The Basics and Beyond: Perpetual Calendar
One of the most hallowed complications in watchmaking and the ingenuity involved in simplifying their complex makeup.
The calendar has, across the gulf of centuries, given our species a sense of the big picture, of dailiness and eventfulness as well as a vital historical consciousness. By contrast, time measured by the clock seems to be abstracted from the motion of the universe, as if time was granular or a series of short, disjointed phases rather than continuously hastening the frantic world on with each tick.
In our modern technological age, the calendar serves as a connection to the natural world, whereby the rhythms of the earth and sky coordinate with that of human life. Naturally, a mechanical watch that denotes the progress of the calendar has to embody these cosmic rhythms in gear patterns and ratios, namely the speed at which the Earth rotates on its own axis, forming the day, the Moon’s orbit around the Earth, giving us the month, and finally, the Earth’s orbit around the Sun, which marks a year.
The highest order of such watches is the perpetual calendar as it needs to accomplish a high degree of automation, namely a yearly reiteration, hence having to obey the dictates of the Gregorian calendar in which each leap year has 366 days instead of 365, giving February 29 days instead of the common 28. This means that the mechanism has to correctly care for all irregularities of the calendar, acting at the end of each month, changing itself to the first day of the following month, sometimes after 30 days and other times after 31 days, changing once a year on the 28th day of February and finally, extending February by a day every four years. The complexity of encoding this nest of rules makes the gear train of a perpetual calendar one of the most intricate, perhaps second only to that of a minute repeater — both of which, with much irony, are driven by the humble motion works that spell out the time in every watch.
However, a perpetual calendar, contrary to the optimism expressed in its name, is not everlasting. By design, a perpetual calendar omits the one rule that makes a calendar Gregorian rather than Julian and that is, centurial years are only leap years if they are divisible by 400. The mechanism will interpret the years 2100, 2200, and 2300 as leap years when in fact they are common years. In other words, a perpetual calendar will recognize the length of every month irrespective of whether it contains 28, 29, 30 or 31 days until the year 2100 before it requires a manual intervention. It is worth noting that the only kind of calendar watch that can account for this principal anomaly is the secular perpetual calendar, making all regular perpetual calendars, more strictly, Julian calendars.
Perpetual calendars are predominantly executed in one of three formats. It can be designed with 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 while working with a separate leap year cam, or a 12-month cam with an integrated Maltese cross satellite to manage the month of February. At the heart of each format is a grand lever that advances the date one day forward every day at midnight and is built to automatically advance past the additional days at the end of months shorter than 31 days.
Most perpetual calendars are modular in construction in that the calendar mechanism is fitted on an additional plate rather than being an integral part of the movement. There are a handful, however, wherein the baseplate was designed to accommodate calendar indications that extend to the back of the watch.
The gear train that encodes the units of the date — day, month and year — are derived from the motion works, specifically the hour wheel of a watch. This makes the gear train of a perpetual calendar mechanism, like the motion works itself, a reduction train. As illustrated in the diagram, driven by the motion works below the calendar plate is a 12-hour wheel that drives a 24-hour wheel. Affixed to it is a finger that indexes a moon gear, typically with 59 teeth to account for two lunar cycles, advancing it by one notch every 24 hours. At the same time, this finger also comes into contact with the arm of a grand lever, lifting it every 24 hours. The grand lever then pivots, and another arm engages a seven-point day star wheel, moving it forward by one step. At this point, a beak on the grand lever also comes into position to actuate a 31-tooth date star wheel, advancing it by a notch.
All this time, the third arm of the grand lever bears on a 48-month cam that is driven at the rate of one rotation in four years. This cam governs the travel of the grand lever. It consists of notches of varying depths, four specifically to correspond to the length of months — 31, 30, 29, and 28 — with the deepest representing 28 days for February and its full diameter representing 31 days. Each slot increases or decreases the length of travel made by the grand lever and the arm that drives the date star. This enables the arm to advance the date wheel further, when necessary, by several days at once so that it can jump from 28 to 1 at the end of February, for instance.
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 fourth arm on the grand lever locks into a date snail cam and forces the date change. Hence, if the grand lever pivots further, it catches the snail cam at an earlier date.
