PERPETUAL PLEASURES: THESTATE ART OF THE OF THE PERPETUAL CALENDAR
Time marches on, but it does so in a regularly irregular way; and so, we have the various forms of calendars by which mankind has for millennia tried to cope with the fact that a year is not evenly divisible by a whole number of days. There are, as it turns out, a lot of different ways to define a year, but the two we’re interested in as watch lovers are the tropical and the sidereal year. Basically, both of these have to do with how long it takes the sun to return to a certain starting point in the sky as seen from Earth, but the tropical year’s usually the one used as a starting point for calendars because it has to do with how long it takes the sun to cross the point in the sky marking the vernal (or spring) equinox. (For the sidereal year, you use the background of stars as a reference instead — it’s similar to the difference between a solar and sidereal day.)
The basic idea is that since you can’t fit a whole number of days into a year, any calendar you can possibly come up with is going to start to drift out of synchrony with the seasons, which is why our ancestors came up with the brilliant idea of having years of different lengths. The basis of any self-correcting calendar — like your favorite and mine, the Gregorian calendar — is that you start out with a calendar that is actually a little shy of a true year in length. If a year is actually around 365.25 days long, then you’ll be a full day short at the end of four years. If, however, you are clever, you just make sure that every four years, you add an extra day to the calendar. This keeps everyone happy because it means that the date doesn’t gradually drift out of synch with the seasons, and that April showers will always bring May flowers, instead of April snow bringing… May snow, depending on your latitude.
(The actual exact length of a tropical year? Bad news: there isn’t one; it varies due to countless tiny celestial wobbles like the rate of precession of the Earth’s axis. The mean tropical year, on 1 January 2000, was 365 days, five hours, 48 minutes, and 45.19 seconds. As you can see, 365-and-a-quarter days is, as they say, close enough for government work.)
Now, consider the watch (or clock). A basic calendar indication is pretty simple,
really. You’ve already got the hour hand going around twice a day. All you have to do is put a reduction gear on the hour-hand wheel that turns that into one rotation a day, and then you use that gear to advance the date one day every 24 hours (you can use a hand or a disc, or whatever you like — we’re concerned here with mechanics, not cosmetics).
As long as the month is 31 days long, you’re good to go; if it’s shorter, you have to advance the date manually on the 30th (if it’s September, April, June, or November) or on 28 February (in a non-leap year) or 29 February (in a leap year). (At this point, some people might ask why you would need an extremely complex and delicate mechanism to save you the unspeakable agony of having to spend 30 seconds turning the crown a couple of revolutions five times a year, but let us not concern ourselves with such benighted persons.)
Generally, the way a perpetual-calendar watch handles this is through the use of something called a “program wheel”, which is a kind of gear (rather large compared to other gears in a watch) that has 48 elevations and depressions on it. The tip of a lever rests on one of these 48 points, and, depending on the depth, will make the calendar advance the date either one, two, three or four days at the end of the month. (The biggest jump corresponds to the deepest notch, and if you look at a classic perpetual-calendar mechanism — a little difficult to do unless your watch has a transparent dial, as the calendar works are usually under the dial, and thus not visible through the display back — you’ll see a total of four deep notches for the four months of February, with one slightly less deep than the others; that’s for February in a leap year, when we add 29 February.)
The invention of the perpetual calendar is generally credited to Breguet, although interestingly enough, one of Breguet’s earliest perpetual calendars didn’t use the abovedescribed system. Dr. George Daniels gives, in The Art of Breguet, a date of 1795 for the invention of the perpetual calendar, and the most famous example of a Breguet watch with a selfcorrecting calendar is Breguet no. 160, better known as the “Marie-Antoinette”. The history of the watch is as complicated as the watch itself, but its perpetual calendar is of interest here, thanks to its use of a very clever — if very complicated — design that’s driven by the kidney-shaped cam for the equation of time (EOT). In this design, the EOT cam, which turns once a year, has four pins on it that strike a lever controlling the number of days advanced at the end of the month; when the lever’s position is changed by one of the pins, the date changes from the 30th to the first (four pins for the four months of 30 days’ length). For the month of February, there is a four-pointed star wheel with four pins attached to each tooth, also mounted on the EOT cam. These pins control the same lever as the ones attached directly to the EOT cam. Three of the pins are set so that the date advances from 28 February to 1 March, and one is set so that, once every four years, the date jumps from the 29th to the first. The star wheel turns one full revolution every four years.
