodic period, as it is the one that is tied directly to the moon phase compication.

The synodic period of the moon is defined as the time it takes for the moon to complete a full cycle of its phases – from new moon to new moon. Now, the moon’s orbit around the Earth takes approximately 27.3 days to complete a full 360-degree arc (this is the sidereal period). The phases of the moon, however, take slightly longer, as the change in position of the Earth relative to the sun must be taken into account. That being said, the amount of time it takes to The synodic period of the moon is defined as the time it takes for the moon to complete a full cycle of its phases.complete a full phase cycle is about 29.5 days. To be more exact, the synodic period is 29 days, 12 hours and 44 minutes (29.53 days).

Since clocks run on a 24-hour scale, there isn’t a simple fix for incorporating a fractional period into the functionality of a watch. Also, 29.53 days don’t translate into watch functionality, because there are no partial days. Every day is 24 hours without fail. This is observable in the Julian calendar as leap years – every four years a day must be added to the calendar to cope for the fact that the Earth actually travels around the sun in 365.25 days. Unfortunately, watches – complex mechanical devices – can’t simply have time added and taken out of their functionality. So watch makers had to get a little creative.

How Does It Work

An Imperfect Time

The first of the moon phase complications actually functioned relatively simply. A geared dial was added beneath the main dial of the clock, which had 59 teeth that would advance via a single mechanical finger one notch every 24-hours – very nearly mimicking one full lunar synodic cycle. “Very nearly,” however, is the key phrase here – as the slight inaccuracy actually causes this complication to go out of sync every couple of years.The slight inaccuracy actually causes this complication to go out of sync every couple of years. Which means it has to be adjusted at fairly regular intervals. And that’s a pretty big flaw in the watch world.

So, some clever minds had the idea to increase the number of teeth to 135 – which guarantees accuracy for about 122 years. For those counting, that’s longer than nearly any human has ever lived (the one exception: a French woman named Jeanne Calment who lived to be 122 years and 164 days). And for those who like to pass down their heirlooms for generations, it means an approximate ratio of less than one correction per owner. Most high-end lunar phase watches still use this particular movement in their functionality.

From there, a few smaller companies and obsessed makers have tinkered further, figuring out how to produce accuracy tolerances in the thousands of years. None, however, compare to the work of Andreas Strehler. Obsessed with accuracy, this master watchmaker has produced the Lune Exacte – a moon phase wristwatch with an accuracy of over 2 million years. As of yet, nobody has outdone this example. And while it might seem absurd to create a complication that may very well outlive humanity, the achievement is an impressive one nonetheless.

Moon phase Display

Finally, there’s the question of how moon phase watches are displayed. There are two answers. The short one is, every brand has figured out their own particular and artful way of displaying the lunar calendar – so it’s difficult to peg it down. The longer answer is that they all have a few things in common, no matter the accuracy, who made them, or how old the are.

Every lunar watch shows the phases of the moon through visual mimicry. That means, if you look up at the sky and see a crescent moon, you should look down at your watch and see the same crescent moon displayed upon its face. If you don’t, your watch needs adjusting. There are a couple of ways this could be done. The oldest required a dial with a cutout of roughly 160-degrees (apart from two curved humps jutting into the cutout) and a secondary dial underneath. The secondary dial features an image of the moon painted or gilded onto it. As the secondary dial rotates beneath the first, the moon shows through the cutout with the appearance of whatever phase it is in –They all have a few things in common, no matter the accuracy, who made them, or how old the are. be it a waxing crescent, full, or waning. And when the moon is not visible, that means it’s new.

Nowadays, the same basic principle is followed, though there are some variations in regards to exactly how the phases are displayed. Certain watchmakers have found new and interesting twists to the way the moon is shown to the wearer, but they all get the same point across: illustrate through a graphic which phase the moon is in at any given time. And unless something happens to the Earth or the moon to drastically change their orbital relationship, we imagine moon phase watches will remain largely the same, as well.

Stargazer Timepieces

Seiko SRX003 Premier Kinetic Moonphase Watch: $999
Montblanc Heritage Spirit Moonphase Watch: $14,700
Rolex Cellini Moonphase Watch: $26,750

While a moon phase complication might seem like a relatively easy thing to add to the function of a watch, it’s hardly ever the only additional feature. In fact, it’s typically regarded as a luxury on top of other perhaps more useful complications – like a 24-hour dial, chronograph, diving bezel, etc. As such, watches worth their weight with a moon phase complication tend to be on the more expensive side. And while there are some extremely superb examples available in the thousands of dollars, that doesn’t mean there aren’t any for those with a slightly tighter budget. The three pictured above are all excellent examples of available moon phase watches.

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