The date star carries a finger that engages a 48-month wheel at the end of every month. An intermediate wheel affixed to it then drives a 48-tooth wheel, consequently rotating the 48-month cam.
It is difficult to determine which format of program wheel is used by looking at the dial as there are ways to achieve a desired design regardless of which is used; watches that adopt a 48-month cam do not necessarily have a 48-month indicator. Known examples that use a 48-month cam system are any of Patek Philippe’s three- register perpetual calendars.
12-MONTH CAM WITH SEPARATE LEAP YEAR CAM
A 12-month cam differs primarily from a 48-month cam in having a rotation speed of one turn per year. As such, a leap year, which occurs once every four years, cannot be encoded in this cam. It consists of notches of only three varying depths, accounting for 31, 30 and 28 days. To account for February 29, the 12-month cam works in conjunction with a separate leap year cam.
In such a configuration, the 12-month cam is sampled by an arm of the grand lever while a second arm bears on a leap year cam. The latter is co-axially mounted on the 48-month wheel, which completes one rotation in four years. The leap year cam has a large plain section that corresponds to a conventional 28-day month of February in three consecutive years, followed by a high step that takes care of a 29-day month. During a leap year, the lever bears on the high step and this prevents the first arm from reaching the full depth of the February notch in the 12-month cam, thereby overriding it.
This limits the travel of the grand lever which then catches the date cam later and advances three steps at the end of the month.
12-MONTH CAM WITH MALTESE CROSS SATELLITE
An alternative to having a separate leap year cam is to incorporate a Maltese cross system in the 12-month cam to manage the month of February. In this format, the 12-month cam is affixed to the month star wheel and consists of two depth profiles, with the full diameter corresponding to months with 31 days, and shallow recesses that take care of months with 30 days. The cam features a cutout where February ought to be and in its place is a leap year cam. The latter has four points — three identical points of the same height and one point that is longer. This cam is fixed to a Maltese cross that is pivoted on the month star.
The Maltese cross, like that used as a stopwork, rotates around a finger cam that is mounted on the month star. When the finger enters a notch in the Maltese cross, it forces it to make a 90-degree rotation. This continues such that the longer, protruding side is exposed to the grand lever in the fourth year, limiting its travel to account for February 29.
ADJUSTMENT VIA CORRECTORS
While a perpetual calendar only requires a correction after every full century when the Gregorian calendar omits a leap year, like a regular watch, it comes to a halt when the mainspring winds down. Thus, correctors are installed so that the day, date, month and moonphase can be manually adjusted with little pushers set into the caseband using a stylus.
Each indicator has its own corrector in the form of a lever that overcomes the tension of a spring when it is depressed, enabling adjustments to be made forward. When changing the month, a lever lifts the grand lever, so the 48-month cam is freed and the corrector lever advances a pinion, which drives the 48-month wheel. The day star can be set independently with its own pusher and corrector lever as well as together with the date via a second pusher that acts on the grand lever.
BREAKING THE MOLD: IWC DA VINCI PERPETUAL CALENDAR
The complex, notably three-dimensional construction of a perpetual calendar characterized by levers and springs results in high cost in manufacturing and assembly. Hence, for a long time, perpetual calendars remained the preserve of top watchmaking companies the likes of the holy trinity.
But in 1985, a new wind began to blow when IWC introduced the Da Vinci, the first perpetual calendar in which all indications can be adjusted via the crown. Designed by the firm’s legendary head watchmaker Kurt Klaus, the calendar mechanism did away with corrector levers and reduced the number of intermediate components by having each display wheel drive the next directly, greatly reducing part count. As such, the movement was notably compact and slim while the calendar display was remarkably complete with the addition of a full four-digit year.
The module was characterized by a uniquely shaped grand lever, which is driven back and forth by a drive wheel linked to the motion works. It has a feeler arm that samples a 48-month cam while its ring-shaped arm has a shift pawl that advances a 31-tooth date ratchet wheel daily. Instead of having a separate finger cam, a tooth that extends further outward radially on the date ratchet wheel advances the 48-month wheel directly.
