Perpetual calendar

The perpetual calendar is doubtless one of the most complex horological complications of all, often likened to a mechanical computer. Watchmakers have indeed imagined and developed a system that integrates and reproduces the vagaries of the calendar, automatically recognizing months with 28, 30 or 31 days, including leap years, until the year 2100.

The solar year – also called tropical year – corresponds to the time required for the Earth to complete its orbit around the Sun. This revolution lasts exactly 365 days, 5 hours, 48 minutes and 45 seconds, meaning 365.2421875 days. Yet the civil year introduced by Julius Caesar in his Julian calendar promulgated in 46BC lasts 365 days.

To compensate for the difference between the solar year and the civil year, and thus to avoid a progressive discrepancy with the seasons, an additional day was decreed every four years (corresponding to 4 x 0.2421875 days), thus instating the so-called leap years.

For a long time, attempting to keep track of the convoluted nature of the current calendar – with months of 28, 30 and 31 days we well as 366-days every four years – proved an impossible task for watchmakers. The solution finally came from a component called a “cam”. A metallic control disk with an uneven radius, the cam is an organ serving to transmit information or to transform motion. In the case of the perpetual calendar, there are two types of mechanisms: a 48-tooth cam or a 12-tooth cam.

The 48-tooth cam has notches alternating with solid sections. Each of these portions corresponds to a different month and, according to the depth of the notch, indicates whether the current period lasts 28, 29, 30 or 31 days. This cam makes a complete turn about its axis in 48 months, meaning four years and thus corresponding to the leap-year cycle. It passes on its information to a large yoke by means of a feeler-spindle, and the said yoke then acts – or does not act – on the day of the week, date and month displays.

Meanwhile, the 12-tooth cam operates in two ways: with a second leap-year cam, or with a Maltese cross stopwork device. Basically, the principle is the same as with the 48-toth cam, although in this case the disk makes one full turn in a year, and the 12 notches and solid sections correspond to the 12 months of a common year. It thus became necessary to find a system which, once the leap year occurred, enabled the large yoke to advance to a degree corresponding to February 29^th.

The first solution consists in fitting the large yoke with a second tip, which comes into contact every four years with a second cam called a “leap-year cam”. The latter then compensates for the absence of a notch corresponding to February 29^th. The other solution features a Maltese cross stopwork: the 12-tooth cam is formed of solid sections corresponding to 31-day months and rather shallow notches corresponding to 30-day months. At the place where February would normally appear, however, the cam features a circular cut-out housing the leap-year cam. The latter comprises three identical parts corresponding to February 28^th, and a larger section corresponding to February 29^th. Each year, it makes a quarter of a turn thanks to the Maltese cross.

These two mechanisms – 48-tooth and 12-tooth cam – are generally recognizable by closer observation of the watch dial: in the first instance, the hand indicating the months performs one full turn in four years and successively points to the 48 months of the leap year; while for the second, the display indicates only 12 months and a second hand provides leap-year information.

The word ‘calendar’ is derived from the Latin word calare, which means ‘to call’. On the first day of the month, the Romans were indeed literally called upon and solemnly informed of the feast days. In 46 BC, Julius Caesar was the first to introduce a mode of time organization closely tailored to the tropical year – the time taken by the Sun between two passages through the equinox. The Julian calendar thus estimated the latter at 365.25 days and established the civil year at 365 days. To compensate for the difference between the two, the emperor also introduced an extra day every four years.

However, progress in mathematics and astronomy soon helped determine the duration of the tropical year more precisely: 365.2421897 days, implying a 0.0078-day difference compared with the Julian calendar. By the 16^th century, the latter was a full 10 days ahead of the equinoxes and solstices. In 1592, Pope Gregory XIII decided to correct this discrepancy by having Friday October 15^th directly followed by Thursday October 4^th. At the same time, he eliminated a leap year every 100 years, apart from century years divisible by 400. That is why 2100, 2200, 2300, 2500, etc., will exceptionally not be leap years. Such are the vagaries of the Gregorian calendar.

At the time, pocket-worn horology was in its infancy. Calvinist Geneva had just forbidden the production of jewelry and horologists set themselves up as a corporation (guild) in 1601. Although certain calendar functions, such as the date and moon phases, appeared at a very early stage on certain models – even before the introduction of the seconds hand –, it would be another two centuries before the ‘governors’ of time would succeed in producing models automatically mastering calendar fluctuations.

In around 1762 and 1764, Thomas Mudge created two automatically triggered perpetual calendar watches including the February 29^th of leap years and programmed for 100 years. In 1770, Jean Antoine Lépine responded to an order placed by Louis XV for a model described in the Tablettes royales de renommée, dated 1772, as an “astronomical repeater watch invented by him with equation of time and perpetual calendar functions [...]. It marks off the moon phases, months, days, hours and minutes, as well as beating the seconds in the center.”

Several remarkable models were created thereafter, some of them indicating the number of the week, the year, the date of Easter, the equation of time, sunrise and sunset times, sidereal time, the line of knots useful in forecasting eclipses as well as Ephemeridae tables (solstices, equinoxes and seasons). Among them, one should mention the “Marie-Antoinette” by Abraham-Louis Breguet. Completed in 1827, it comprises an automatic winding device, a power-reserve indicator, independent deadbeat seconds, a jumping hours hand, a minute repeater, a perpetual calendar with retrograde date, an equation of time and a thermometer.

The first perpetual calendar wristwatch was the work of Patek Philippe in 1925. The Maison equipped it with a previously unused movement from a 1898 ladies’ pendant watch and adapted it to a wristworn model. This horological complication was subsequently associated with other functions. Ebel was the first to launch a perpetual calendar and moon phase chronograph in 1984, powered by a Dubois Dépraz module fitted on the Zenith El Primero movement.