The moon phase complication – The most breathtaking and unnecessary complication of a watch?

Tags: Technology and concepts | Classic

4.10.2021 | 10 MIN

The fascination with the Moon accompanies humankind for hundreds of thousands of years and perhaps even longer. It attracts romantics, dreamers, artists, and also scientists, astronomers, and last but not least watchmakers. That prompted the birth of a complex complication, the Moon phase indicator, which is still very popular in the 21st century. How does it work, and why is it considered as one of the most breathtaking and yet unnecessary watch complications? (Unless, of course, you are a werewolf).

"On the white walls a silvery haze / Poured out by the pale moon’s gaze / But, deep in the tower, is utter dark / For the bright moon’s shining might / Through a narrow window into the long vault / Flitting, is altered into twilit night." Those who remember the Czech poem May by Karel Hynek Mácha know the rest.

Even today, the Moon is still a romantic, mystical, and infinitely fascinating motif. And although we have subjected it to thorough astronomical research over the centuries and even walked on it, many of its secrets still remain hidden from us. So for thousands of years, we have been left with one thing only, to humbly observe it from a distance and reflect on its image in legends, music, literature, art... And of course also on the watch.

Snoopy a Měsíc na hodinkách Omega. Zdroj:

Snoopy and the Moon on the Omega watch. Source:

Moon phases and synodic month

The Moon phases indicate the shapes of the Moon's directly sunlit portion as viewed from Earth. We traditionally recognize four main phases: the new moon, when the Moon is facing the Earth by the unlit side.  After the new moon follows the first quarter moon, which resembles the letter D, as the Moon "grows" into the third phase, the full moon when the Moon appears fully illuminated from Earth's perspective. The full moon is then followed by the last quarter moon, which resembles the letter C, and the Moon "recedes" back to the first new moon phase.

The whole four-phase cycle, also called the lunar cycle, lasts about 29 and a half days, and although there are several types of astronomical cycles that we can distinguish, the most important cycle for the watchmaking world is the synodic month.

The synodic period determines the time (measured from the point of view of the terrestrial observer) it takes for the Moon to go through all the phases before returning to the same position relative to the Sun. The duration of such a cycle varies between 29.27 and 29.83 days, of which the long-term average is 29 days 12 hours 44 minutes 2.8 seconds (let's remember this exact interval for now because it is very important for watchmakers and the Moon phase complication).

Knowing the phases of the Moon was important centuries ago mainly for navigating the sea and determining the tides (at the full moon and the new moon the effects of the Moon and Sun are added and subtracted in the first and last quarter moon, so the height of the tides fluctuates during the month).

The effect of a certain mystical power of the Moon on man and nature was also not entirely negligible, especially if people truly believed in it. Various pagan rituals were also followed according to the phases of the moon, as well as some secular matters (for example, the phases determined when is the best time to sow grain, when is the time to start longer journeys, or even when is the best time to cut the baby's hair for the first time).

Few interesting facts: the word "lunatic", meaning insane, comes from the Latin word "luna", which describes the unpredictable behavior that many individuals often exhibit during the full moon period. In ancient civilizations, it was also believed that the "blood moon" was a harbinger of something horrible. Recent studies have also shown that the phases of the moon affect sleep cycles, and many animals and organisms use the moon cycle as their "internal clock."

What is the moon phase complication, and how does it work?

A complication is usually defined as any above standard watch function showing anything other than prime time. Most complications (such as date, day of the week, GMT, chronograph, ...) display absolutely accurate data that you can rely on during the day and often serve as a purely rational function on the watch (well, we can argue about the chronograph).

Hodinky Vacheron Constantin Egérie Moon Phase. Zdroj:

The Vacheron Constantin Egérie Moon Phase watch. Source:

That cannot be said about the moon phase complication, as it copies the moon cycle faithfully, but nevertheless has a rather symbolic, aesthetic and emotional function on the watch. In modern times, it has no special practical use.

The moon phase complication is among the oldest watch complications in the world and it has a really extensive and interesting history. Before we get to that, let's take a look at how the complication looks on a modern wristwatch. Here, the complication commonly appears as a semicircular section imitating the silhouette of the curvature of the Earth, in which the drawing (or any other representation) of the Moon moves during the synodic period.


This complication can be a great design challenge. For the indicator to look authentic and for it to radiate the desired, almost poetic atmosphere, the use of night sky colors (blue, black, gold, and silver), rich and discreet decoration, and unusual protruding materials predominate in designing the look (the most interesting is the use of a remodeled moonstone in the form of the Moon on the dial).

At the beginning of the synodic cycle (the new moon phase), the disk portraiting the Moon is hidden behind the left edge, during the cycle, the moon travels through the sector to simulate the exact phases in which the Moon is in the sky. For this complication to work correctly, there are two Moons on a rotating wheel, in an opposite position from each other. Technically, the movement of the Moon is constructed in connection with the movement of the hands of the main time, but it is also often associated with the function of the date and day of the week. However, the connection of the moon phase with time is more accurate, because of its constant and smooth movement.

Below the semicircular section on the dial, there is a rotating wheel, on which are usually mounted two round discs representing the Moon. Source:

Below the semicircular section on the dial, there is a rotating wheel, on which are usually mounted two round discs representing the Moon. Source:

Setting up the moon phases is not that difficult. Press the corresponding button, which controls the moon phases on your watch and set it for the full moon phase. Look in the calendar for when was the last full moon and count the days. Calculate how many days have passed since then and press the button as many times as it have been days. Done.

How accurate is the complication?

Older mechanisms of the moon phase complication, as well as those in today's more affordable watches, track the moon phases with a 59-tooth driving wheel, where the wheel moves by one tooth every 24 hours. This driving wheel is connected to a disc, which presents two moons.  Thus, one complete complication cycle lasts 29 and a half days (and not 29 d, 12 h, 44 min and 2.8 s).

