JULY 25, 2022

From Here To Eternity:
THE SECULAR PERPETUAL CALENDAR

A deep dive into the most accurate form of perpetual calendar.

WORDS Cheryl Chia

The monumental Calibre 89 created to mark Patek Philippe’s 150th anniversary in 1989

Perpetual calendars are the ultimate expressions of date complications. When driven under constant power, it stands up to extended scrutiny, taking into account larger passages of time, notably of leap years. Unlike an annual calendar, which needs resetting once a year at the end of February, a perpetual calendar only requires a correction after every full century when the Gregorian calendar omits a leap year.

This anomaly was the result of Pope Gregory XIII’s reform in 1582, which reduced the margin of error under the Julian calendar by omitting a leap year three times in the course of every four centuries, with an average year comprising 365.2425 days instead of 365.25 days. As such, centurial years are only leap years if they are divisible by 400. By design, a regular perpetual calendar, however, will interpret the years 2100, 2200, and 2300 as leap years when in fact they are common years (at least in all parts of the world that had adopted the Gregorian calendar), thus requiring manual adjustment by one day on March 1 of these years.

As such, a superior form of perpetual calendar was birthed to accommodate this nuance — the secular perpetual calendar. This mechanism can accurately account for three consecutive centurial years that are non-leap years in a 400-year cycle.

Mechanically, as movements are designed to keep track of larger passages of time, from seconds and minutes, which are directly driven by the gear train, to 100- and 400-year cycles, derived from the humble motion works of a watch, they evolve in complexity, with a number of increasingly elaborate mechanisms in between to compensate for variances in month length, leap years and centurial years.

The first portable perpetual calendar timekeeper was made in 1762 by English horologist Thomas Mudge (1715–1794) in the form of a pocket watch while Patek Philippe was the first to file a patent for a perpetual calendar mechanism in 1889, subsequently creating the first perpetual calendar wristwatch in 1925. But more notably, Patek Philippe was also one of the first few manufactures to produce a secular perpetual calendar when it created a unique pocket watch for American industrialist Seth Atwood in 1972. Later, this complication was further accompanied by the date-of-Easter complication in the caliber 89, created to mark the brand’s 150th anniversary in 1989.

The Secular Calendar in the Caliber 89

Nine years in the making, the caliber 89 boasted an astounding 33 complications on tap, including a grande and petite sonnerie, minute repeater, split seconds chronograph, sidereal time, equation of time, sunset and sunrise, a secular perpetual calendar, display of the seasons, equinoxes and solstices, including houses of the zodiac, simultaneously becoming the first watch to display the precise date of Easter (for a period of time as dictated by the limitations of encoding a 5.7-million-year cycle into the program wheel). The 1,728-part movement was built on four layers contained in three plates. With the staggering masterpiece, Patek broke its own record of the world’s most complicated timepiece — previously held by the Henry Graves Supercomplication — by nine complications.  

The back of the Caliber 89 shows a sidereal dial with the equation of time displayed on a penannular scale at six o’clock, the hours and minutes of sunrise and sunset at eight and six o’clock respectively and the date of Easter right above the star chart; Displayed on the front is the rattrapante chronograph, the full secular perpetual calendar showing the year in an aperture and the retrograde date indication right above it, the age and phases of the moon and a second time zone.

The secular calendar in the caliber 89 is notably complete, displaying the full suite of calendar information in a concise manner — the four-digit year, the year in the four-year cycle, a retrograde date, the month and the day of the week. The four-digit year is displayed in an aperture at 12 o’clock while the number of the year in the four-year cycle is shown in an aperture right next to it. The century and decade are depicted by two concentric disks on the left while the year and the year in the four-year cycle are on a separate pair of concentric disks on the right. Abbreviations for the month and day are displayed in apertures at four and eight o’clock respectively. 

As a recap, the rule for leap year dictates that a year has an extra day at the end of February if it is divisible by 4. However, if the year is also divisible by 100, it becomes a common year with only 365 days. But if the year is also divisible by 400, then it becomes a leap year again. Encoding this nest of rules into the gear train gets even trickier as the date indication in the caliber 89 is a retrograde system.

The date is indicated by a blued centrally mounted hand that moves across a graduated arc, accounting for 29 days in February every leap year and in contrast to a regular perpetual calendar, accords 28 days for centurial years that are not leap years. As it reaches the end of each given month, the hand flies back to the starting position to indicate the first day of the following month.  

The secular perpetual calendar mechanism is hidden under the month disc at four o’clock while the secular cam that makes a revolution every 400 years is under the bridge plate at six o’clock

The secular mechanism responsible for this and for which a patent was granted in 1986, is extremely complex in construction and ingenious in its solution, though admittedly pitted against an extremely narrow field of watches, within which none are pocket watches. The patent attributes the invention to François Devaud and Jean-Pierre Musy; the latter headed the development of the caliber 89 at a tender age of 28 and has been at the company for more than four decades since. 

