Variations On The Moonphase

<h4 style="text-align: center;">One of watchmaking’s oldest complications is still evolving.</h4>
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The <a href="../watches/moonphase-watches/">moonphase complication</a> is as old as watchmaking itself. In fact, it’s older – public clocks, which preceded the invention of mechanical watches by at least two centuries, often had astronomical displays that showed the phase of the moon. The oldest astronomical clock in existence, in Prague, was installed in 1410 and features the oldest extant moonphase display anywhere in the world (and it’s a spherical moonphase to boot).</p>
<p>The moonphase shows the appearance of the Moon as it progresses through the lunar month – specifically, the synodic lunar month, defined as the number of days between successive New Moons. The lunar month is approximately 29.5 days, but this is only an approximate number. The length of any given lunar month can vary by several hours, depending on the positions of the Earth and the Moon in their orbits, but the current average length of a lunar month is 29 days, 12 hours, 44 minutes, and 2.8 seconds. A moonphase display therefore will show an <i>approximately </i>correct moonphase, and different watches will show what’s actually visible in the sky with varying degrees of precision.</p>
<div id="attachment_1166441" style="width: 1410px" class="wp-caption alignnone"><img aria-describedby="caption-attachment-1166441" decoding="async" loading="lazy" class="wp-image-1166441 size-full" src="https://watchbox-blog.imgix.net/wp-content/uploads/2023/02/Praugue-Clock.jpg?max-w=1024&q=50&auto=format,compress&usm=10&usmrad=3" alt="" width="1400" height="2100" /><p id="caption-attachment-1166441" class="wp-caption-text"><em>The Great Astronomical Clock, Prague, 1410. Spherical moonphase at 1:00 on the upper dial, showing a waning crescent Moon.</em></p></div>
<p>The moonphase was once a somewhat practical complication – especially  before the invention of artificial lighting, when after sundown, it got dark in a way most of us will never experience. Today, it’s a nod to tradition, but it’s also more than that – it’s a piece of visual poetry on the wrist, and a reminder that we’re part of a larger universe.</p>
<h2>The Standard Moonphase Display</h2>
<p>The overwhelming majority of moonphase watches show the moonphase in pretty much the same way. The moonphase is shown in a subdial – most often at 6:00, although other placements are possible as well, depending on the design of the watch and the presence or absence of other complications – with a cut-out in the upper half, more or less in the shape of an “m”.  Under the dial, there’s a disk with two moons printed on it. The disk has 59 teeth on its edge and it’s advanced one tooth per day (59 divided by two is 29.5) so it makes one half turn per lunar month. At the New Moon, one Moon disk has just sunk out of sight and the other is just about to rise above the opposite edge of the aperture.</p>
<div id="attachment_1166434" style="width: 1410px" class="wp-caption alignnone"><img aria-describedby="caption-attachment-1166434" decoding="async" loading="lazy" class="wp-image-1166434 size-full" src="https://watchbox-blog.imgix.net/wp-content/uploads/2023/02/Patek-Philippe-Moonphase-5270-01R-001-1.jpeg?max-w=1024&q=50&auto=format,compress&usm=10&usmrad=3" alt="" width="1400" height="1400" /><p id="caption-attachment-1166434" class="wp-caption-text"><em><a href="../shop/Patek-Philippe-Grand-Complications-5270R-001.html">Patek Philippe 5250R Grand Complication, Perpetual Calendar Chronograph.</a></em></p></div>
<p>A variation on this is a single, circular moonphase aperture, with a dark circle representing the side of the Moon’s disk which is in darkness that sweeps across the aperture once every lunar month.</p>
<p>The traditional moonphase complication only shows what’s actually in the sky accurately part of the time but it has tradition and custom on its side. The biggest practical drawback to this most basic moonphase complication is that it’s based on a lunar month of 29.5 days, which is only an approximation. The moonphase goes out of sync with the actual phase of the Moon by a full day after about two and a half years. Given the fact that the recommended service interval for watches is five to eight years (assuming they are worn daily) this is not an enormous practical problem, and moonphase watches have correctors which let you adjust the display as needed.</p>
<p>However, watchmakers are nothing if not perfectionists. The perpetual calendar doesn’t offer all that much of an advantage over a standard calendar – you need to adjust the calendar manually only five times per year and it takes less than a minute, and perpetual calendars can be delicate, difficult to set, and extremely expensive. This however has not kept them from becoming one of the most respected of all watch complications.</p>
