The Arrant Thief: A History of the Moonphase Complication

The Moonphase disc from an Arnold & Son watch. Photo courtesy of Arnold & Son.

The Moonphase disc from an Arnold & Son watch. Photo courtesy of Arnold & Son.

This article was first published in Arabic in the January/February issue of Alam Assatt wal Moujawharat

For thousands of years, Humanity has been enraptured by the Moon's heavenly dance

The Astronomical Clock that sits on the Southern side of the Old Town Hall in Prague, Czech Republic. It was finished in 1410 and remains the oldest operating astronomical clock. Photo credit: Wikipedia.

Its regular orbit across the skies birthed our twelve month calendar and is crucial to religious ceremonies across the world. The earliest known example of a mechanical moonphase is the Ancient Greek Antikythera mechanism built in 205 BC which displayed the position of the Moon, known planets, the dates of the next Olympic games and the current day, month and year. The complexity of the Antikythera mechanism would remain unmatched for over a thousand years until the invention of astronomical clocks.

These clocks, usually built inside cathedrals, displayed the orbit of the known planets and the phases of the moon, along with the time and month of the year. Built during the Renaissance, these clocks placed God's creation of Earth at the center of the universe with the Sun orbiting around it. These mechanical sermons to religious doctrine were also monuments to the commissioning patrons. Astronomical clocks fell out of favor as scientists discovered that Earth was not the center of the Universe, much to the chagrin of the clergy.

However, the moon phase complication stuck around and was first seen on its own in 16th Century German and English longcase clocks.

An 18th Century British Longcase clock with traditional moonphase aperture. Photo credit:

The method of displaying phases in these clocks would remain unchanged for hundreds of years. A metal finger connected to the hour hand of a movement would turn a 59 toothed gear once every 24 hours. Connected to this gear was a disc painted with two moons, one at 12 o'clock and the other at 6 o'clock. An aperture would reveal part of this rotating disc so that only one of the moons was visible at any one time. Two 'humps' on either side of the aperture block the approaching moon and mimic the waxing and waning phases of the moon. The disc takes 59 days to rotate with each moon seen once every lunar month, approximately 29.5 days. Some modern wristwatches have a double aperture that displays the Moon's position in both hemispheres. Whilst beautiful, these watches are identical to their single hemisphere brothers, the only difference being that more of the Moon disc is displayed.

The moon phase complication continued to charm watchmakers and clients as it went from longcase clocks to pocket watches. Though its usefulness in a modern age was questionable, the moon phase would become a key part of the perpetual calendar complication. In 1925, Patek Philippe made the first perpetual calendar wristwatch with a movement intended for a ladies pendant watch. Breguet and Audemars Piguet would follow with annual and perpetual calendar watches of their own in 1930s and 1940s.

As the styles of the period changed, so did the design of the Moon disc.

The 17th Century rosy cheeked lunar cherubim aged into the mature and winking 'Man in the Moon' in the 18th and 19th Centuries. Wristwatches from the 1920s put an Art Deco spin on the winking moon and watches from the 1950s and 60s opted for a simple polished disc. Modern design now favors a 'semi-realistic' style where engraving or 3D stamping aims to resemble the Moon's craters.

If there is one thing that watch brands like to brag about more than being the first at something, it's accuracy. The 59 toothed gear system is only accurate to one day every 2 years, 7 months and 2 weeks and watchmakers would not stand for such inaccuracy.

A temporary victory was achieved when watchmakers calculated that a 135 toothed gear would be accurate to within one day every 122 years. Yet when astronomers discovered the true length of a lunar month was 29.530587981 days, watchmakers had a new ideal to aim for. The more decimal places that watchmakers can include in their calculations and their movements, the more accurate their moonphases would be.

The quest for accuracy defines the moon phase complication in the latter half of the 20th Century.

H. Moser & Cie, A. Lange & Sohne and Patek Philippe all have watches with over 1000 years of accuracy, yet other smaller brands have gone much, much further. Ochs und Junior is a small brand based out of Lucerne, Switzerland and their minimalist design allows for the use of a unique moonphase system. Ochs place their in-house planetary gear system, made up of five gears, on top of a base ETA 2824-2 movement.