There are three main ways in which the heart of the perpetual calendar — the month cam — can be mechanically executed. It can be designed as a 48-month cam that encodes the length of the months in a four-year cycle with notches of varying depths, a 12-month cam which functions the same way but makes a revolution once a year, or a 12-month cam with a Maltese cross satellite to manage the month of February.

The secular calendar employs a classical 12-month Maltese cross cam system. The 12-month cam features notches of varying heights that encode 30- and 31-day months. Meanwhile, a small four-point cam that is fixed to a Maltese cross satellite accounts for the month of February. The four-point cam is made up of three points of the same height while the fourth point is longer to correspond to February 29.  

Logically, this principle was extended to account for three consecutive centuries in every four centuries that are non-leap years. A secular cam makes one revolution every 100 years while a four-point satellite cam on its circumference makes one revolution every 400 years with the longer point corresponding to the leap year in the fourth century.

While it is all reasonably straightforward up to this point, the complexity lies in how the secular cam overrides the first 12-month cam in the event of a century year that is not a leap year and how it integrates with the mechanics of a retrograde system. 

The solution lies in a lever with four steps, but it is worth zooming out for a bit. As with a standard calendar, a 24-hour wheel drives a 31-tooth date wheel, which carries the retrograde hand. Meanwhile, a rack lever engages with the pinion of the date wheel, causing the retrograde action at the end of the month by means of a spring. The date wheel is indexed by a finger every 24 hours and is locked by a pawl. Mounted on the date wheel is an index which engages with the steps of a special lever that is articulated on the free end of the pawl.

Patek Philippe’s secular calendar mechanism in the Caliber 89 from the original patent

This lever has two triangular protrusions: one in the middle that interacts with the month cam and one at its tip to lock into the teeth of the secular cam in February of a centurial year. The month cam is driven at the rate of one revolution per year by a finger mounted on the rack lever while the secular cam is driven at the rate of one revolution in a hundred years by the month cam via a mobile reduction wheel carried by the rack lever.  

This lever has two triangular protrusions: one in the middle that interacts with the month cam and one at its tip to lock into the teeth of the secular cam in February of a centurial year. The month cam is driven at the rate of one revolution per year by a finger mounted on the rack lever while the secular cam is driven at the rate of one revolution in a hundred years by the month cam via a mobile reduction wheel carried by the rack lever.  

On a regular 30- or 31-day month, the first protrusion on the lever locks into the teeth of the month cam, while in February in a regular year, the protrusion locks into the satellite cam on the month cam. But in February of a centurial year, be it a leap or common year, the satellite mounted on the secular cam would have completed a full turn returning to its position at a 60-degree angle meeting the second protrusion at the tip of the lever, stopping it before the first protrusion engages the satellite on the month cam. Thereby, the lever bypasses the month cam on every centurial year for four centuries with the longer point of the satellite accounting for a leap year in the fourth century.  

This unique solution to overriding the month cam is directly tethered to the uniqueness of the indicator placement on the dial, which makes reinterpreting it, let alone redesigning it for a wristwatch, a daunting challenge. However, it did not deter two watchmakers from devising their own solutions and building the mechanism for a wristwatch — Svend Andersen and Franck Muller. Naturally, they differ starkly from Patek’s secular calendar, as given its almost unadaptable nature, to reinterpret it was to design it from scratch.

SVEND ANDERSEN PERPETUAL SECULAR CALENDAR

Svend Andersen Secular Perpetuel Calendar from 1996

Having spent nearly a decade working in the Grand Complications workshop of Patek Philippe in the 1970s, Svend Andersen developed an expertise in various complications, most manifestly in the dial-side variety including the Louis Cottier’s world-time system and went on to develop his own world-time watches when he founded Andersen Genève in 1980. 

A more significant but lesser-known accomplishment was his creation of a secular perpetual calendar for the wrist in 1996. Though Svend parted ways with Patek Philippe in 1979, he remained in contact with the watchmakers of the brand and was consulted on the secular calendar complication during the development of the caliber 89.  

Svend, however, felt that the secular calendar module was too complicated and began devising a more practical construction that allowed it to be fitted in a wristwatch. During this time, he also developed the first Hebraic calendar for a wristwatch, which was used in Alain Silberstein’s Hebraica. With his vast expertise in calendar complications, Svend discovered a simpler way to encode the peculiarity of the Gregorian calendar into the gear train that would keep the part count at a minimum. 

In the Andersen Genève Perpetuel Secular Calender "20th Anniversary", the day of the week is represented by the sun, the moon and five planets and the date is displayed in an aperture at three o’clock; The reverse shows the month and the year number in a four-year cycle in a co-axial counter, the secular indicator in a separate sub-dial while the central hand indicates the year on a 50-year scale.