<div id="attachment_1166438" style="width: 1410px" class="wp-caption alignnone"><img aria-describedby="caption-attachment-1166438" decoding="async" loading="lazy" class="wp-image-1166438 size-full" src="https://watchbox-blog.imgix.net/wp-content/uploads/2023/02/Lange_Moonphase_Hommage_231.035-2.jpg?max-w=1024&q=50&auto=format,compress&usm=10&usmrad=3" alt="" width="1400" height="934" /><p id="caption-attachment-1166438" class="wp-caption-text"><em><a href="../shop/A--Lange-Sohne-1815-231-035.html">A. Lange & Söhne Hommage To Emil Lange, with a precision of one day’s error in 1.058 years.</a></em></p></div>
<p>In the same spirit, watchmakers have worked hard over the last three or so decades to improve the precision of moonphase displays. In the 1980s, IWC’s Kurt Klaus calculated a gear train for the Da Vinci perpetual calendar chronograph that was accurate to one day every 122 years (IWC’s perpetuals are today accurate to one day in every 577.5 years). Today, a precision of one day in 122 years is more or less the industry standard in fine watchmaking.</p>
<p>You can go really bananas in terms of precision if you are so inclined. The current record holder for moonphase precision is Andreas Strehler’s Sauterelle à lune perpétuelle, with a precision of one day’s error every 2.045 <i>million </i>years. This is obviously pretty conspicuously outside the realm of any practical advantage, but it is also, equally obviously, an incredible technical achievement.</p>
<h2>Two Hemisphere Moonphase Displays</h2>
<p>We all tend to think that the Moon we see – <i>our </i>Moon – is everyone’s Moon but in fact the Moon looks different depending on where you are on Earth. If you travel from the Northern to the Southern hemisphere (or vice versa) the Moon is going to look upside down compared to what you see at home. The giant impact crater Tycho, for instance, one of the most prominent lunar features, is on the bottom half of the Moon’s disk when you look at it from the Northern Hemisphere, but it’ll be on the top half of the disk if you’re in the Southern Hemisphere.</p>
<div id="attachment_1166439" style="width: 1410px" class="wp-caption aligncenter"><img aria-describedby="caption-attachment-1166439" decoding="async" loading="lazy" class="wp-image-1166439 size-full" src="https://watchbox-blog.imgix.net/wp-content/uploads/2023/02/Moon-Phase-Perspectives.jpeg?max-w=1024&q=50&auto=format,compress&usm=10&usmrad=3" alt="" width="1400" height="686" /><p id="caption-attachment-1166439" class="wp-caption-text"><em>Waning crescent Moon as seen from the Northern Hemisphere (left) and Southern Hemisphere (right). Image, NASA</em></p></div>
<p>The moonphase you’ll see if you travel is its mirror image as well. If you’re looking at a waning crescent Moon in the Northern Hemisphere, the lit part of the Moon is on the left, and the area in shadow is on the right, but if you’re in the Southern Hemisphere, it’s the other way around.</p>
<div id="attachment_1166436" style="width: 1410px" class="wp-caption aligncenter"><img aria-describedby="caption-attachment-1166436" decoding="async" loading="lazy" class="wp-image-1166436 size-full" src="https://watchbox-blog.imgix.net/wp-content/uploads/2023/02/Hermes-Arceau-Meteorite-Moonphase.jpeg?max-w=1024&q=50&auto=format,compress&usm=10&usmrad=3" alt="" width="1400" height="1400" /><p id="caption-attachment-1166436" class="wp-caption-text"><em><a href="../shop/Hermès-Arceau-Arceau-Meteorite.html">Hermès Arceau L’heure De La Lune Meteorite Edition, with Northern and Southern Hemisphere Moons.</a></em></p></div>
<h2><b>The Spherical Moonphase</b></h2>
<p>Spherical moonphase displays are often thought of as an attempt to solve the basic problem of a traditional moonphase, which is that the latter doesn’t accurately show what’s visible in the sky. While it’s true that the spherical moonphase can indeed show what the Moon really looks like more accurately than a traditional moonphase display, I’m not entirely sure that was the original inspiration behind it – the spherical moonphase is very old and may well predate what we’ve been calling the traditional moonphase although very early in the history of watch and clockmaking you can see both (the flat moonphase display seems to be more common in watchesk and the spherical moonphase, more often seems to have been used in larger clocks).</p>
<p><img decoding="async" loading="lazy" class="alignnone size-full wp-image-1166435" src="https://watchbox-blog.imgix.net/wp-content/uploads/2023/02/DeBethune_DB28_Digitale.jpeg?max-w=1024&q=50&auto=format,compress&usm=10&usmrad=3" alt="" width="1400" height="1400" /></p>
<p>Spherical moonphase displays are rare in modern watchmaking for several reasons but the main reason is probably that they are simply harder to make. The sphere that represents the Moon has to have, obviously, two hemispheres – one brighter and one darker. One interesting solution from an independent watchmaker is Paul Gerber’s spherical moonphase, which has 54 tiny diamonds on the bright side, and a lapis lazuli hemisphere for the dark side.</p>
<p>Probably the best known maker of spherical moonphase displays is De Bethune. Denis Flageolet developed the first spherical moonphase for De Bethune for the 2004 DB15 Perpetual Calendar, and it can today be found across a wide range of <a href="../watches/de-bethune/">De Bethune watches</a>.</p>