Although the interval of 29.5 days is acceptably accurate for most people, a slight deviation from the true time of the synodic cycle causes the phase on the watch to be delayed by one day after two years and 7 months. And as we all know, the watchmaking breed is not the one to simply accept such inaccuracy and let it be.

So the watchmakers worked hard on refining this complication and subsequently managed to create devices that would show the exact lunar phases for the next 122 years. For many, this was indeed sufficient time, but not for all.

The luxury German brand A. Lange & Söhne has developed a complex complication of the Richard Lang Terraluna L096.1 movement, which allows the resetting of the moon phase to be necessary only once every 1058 years (when the interval is shifted by 1 day). The interesting thing about a watch with this movement is that the Moon on the dial also completes the orbit around the disc depicting the Earth once a day. Beautiful...


But even 1058 years were not enough for some people. And so Ochs und Junior, a manufactory of a Swiss family business from La Chaux-de-Fonds, came up with a unique system running on the ETA 2824-2 movement, which extended the accuracy of the complication to more than 3,000 years.

Hodinky Och und Junior. Zdroj:

Ochs und Junior watch. Source:

Dutchman Christiaan Van der Klaauw, who primarily focused on astronomical clocks, took precision to an even higher level when he came up with the Real Moon Joure watch, which had a 3D moon phase indicator on the dial that deviates only once every 11,000 years. At this moment, this is the most accurate mechanical 3D moon phase complication ever created.

Hodinky Christiaan Van Der Klaauw. Zdroj:

Christiaan Van Der Klaauw watch. Source:

And if you thought that 11,000 years are enough for watchmakers... Andreas Strehler took everyone's breath away when he developed the Sauterelle and Lune Perpetuelle watch in 2014, whose lunar phases deviate only once every two million years!



History of moon phase observation

Observing and recording the phases of the Moon is not a matter of the last few hundred years. The first lunar calendar was discovered by archeologists on murals in the French cave of Lascaux – dating from around 15,000 years ago. Though to this day, it is a subject of speculations, if those 29 blots (for 29 days long synodic cycle) are really the first documented record of the moon phases.

Nástěnná malba v jeskyni v Lexcaux stará 15 000 let. Zdroj:

A 15,000-year-old mural in a cave in Lascaux. Source:

But if we want to find the first more complicated device, we have to move a few thousand years later, to about 150 BC. The mechanism was found on the Greek island of Antikythéra, and according to archaeological research, it was supposed to be a device that could show the annual pilgrimage of the Sun and Moon by turning the handle. The mechanism supposedly predicted their eclipse and also mimicked the irregular movement of the Moon around the Earth.

Rekonstrukce mechanismu z Antikythéry. Zdroj:

Reconstruction of the mechanism from Antikythéra. Source:

This ancient mechanism was so complex that no one could outdo it for centuries. The watchmakers did not succeed until almost a thousand years later, during the Renaissance. From this period (15th and 16th century), there are the first types of astronomical clocks, which were usually attached to the towers of monumental cathedrals.

Nejznámější renesanční astronomické hodny najdeme v Praze. Staroměstský orloj mimo jiné ukazuje Měsíční fáze a postavení Měsíce vzhledem ke Slunci. Zdroj:

The Prague astronomical clock displays, among other things, the moon phases and the position of the Moon relative to the Sun. Source:

The Enlightenment in the 16th century brought a revolution in astronomy, which changed not only the understanding of the solar system but also the concept of mechanisms for displaying the lunar phases, whose popularity grew in the following centuries.

The development of the mechanism then gradually shifted from large, complicately designed wall clocks to small pocket watches. The first moon phase wristwatch came from Patek Phillippe in 1925. The feature was incorporated as part of the complication of displaying the perpetual calendar. Soon after that, the Breguet brand came up with a "moon" watch, and the popularity of this complication persists to this day.

Patek Philippe 97975, první náramkové hodinky s věčným kalendářem a ukazatelem fází Měsíce, 1925. Zdroj:

Patek Philippe 97975, the first wristwatch with a perpetual calendar and a moon phase indicator, 1925. Source:

Why is this complication still popular today?

Nowadays, not so many thing and not so many people follow the moon phases. There might be some who still perform paganish rituals and gather hebrs during the full moon, but for most people this complication has no practical use...

So to answear, why is it still so popular, there is one simple explanation. It just looks beautiful.


That is also why we can usually see it on really elegant, dress watches, as the moon phase indicator adds a romantic, almost mythical scent. (This feature can, of course, be found in both, in analog watches and modern digital watches, but that does not really belong here). Even the world of watchmaking is probably under the influence of the Moon's magic. ;)

Our watches with the moon phase indicator

There are women's and men's analog watches with a Moon phase indicator in our offer, both with a mechanical and a more affordable quartz movement.

The one brand that really likes the Moon on its dial is the Swiss brand Certina. In our offer, you can find for example the Certina DS-8 Lady Quartz COSC Chronometer Moon Phase watch with a white leather strap.

This model also have the COSC accuracy certification. But you can also choose from models with more practical steel bracelet – Certina Moon Phase with bicolor PVD treatment, black dial and all-silver with mother-of-pear in the backgroud of the dial, which nicely supplements the picture of the wandering Moon.

The watch has also a men's equivalent, the Certina DS-8 Quartz COSC Chronometr Moon Phase with a brown lether strap with a imprinted aligator motif.

Certina may also interest men with their Certina DS-8 Chronograph Moon Phase watch.

Another Swiss brand, the Ball, offers the Ball Trainmaster Automatic Moonphase watch with an elegant black dial on which a nicely structured moon complication stands out.