This gear system consists of four small gears placed in a line inside a larger gear that rotates around a groove cut inside the dial. These gears might be small with a low number of teeth, but when meshed together with the connecting pinions, they create a very large gear ratio. This gear ratio makes the moon phase accurate to 3,478.27 years before adjustment. All Ochs und Junior watches are customisable by the client and despite the accuracy of the complication, the moon phases 'only' cost $8,000.

Christian Van Der Klaauw's Satellite du Monde. Photo courtesy of CVDK.

Christian Van Der Klaauw's Satellite du Monde. Photo courtesy of CVDK.

Christian Van Der Klauw's Real Moon Joure with Scorpion dial. Photo courtesy of CVDK

Further north, Christian Van Der Klaauw obsessed over the stars from a young age and has devoted his life to making hand-made astronomical watches. The Dutch watchmaker first began making moonphase and astronomical clocks in the mid 1970s and it was in 1994 that he made his first wristwatch. The Satellite du Monde's dial is instantly striking with a blue to black ring running around the edge that mimics the changing color of the sky. An indicator on this ring shows the current phase of the Moon and another shows where on Earth it is currently midday.

The crowning achievement of the CVDK is the Real Moon Joure, a three dimensional rotating moon accurate to within one day every 11,000 years. A rod pierces the moon along its vertical axis, allowing it to be connected to a more traditional moon phase gear set up. Half the Moon's surface is painted silver with the other half painted black so that its rotation mimics the Earth's shadow cast upon the real moon.

The 11,000 year accuracy claim is impressive but is beat by the work of Andreas Strehler

The Andreas Strehler Sauterelle a lune Perpétuelle. Photo courtesy of Andreas Strehler

The Sauterelle a lune Perpetuelle is accurate to within one day every 14,189 years and when Strehler announced the watch, he joked that any further accuracy achievements would be overkill. Next year he debuted the Sauterelle a lune Perpetuelle 2M which is accurate to within one day every 2,060,757 years.

For the 2M, Strehler used a four part planetary gear system, whose gears had teeth on the inner and outer circumferences. These teeth add up to large prime numbers which can achieve complex fractions. These complex fractions allow the moon phase to achieve its high level of accuracy. (This writer is not a mathematician so will have to take Strehler's word that it's not magic.)

Not only is the 2 million year accuracy of the 2M 145 times better than the original Perpetuelle, but it is impossible to beat. In 2 million years time, the orbit and rotation of the Earth and Moon will have changed enough to increase the length of a day by 36 seconds. There cannot be a more accurate moon phase because we can't work out where the Moon will be that far into the future.

This writer, whilst impressed at the achievements of CVDK and Strehler, cannot help feel that they've cheated a little bit. Watchmakers of yesteryear used to calculate the position of the stars on paper and would make every component by hand. Today computers calculate complex ratios in minutes with laser cut gears possible at the press of a button.

It's amusing to think that the quest for extending the accuracy of a moon phase is pointless, in every sense.

"Any of you earthlings know how to service Andreas Strehler? No? Well, onto Alpha Centauri then". Photo: Watch #1011 by OneHourWatch

No watch can work in perpetuity without a service and dismantling the movement 'resets' the length of accuracy every 5-10 years. A lazy owner unwilling to store their watches in a winder, or take the time to wind them, will also render the feats of all these brands meaningless. Not to mention the ever decreasing likelihood that society will last to the next millennium, let alone the next two million years. Let us hope that any intergalactic visitors our descendants encounter will have a watchmaker or two in their ranks.

No amount of fatalistic thoughts about the pointlessness of it all should stop the creation of such wonderful mechanisms. These pioneers represent what luxury watchmaking can be when it elevates itself above the bravado. The mechanical moons of Ochs und Junior, CVDK and Andreas Strehler speak to a greater purpose that is superior to carat weight or precious metals. Their inventions resonate with us because deep down, we all share the same fascination with the Moon.

If you were to travel in time back to the construction of the grand astronomical clocks, language would be an impassable barrier. Yet a shared glance towards the night sky would forge a bond between strangers as a shared fascination with that familiar, silvery orb transcends language and time.