Svend’s Perpetual Secular Calendar displays the calendar on two dials. The front dial displays the date at three o’clock while the reverse shows the month and the year number in a four-year cycle in a co-axial counter, the secular indicator in a separate subdial and the central hand indicates the year on a 50-year scale. To mark the 20th anniversary of his perpetual secular calendar in 2016, he added a day-of-the-week complication on the dial. Given that the names of the days of the week are derived from the names of the sun, the moon and five planets, these seven bodies are depicted as hand engraved gold buttons on the dial.

As with a traditional perpetual calendar, his mechanism relies on a 48-month cam that makes one revolution every four years. This cam has 48 teeth with four steps of varying depths that corresponds to the length of each month, be it 31, 30, 29 or 28. The grand lever, thus, pivots varying amounts based on the month — further for short months and lesser for long months. At the end of the month, the grand lever locks into the teeth of the date cam and forces the date change. Thus, for a shorter month, the grand lever pivots further, locking the cam at an earlier date.

To account for centurial years that are not leap years, this 48-tooth cam drives a reduction wheel one tooth forward every four years. The reduction wheel, in turn, rotates a 50-tooth secular wheel one tooth forward every eight years. Thus, the secular wheel achieves the speed of one rotation in 400 years. There is no patent protecting his mechanism, hence the bulk of his ingeniously simple solution, in terms of how the secular wheel overrides the month wheel, remains a closely guarded secret.  

Franck Muller Aeternitas Mega 4

Before striking out on his own, Franck Muller spent his formative years working at the workshop of Svend Andersen in the 1980s. Together they restored 60 Patek Philippe watches which reside in the Patek Philippe Museum today. It is thus no coincidence that their sources of influence and interests were intertwined. Franck Muller would also go on to build his own world-time watches based on Louis Cottier’s world-time system and together, they even co-developed a perpetual calendar module (2-30) in Svend’s workshop, which Franck implemented in his early watches.

Franck Muller’s secular calendar made its debut in 2007 in the Aeternitas Mega, the watch that ousted the caliber 89 as the most complicated timepiece in the world with a total of 36 complications. They included a tourbillon, a split seconds chronograph, equation of time, a Carillon-Westminster minute repeater as well as grand and petite sonnerie. Like the caliber 89, Franck Muller’s secular calendar featured a retrograde date function. However, it more closely resembles a traditional perpetual calendar with its reliance on a grand lever that samples a 12-month cam. 

The Aeternitas Mega 4 boasts a staggering 36 complications, 25 of which are visible on the dial

In reference to the secular calendar in the caliber 89, Franck Muller’s patent states, “On this gear train making one revolution in one hundred years is mounted a satellite with four branches which makes one quarter turn every one hundred years. This satellite takes the form of a Maltese cross, which can cause some problems in functioning.”

Thus, in the Aeternitas Mega, the 12-month cam takes on a more conventional form with plain sections and notches of varying depths corresponding to the length of each month in a single year with the deepest being February. A key difference between this format and the 48-month cam as well as the 12-month Maltese cross cam is that February 29 cannot be encoded in this cam, as it makes a full rotation once a year. As such, the grand lever has a secondary arm that carries a wheel with a leap year satellite cam. In this configuration, there is no need to override the month cam as the leap year is already separated. Instead, a secular cam has to correct the leap year cam. 

The split seconds mechanism visible on the back of the Aeternitas Mega 4; The flying tourbillon with an elegantly shaped cage

A quarter of the circumference of the leap year cam has a greater diameter to account for 29-day months. It completes one rotation around the pinion in a year, rotating on its own axis at a rate of a quarter turn every year. Every February the leap year cam is in such a position that a feeler on the central portion of the grand lever bears on it, accounting for leap years and non-leap years like a perpetual calendar. However, at the same time, the tail of the secondary arm bears on a notch of a secular cam, which makes a quarter turn every century or a full turn every 400 years. It is designed with three notches, enabling the lever to move back in such a way that the leap year cam is in the same position for a conventional 28-day month of February in three consecutive centuries, correcting it, and then a high step to account for February 29 of the year 2400. With the exception of the three consecutive centurial years, the tail of the lever always bears on the high step of the cam. It governs the entire calendar mechanism and as such, the beak of the grand lever is prevented from ever reaching the full depth of the February notch in the month cam. In comparison to the only other patented secular calendar in modern watchmaking, this is an ingeniously practical solution to overriding the leap year cam. 

Frank Muller’s secular perpetual calendar mechanism from the original patent

Without a doubt, the gear train of a secular perpetual calendar is one of the most elaborate in watchmaking. As longer subdivisions of time are taken into account, the gear train grows in complexity. However, it goes beyond good math when a movement is designed to account for the anomalies of the Gregorian calendar. Due to limitations of encoding a long interval of time in a single gear, myriad auxiliary mechanisms have to be devised to override the standard month cam in a perpetual calendar mechanism and that’s where the magic lies, making the secular calendar, apart from its supreme function, one of the most fascinatingly difficult mechanisms.

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