It’s an extremely clever mechanism, and obviously lends itself to simplification. The 48-step program wheel was most notably used in the early 19th century by the French watch and clockmaker, Achille Brocot, and is obviously conceptually connected to the
mechanism Breguet used for the Marie-Antoinette (the latter was further complicated by Breguet’s use of a retrograde date hand). However, Breguet’s idea of using a four-pointed star wheel mounted on the program disc was resurrected toward the end of the 19th century, to reduce the number of steps on — and thus the size of — the program wheel.
Again, this is pretty simple if you think about it — the only reason you need 48 steps to begin with is because you have, once every four years, an extra day on the month of February. If you use the star-wheel system, you can get away with a program wheel with only 11 steps, plus the point on the program wheel where you put the star wheel.
This system appears in a patent granted to Patek Philippe in 1889. The major difference is that, rather than a star wheel, the Patek design used a four-faced block mounted on a Maltese cross, which is placed on the now 11-step (plus the block) program wheel. Once every four years, the block is turned so the lever controlling the length of the month is resting on a face slightly higher than the other three — and, hey presto, the date advances to the first on February the 29th, rather than the 28th. Using this system, it’s possible to reduce the diameter of the program wheel considerably.
So matters stood for some time. The perpetual-calendar mechanism, at least in its classic versions (whether the sort used by Breguet for the no. 160, the Brocot design, or the type shown in the Patek patent of 1889), was not a common complication. It wasn’t so much that there were a lot of parts per se, but rather that the complication (which was, and is, often combined with a moonphase) has a number of blade springs, jumpers (springs with a tooth on the end that holds a wheel in place unless it’s specifically meant to move) and levers that are relatively difficult to make. The classic versions are also delicate, easily damaged if mishandled, and troublesome to set correctly, as each of the indications has to be adjusted through pushers in the case (manufacturers often give you a nice little stylus for this purpose, which you will, of course, proceed to lose immediately, and there is something jarring about adjusting your mechanical paean to the highest pinnacle of horological tradition with a toothpick). For many years, the perpetual calendar was rare enough that this didn’t matter very much, and it often traveled along with other complications in pocket watches made for ultra-wealthy and ultra-serious collectors — King Farouk, Henry Graves, James Ward Packard — who were not going to be deterred by little things like case pushers (and who most probably had someone on staff full-time just to attend to their watches). But eventually, the wristwatch began to supplant the pocket watch.
The very first perpetual calendar to find its way onto a wrist, at least as far as anyone can tell for certain, was made by Patek Philippe, and it started out as a movement completed in 1898 and originally intended for a ladies’ pendant watch. In 1925, however, Patek Philippe placed the movement in a wristwatch case, as ref. 97975, and two years later sold it to a well-known Patek client named Thomas Emery (Patek later bought back the watch from his estate and it now resides in the Patek Philippe Museum). The watch also featured an instantaneous jumping mechanism for the perpetual calendar (the main subject of Patek’s patent from 1889).
Instantaneous jumping of perpetual-calendar indications makes an already challenging complication even more so, as the energy necessary to jump all the indications at midnight has to be drawn from the mainspring. The traditional way to do this is to put a cam on the drive wheel for the perpetual calendar, with a lever pressing against it under the force of a fairly strong straight spring. The cam works in the same way as the return-to-zero cam on a chronograph; at midnight, the tip of the lever rides over the high point of the cam and instantly (well, practically instantly) rotates the cam so that the lever now rests on the lowest point. This causes a foot mounted on the cam to flick the date-change lever, and the indications all switch over. The system must have been difficult to miniaturize at first — for one thing, it takes a lot
of energy, relatively speaking, and shrinking everything down to wristwatch size was entering terra incognita. The attraction of such a mechanism must have been even greater, though, because the first wristwatch movement we know of that was really designed from the ground up as a perpetual calendar, was made by Breguet and completed in 1929: the Breguet no. 2516, sold in 1934 to a Monsieur Jean Dollfus. (Jean and his brother Louis were Breguet enthusiasts; the Breguet archives list nine watches sold to the brothers.) No. 2516 had a moonphase, as well as indications for the date, month, and day of the week, all changing instantaneously at midnight.
The technical evolution of the perpetual calendar began to slow after World War II, and it’s not hard to understand why — always expensive and difficult to make, they were never in particularly high demand outside serious collectors’ circles, and in the 1970s and early 1980s it was far from clear that anyone would be interested in a mechanical perpetual calendar ever again. They did continue to be made, however — Audemars Piguet, for example, came out with a perpetual-calendar version of the Royal Oak as early as 1984. And, despite the shaky ground on which mechanical watchmaking still stood in the ’80s, it was in that decade that a major leap forward in perpetual-calendar design came — from IWC.