<p><a href="https://www.thewatchbox.com/blog/what-is-heat-bluing.html">Flame tempering metals to various colors</a> is a big part of De Bethune’s design language and its spherical moonphase uses heat-bluing of steel in an interesting way. The Moon sphere has two hemispheres – one of them is steel and the other is palladium. The sphere is held over a spirit lamp flame in the traditional fashion, until the steel half turns blue. The palladium half is exposed to the same heat but doesn’t change color, because palladium doesn’t change colors at the same temperature as steel.</p>
<h2>Perpetual Calendars And The Moonphase Complication</h2>
<p>The moonphase complication doesn’t need any excuse to be present in a watch in its own right, but there is one other complication with which it often travels – the perpetual calendar. In fact you can hardly think of a perpetual calendar watch that <i>doesn’t </i>have a moonphase.</p>
<p>There are several reasons for thinking of the perpetual calendar and the moonphase as complications that are joined at the hip. One reason is purely historical. The first documented perpetual calendar watch was <a href="https://www.sothebys.com/en/auctions/ecatalogue/2016/john-harrison-enduring-discovery-l16055/lot.28.html" target="_blank" rel="noopener">completed in 1762 by the English watchmaker Thomas Mudge</a>, and that watch had a moonphase display – and ever since, watchmakers have overwhelmingly tended to include a moonphase in their perpetual calendar watches. Another reason for including a moonphase with a perpetual is that both are astronomical complications. A perpetual calendar is useful because it accounts for the four year Leap Year cycle in the Gregorian calendar – which is necessary because the orbit of the Earth doesn’t take up a whole number of mean solar days.</p>
<p>If you had to pick out a single watchmaker whose name is synonymous with perpetual calendars – well, if you tried to with a bunch of fellow watch enthusiasts you would have the makings of a very heated argument but one name that would be sure to come up would be <a href="../watches/patek-philippe/">Patek Philippe</a>.</p>
<p><img decoding="async" loading="lazy" class="alignnone size-full wp-image-1166440" src="https://watchbox-blog.imgix.net/wp-content/uploads/2023/02/Patek-Philippe-5207-Moonphase.jpeg?max-w=1024&q=50&auto=format,compress&usm=10&usmrad=3" alt="" width="1400" height="1400" /></p>
<p>The company made its first perpetual calendar wristwatch in 1925 but the movement wasn’t originally intended for a wristwatch – it was made in 1898 and it was meant at first to go into a ladies’ pendant watch but that was not to be its fate. (One place you might get an argument about primacy in perpetuals, is from a Breguet fan – Breguet’s rectangular ref. 4244, from 1929, is probably the first perpetual calendar watch intended from the beginning to be a wristwatch). Patek Philippe also made the first series produced perpetual wristwatch ever. This is the ref. 1526, which is a watch that might have been responsible for coining the usage of “holy Grail watch” among collectors. It’s beautifully spare in design. There are discrete windows for the day and month, and the date, moonphase, and running seconds all share a sundial at 6:00. It’s also a quite rare watch – Patek only made 210 examples over a twelve year period.</p>
<h2>The Future Of The Moonphase Complication</h2>
<p>Where the moonphase complication will go next is anybody’s guess. The race for better precision seems to have more or less petered out (which is probably just as well; pursuing greater precision than one day in two million years and more seems kind of pointless) but where there is a lot of room for further experimentation, is in the display itself. How the moon looks in the sky on any given night is the result of a lot of different factors. Over the course of a lunar month, the Moon changes in size, the line between shadow and light changes in shape as it advances and retreats, and the Moon even wobbles up and down and back and forth (a phenomenon called libration, which for understandable reasons no watchmaker has cared to tackle).</p>
<p>With the spherical moonphase now a niche, but well established feature of modern watchmaking, perhaps the next problem to tackle will be making what’s on your wrist look more like what’s in the heavens, than ever before.</p>
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Jack Forster

WatchBox Global Editorial Director Jack Forster has been fascinated by every aspect of watches, clocks, and watchmaking for his entire life. He began systematically studying and learning about watches over 20 years ago, and during the first explosion of watch enthusiast forums, he became a moderator on thepurists.com. Jack became a full-time watch writer in 2007, when he joined Revolution Magazine, first as group technical and features editor, and then as editor in chief of Revolution Magazine USA. He joined the enthusiast website Hodinkee in 2015 as managing editor and then as Editor in Chief. In 2022, Jack joined WatchBox as the Global Editorial Director. He is the author of Cartier Time Art: Mechanics of Passion, and has written about watches, and other subjects, for many other venues including CNN, American Express Departures, and Vanity Fair.

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