The ring-shaped arm has an additional pawl that rides against the date cam. As with a standard perpetual calendar, this pawl catches the cam earlier or later, as governed by the depth of penetration in the 48-month cam, and forces the date change. At the end of a 28-day February, for instance, the swing of the grand lever leads to a turning of the date wheel by three teeth by the pawl and the turning of another tooth by the shift pawl, thereby progressing by four teeth to March 1.
In effect, the date wheel indexes the 48-month wheel directly and the latter drives a 12-month wheel on which the month hand is mounted. The 12-month wheel has a finger affixed to it that engages a pair of co-axially mounted intermediate wheels of the same diameter and pitch. The first wheel has 10 teeth while the second has a single tooth. They mesh with another pair of identical co-axially mounted wheels to achieve a digital year display. One gears with the 10-tooth wheel, giving us the year and the other, the single-tooth wheel, forming the decade wheel. Thus, the year wheel is moved once a year by one tooth and the decade wheel is advanced once every 10 years by one tooth.
Along the periphery of the decade wheel is a protrusion that engages the horizontal recesses in a century slide such that after each revolution of the decade display ring, the century slide is moved further by an amount equal to one recess. A detent device consisting of a disk that is tensioned by a spring into the edge of the recess secures it in place after each displacement.
The daily swinging motion of the grand lever also advances a day star by one tooth. The day star, pivotably mounted on a moonphase wheel, drives a reduction train at a ratio of 8.4375:1, enabling the moon disk to make one rotation every 59.0625 days.
REINVENTING THE WHEEL: ULYSSE NARDIN PERPETUAL LUDWIG
Following IWC’s breakthrough, developments that followed the same path of simplicity slowly began to emerge. The most significant was the Ulysse Nardin Perpetual Ludwig launched in 1996 to mark the brand’s 150th anniversary.
The watch was named after Ludwig Oechslin, a Vatican clock restorer and polymath, who spearheaded the rebirth of Ulysse Nardin, along with its visionary owner, Rolf Schnyder. The Perpetual Ludwig was the first perpetual calendar that enabled forward and backward date adjustments over a single crown.
This was made possible by eliminating the conventional 48- or 12-month cam with deep notches, which immediately quashed the need for a grand lever. However, this is not to say that the resulting mechanism was simple. On the contrary, its structural simplicity — composed of nothing but precise gearwork — is exceedingly hard to achieve, and the genius by which the solution was produced only inspires awe.
At the heart of the mechanism is a pair of co-axial wheels — a 31-tooth date wheel that completes one revolution per month and a co-axial fixed wheel. The date wheel is driven forward by a 24-hour wheel, which, as with a standard perpetual calendar, is controlled by the hour wheel. However, this 24-hour wheel comprises of two stacked wheels — the first wheel with a single long tooth to advance the date wheel by one tooth at the end of months with 31 days and the second wheel, superimposed on the first, with three successive long teeth to take care of months with 30, 29 or 28 days. The wheels are offset such that the long tooth of the first wheel is immediately followed by the three long teeth of the second wheel to advance the date wheel by additional steps at the end of months with less than 31 days.
As the date wheel turns, a system of planetary gears rotates around the fixed wheel. It consists of three wheels — the first for months with 30 days or less, the second for February and the third for leap years. They are rotatably mounted on individual axles on the date wheel. Each of the three wheels have one or more teeth that are longer than the others — four (out of 24) teeth on the first wheel, one (out of 24) on the February wheel, and three (out of eight) protruding teeth on the leap year wheel. The longer teeth protrude at the periphery of the date wheel as the latter rotates and are retracted inwards the rest of the time. As the longer teeth emerge on periphery at the end of the month, they come into contact with three additional teeth on the 24-hour wheel to generate the additional steps on the date wheel at the end of a month of less than 31 days.
The date wheel itself is mounted on a large finger cam, which indexes a pinion at the end of the month. This pinion in turn drives the month wheel.
As a result of being entirely made of gears, the calendar can be adjusted both forward and backward and is eminently more resistant to shocks, making it more robust.