In 1985, IWC showed the Da Vinci wristwatch, ref. 3750, at BaselWorld. The watch contained a movement developed by IWC’s Kurt Klaus, and the base caliber was IWC’s version of the then-already-venerable Valjoux ref. 7750, the IWC cal. 79061. But it was the Kurt Klaus-developed module that was the big news. The perpetual-calendar module was designed in such a way as to make it possible to set all the calendar indications simply by pulling the crown out to its first position and turning it. You can, of course, set a traditional perpetual calendar by turning the crown and moving the hands around the dial, but the Klaus perpetual module allowed you to quick-set the perpetual calendar just as you would be able to quick-set the date on an ordinary watch — quickly and without having to move the hands. No perpetual calendar had ever offered this ability before.
The key to the system was the use of a rather unusual circular lever that tilted on a pivot on its edge once a day when triggered by a drive wheel and lever in the movement. This was the only point of mechanical linkage between the base movement and the perpetual- calendar plate. Because the change of all the indications was controlled by the movement of a single part, it was possible to coordinate all the date indications — something impossible in a conventional perpetual calendar, which is driven directly by the hour wheel from which it cannot be mechanically uncoupled.
The Klaus perpetual module also featured a full four-digit indication of the year, as well as an indication of the phase of the moon. As with a conventional perpetual calendar, its heart was a 48-step program wheel. The only disadvantages to the system was that it could not be set backwards — and that it required care on the part of the watchmaker servicing or assembling it to make sure all the indications were properly arranged before closing up the case, as the system doesn’t allow for correctors for individual indications (of course, that all indications are linked and can be set
through the crown alone is part of the point of the design in the first place.)
It is a clever, versatile system that is still in use by IWC today, and over the years, the firm has used it on base calibers ranging from the Valjoux cal. 7750 to IWC versions of the ETA ref. 2892, the Frédéric Piguet cal. 953, and the Jaeger-LeCoultre cals. 960 and 849; the module was also used by Jaeger-LeCoultre in several watches, and it’s a testament to the versatility of the module (and to the close historic relationship between IWC and JaegerLeCoultre under the late Günter Blümlein) that Jaeger-LeCoultre uses a variant of the module right down to the present day. Both the Master Eight Days Perpetual and the Master Ultra Thin Perpetual use this module — right down to the original four-digit display of the year. While IWC has omitted the four-digit date in recent years (the last IWC watch to use it was the Da Vinci Perpetual Calendar Edition Kurt Klaus), it also remains the only company in the Richemont Group to offer the module integrated with a chronograph function.
The next step forward was not taken until 1996, but it was a big one — the first perpetual-calendar watch which could not only be set by the crown, but which could also be set both forwards and backwards. This was the Ulysse Nardin Perpetual Ludwig, which was designed by Dr. Ludwig Oechslin, and which was part of the same period of creative fertility that produced, between 1985 and 1992, the Trilogy of Time astronomical complications. The design, like the Kurt Klaus perpetual calendar, was modular, but unlike any other perpetual calendar before it, there was no program wheel. Instead, the Ulysse Nardin perpetual-calendar mechanism uses a group of stacked program wheels, which effect the change of the date.
The mechanism uses a driving wheel that turns one full revolution per day. This wheel has 24 teeth. Four of the 24 — corresponding to the last four hours of the day — have slightly longer teeth than the other 20. These teeth engage the teeth of the date-change program wheel. The date-change program wheel is actually a stack: a bottom wheel with 31 teeth; one above with 24 teeth — five of which are longer than the others; one small one on that, with four teeth, with only one shorter than the others; and finally, one stacked on that, also with 24 teeth, but only one longer than the others. Thus, you have a layer cake: the bottom layer has 31 teeth and turns once a month, and the top three layers are as described. The two 24-toothed wheels turn once a year, but the four-toothed wheel turns only once every four years.
Now, here is the clever part: the four long wheels on the driving wheel are of different thicknesses, to catch — or miss — different layers on the date-change program-wheel cake. The thinnest catches a tooth of the date-change program wheel — on the bottom layer — once a day, advancing the wheel one tooth. This advances the date one day. If the month is 31 days long, then the change from “31” to “1” is no different than any other day.
However, on top of the base date program wheel of 31 teeth is the next layer of the cake, layer two — the smaller wheel with five teeth longer than the rest. If, at the end of the month, it’s necessary to jump from the “30” to “1,” the second longer tooth on the driving wheel — which is at a level to only engage layer two of the program-wheel cake — also catches, and instead of advancing one tooth at the end of the day, the date program wheel advances two teeth at the end of the day — jumping the date from “30” to “1.”