THE MOST ADVANCED PERPETUAL: H. MOSER & CIE. PERPETUAL CALENDAR
In 2005, H. Moser & Cie. went a step further and developed an instantaneous perpetual calendar that enabled forward and backward adjustments of the date via a single crown. Aside from being predominantly made of gears, Moser’s perpetual calendar introduced two more factors to the equation that made its construction inherently unusual — a centrally-mounted month hand and an instantaneous date wheel — hence, differing drastically from Ludwig’s invention. While the resulting mechanism is exceedingly simple and practical, it required nothing less than the genius of Andreas Strehler, an independent watchmaker who is best known for having developed the most precise moonphase indicator in the world with an error of only one day in 2,060,757 years.
The date mechanism consists of two disks, arranged one over the other and operates in a relay. The top date disk bears the date numerals “1” to “15” followed by an aperture, through which the bottom disk with “16” to “31” can be read. In the second half of the month, the top disk is stationary with the aperture positioned at three o’clock and the bottom disk advances. At the end of the month, the top disk makes a single jump to the first day of the following month.
This is made possible by a highly unusual switching mechanism that indexes the top date disk out of its stationary position at the end of any given month. It consists of an index and a switching spring located on the circumference of the lower disk that acts on a series of four latches on the circumference of the top disk spanning from the aperture through to the numeral “3” on the date wheel. This switching spring can be moved outwardly in three positions, accounting for the four varying lengths of months, with its starting position, the innermost position representing 28 days. This is controlled by a cam with two depth profiles representing days with 30 or 31 days and a lever that pivots in three positions with its innermost position accounting for February, with 28 days, middle position for 29 days and outermost position for months with 30 as well as 31 days.
In a month with 28 days, the control lever is in its innermost position and the switching spring assumes its initial position while in a month with 29 days, the control lever is in the middle position where it acts on the switching spring so that it assumes the first position. However, on months with 30 or 31 days, the lever is in its outermost position, and it is the two-step cam that will determine whether the switching spring occupies the second or third positions. Depending on the position of the spring, it acts on one of the latches, thus moving the upper date disk to the 1st of the following month. For instance, on a 28-day February, the spring, which is in its innermost position locks on the first latch, and the date disk shifts to display the 1st of March.
In addition, a pin on the top date disk indexes a ratchet wheel — their engagement akin to a Maltese cross system — which drives the central month wheel via an intermediate gear.
EXTREME REDUCTION: OCHS UND JUNIOR PERPETUAL CALENDAR
When Ludwig Oechslin founded Ochs und Junior in 2006, he began streamlining the complication even further by rethinking its basic design and in 2016, arrived at a perpetual calendar module that was made up of just nine components. The novelty lies not only in the way the calendar mechanism was constructed, but also the way the indications were displayed, which made the severe reduction possible.
The date is displayed using an outer ring of 31 perforations with an elongated colored date marker printed on a rotating disk beneath it. Thus, the marker appears through the perforation to indicate the date, which can be conveniently read against the 10-minute markers, each representing five days. The month, on the other hand, is indicated by a set of four perforations on a centrally mounted disk that makes a complete rotation in 12 months, while leap year is indicated by a colored dot that emerges on the outward facing perforation in February of a leap year. In other words, in February of three common years, the colored marker is on one of the three inward perforations.
The mechanism is ingeniously simple. In fact, the best way to understand it is to forget all other ways a standard perpetual calendar accounts for leap years. A 12-hour wheel mounted on the hour pinion drives a 24-hour wheel which drives the date disk forward by four teeth every day. The date wheel has 124 teeth, hence four per day. During the three final days of the month, the date wheel starts advancing the month ratchet wheel, which drives the month disk via an intermediate wheel. The month disk carries a four-year Maltese cross wheel, which has four arms of equal length that are interspersed by four fingers, of which one is shorter than the rest. The arm beside this shorter finger has a colored dot to indicate leap year. As this four-year wheel rotates, the longer finger pushes against a small lever, causing it to protrude on three consecutive years that are not leap years, and on the fourth year, the shorter finger causes the lever to retreat, allowing the date disk to advance by a day to 29 February while the colored dot appears on the periphery to indicate leap year.
These examples are particularly enlightening in demonstrating that the ease of use and simplicity in design, construction and assembly of a modern perpetual calendar belie some seriously creative and complex solutions. As such, despite their obvious utility and necessity, these advancements remain a rarity in watchmaking.