This takes care of every month except February. On 28 February, the remaining two of the four longer teeth on the driving wheel catch on the teeth on the last two layers of the programwheel cake: four teeth on the driving wheel pass at the end of the day on the 28th, catching on four teeth on all four layers of the program-wheel cake. Since the date advances one day per tooth on the program wheel, having four teeth means the date jumps four days at the end of the day on 28 February to 1 March.
What about 29 February? Well, remember that comes once every four years. Remember one of the wheels making up a layer of the cake has four teeth, but one shorter than the others, and
turns once every four years. On the fourth year, the shorter tooth sticks out — and it’s too short to catch a tooth on the driving wheel. That means, once every four years, at the end of February, only three teeth catch — and the date advances only three days, from 29 February to 1March.
Though it’s complicated to explain, it’s a surprisingly mechanically simple and elegant solution, and something of a Gordian-knot solution to the problem of perpetual calendars. Gone are the springs, jumpers and levers of the traditional perpetual calendar, as if in a puff of smoke; the entire system consists of gears just as happy to rotate forwards as backwards, and you have a perpetual calendar you can not only reset as blithely as you like, but also travel with, as setting the date back a day is no longer a problem. Not only that, but the watch won’t hiccup on 2100AD, when — in a further correction to a correction — the Gregorian calendar does NOT add a day to February. Instead, you just set the watch back one day at the end of February. The only remaining common element with the classic stepped program-wheel perpetual calendar is the four-toothed February wheel — the latest incarnation of the February star wheel of the classic 12-step program-wheel perpetual calendar, which, in turn, harkens back to the star wheel used by Breguet on the Marie-Antoinette. Why, if you were a romantic, you might call it the Queen’s Star.
One of the most fascinating recent developments in perpetualcalendar design comes from Cartier, and was introduced in the Rotonde de Cartier Astrocalendaire, which debuted officially at the SIHH last January. In certain respects, it’s conceptually similar to the Ulysse Nardin design: there is no conventional 11- or 48step program wheel, and the mechanism is designed so that it can be set either forwards or backwards. Like the Ulysse Nardin system, the mechanism used in the Astrocalendaire uses a drive wheel that turns one full revolution per day, and that advances a “layer cake” date program wheel. As in the Ulysse Nardin design, the date program wheel is advanced, at the end of the month, anywhere from one to four teeth — and thus, one to four days, depending on which month it is.
The big difference, however, is that the Cartier date program wheel doesn’t use a system of stacked gears. Instead, it uses a very clever system consisting of two cams with lobes on their peripheries, which extend and retract extendable gear teeth, set on circular springs above the 31 teeth of the base date wheel. The cams rotate in such a way that, depending on the month, anywhere from one to three extra teeth are extended. The lower of the two cams controls the positions of two teeth and can extend either one or both of the teeth it controls. The upper of the two cams controls one tooth, which is extended once a year at the end of the month of February. When all three extra teeth are extended, the date jumps from 28 February to 1 March — except once every four years, when the tooth controlled by the upper cam does not extend, for the change on 29 February in a leap year.
Not only can it be set either forwards or backwards, but it’s got no levers or jumper springs (except for one jumper for the program wheel) and the cam system ensures that the entire system operates with very low energy cost to the mainspring and very little friction; it’s also quite flat. It’s a beautifully compact piece of engineering and one of the most technically advanced solutions to the problem of mechanical perpetual calendars in existence.
Serious students of the perpetual calendar have probably been waiting for the other shoe to drop — that being the fact that even a perpetual calendar will hiccup once in a while, that is, once every hundred years. The insertion of an intercalary day at the end of February is actually not a perfect correction, and in order to keep the calendar from drifting even slightly, once every century (basically, in any year evenly divisible by 100) you do not add the 29th day for February, when normally you would (as we mentioned a bit earlier).
For most normal people, this gets a big “who cares”, but for hardcore watch enthusiasts, this means you either hope you have a perpetual calendar that can be manually adjusted without issues, or you have to stop your watch, wait until it’s 2 March, and then set it ahead. And, it gets better (or worse, maybe) — the final hurdle in the perpetual-calendar Olympics is making a watch that addresses not just the four-year leap-year cycle, but the 400-year leap-year cycle, in which, every hundred years, you don’t add an extra day to February, but on the 400th year, you do (we’re at the beginning of such a cycle now; it ends in 2400AD, when you should add an extra day to February even though, as it’s a year evenly divisible by 100, you might think you shouldn’t. How wrong you’d be.) To sum up: every fourth year is a leap year, but every 100th year is a common year; and once every 400 years, it’s a leap year.
A watch that can keep track even of these very extended cycles of intercalary days is called a secular perpetual calendar, and
precious few of these watches exist. One of the most famous secular perpetual calendars is Patek Philippe’s magnum opus, the Calibre 89 pocket watch. Patek received a patent for the secular calendar of the Calibre 89 in 1986, and it is described by Patek as “one of the slowest mechanisms in horology” — and indeed it is.
The secular-calendar part of the mechanism consists of a star wheel with four points that rotates once every 400 years — there is a quarter-turn every century. Once every 100 years, the longer teeth of the star wheel change the position of the date wheel at the end of February so that the date changes on the 28th — and once every 400 years, a shorter tooth passes the date lever without engaging it, so that the 29th is added.
Svend Andersen, who at one point had Franck Muller as an apprentice and Patek Philippe as a client, makes another one of the very few secular perpetual-calendar watches in existence. Andersen takes a somewhat unusual approach in building his secular perpetual-calendar mechanism on an ETA ref. 2892 base, but it’s certainly (well, we’re almost certain) the most affordable secular perpetual calendar out there. Especially since your only other choice (unless you happen to be in the market for a Calibre 89) is from Franck Muller — the Aeternitas Mega 4. This ultracomplicated watch uses a patented mechanism for the secular calendar, consisting of a second program wheel (rather than a star wheel) that encodes the full 400-year cycle for the month of February. Along with the main perpetual-calendar program wheel, it ensures that at no point will either you, or your descendants, need to exert yourselves to manually correct the date.
Although the Gregorian calendar is overwhelmingly the one around which perpetual calendars are built, there are many other calendar systems and we feel the subject of perpetual calendars shouldn’t be closed without mentioning one of the most interesting calendar wristwatches ever made: the Blancpain Traditional Chinese Calendar watch.
It’s not a perpetual calendar in the ordinary sense of the word, but it does something no other wristwatch does: show the Gregorian calendar along with indications for the traditional Chinese lunisolar calendar. The Chinese calendar is based on a succession of lunar months of either 29 or 30 days’ length, which yields a lunar year about 11 days shorter than a tropical year; for this reason, an intercalary month is occasionally inserted (this is the reason for the varying date of the Chinese New Year). The Chinese calendar also has a “12-double-hour” day (corresponding to the Gregorian 24-hour day). The Blancpain Traditional Chinese Calendar watch thus has two hour hands, and shows the correct date and month for the Chinese calendar, as well as having a leap-month indication and showing which sign of the Chinese zodiac corresponds to any given year. This requires some 434 parts in all, with the movement offering a seven-day power reserve. The watch also features Blancpain’s innovative underthe-lug correctors, which allow one to correct the calendar without resorting to the inconvenient caseband correctors necessary in many other watches.
Though perpetual calendars have traditionally been demanding to build, and are therefore rare and relatively costly, they have recently started to become available more widely; such watches as Jaeger-LeCoultre’s Master Ultra Thin Perpetual and, most recently, Montblanc’s Meisterstück Heritage Perpetual Calendar, have made the complication more widely available.
Today, there’s an ever-expanding range of choices and prices for lovers of this complication, and whatever your tastes, it’s more interesting now than at perhaps any other time in the history of watchmaking to choose a timepiece that will save you the terrible burden of touching the crown of your watch at the end of February.
ANTI- CLOCKWISE FROM LEFT The first known wristwatch perpetual calendar, by Breguet; the Breguet no. 160, the ‘ Marie-Antoinette’, whose complex perpetual-calendar mechanism was
described by Dr. George Daniels, above, in TheArtofBreguet
ABOVE Patek Philippe cal. 240 Q (above), a modern self-winding perpetual- calendar movement LEFT The perpetual-calendar mechanism with its 48 steps for each month of a full four-year leap-year cycle
01 The Grand Complication wristwatch by A. Lange & Söhne; 02 The Valjoux cal. 7750, on which the IWC perpetual-calendar chronograph cal. 79061 is based 03 Kurt Klaus, inventor of the IWC coordinated perpetualcalendar module
The IWC Novecento perpetual calendar, featuring the Kurt Klaus-designed perpetual- calendar module;
below, the Ulysse Nardin Perpetual Ludwig
The Montblanc Meisterstück Heritage Perpetual Calendar
Only a handful of secular perpetual calendar watches
exist; these include: ( from left) the Franck Muller Aeternitas Mega 4, the Patek Philippe Calibre 89 and
the Svend Andersen Perpetual Secular Calendar