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The Perpetual Calendar by Roffensian

The Perpetual Calendar

This is a very simplified summary of the topic. It is designed to provide an overview of the subject for those who are interested, but will fall far short of a comprehensive discussion. I hope that readers will have a better understanding of the fundamentals, but they should not expect to receive a detailed description of all elements.

Thanks to everyone for the feedback on the threads so far. I know that some of you want something a little more technical, and I’ll try and put a more complete version together that does address a few more of the 200 or so parts that make up a ‘simple’ watch, but forgive me for being fairly basic initially – it’s a little bit of work to pull these together and I want to try and make them accessible to as wide an audience as possible.

One of the topics I got asked about was perpetual calendars, so let’s try and address that. It’s probably helpful to understand the basics of a watch first – I covered that here to some degree - Article on how a watchmovement works

I haven’t yet covered a normal date feature (I’ll try and get to it), but I think everyone understands that it’s simply a mechanism that is triggered once every 24 hours and counts from 1 to 31. If a month has less than 31 days then it needs to be manually adjusted.

A perpetual calendar gets around that by ‘knowing’ how many days there are in a month. There are two types of perpetual – the semi perpetual which can handle everything except leap years, and the full perpetual which can also handle the 29th of February. Let’s not worry too much about the non leap years at the end of the century 3 times out of 4 – because let’s face it, we’ll have forgotten this by 2100!

Most perpetual calendars also include a day, month and year feature in addition to the date – they have to know which month and year it is, so they may as well show it, and the day of the week doesn’t make things much more complicated. It’s also important to understand that while the vast majority of watches use the same mechanism to tell the time, there are a number of different ways to build perpetual movements. I’ll therefore try and focus this article on the process rather than the design.

This is going to get complicated....

Most perpetuals have separate mechanisms for date, day, month, etc – it’s why you need to press a number of buttons in the side of the case to adjust them if they stop. However, all of these are obviously linked together for regular operation – it is the ‘power’ that comes independently from the movement.

The secret to a perpetual is the cam that drives the months. This cam has three different types of markings on it – generally 7 ridges (for months with 31 days), 4 gaps / dents (for months with 30 days), and a larger dent with an additional rotating piece for February.

A lever pushes against this cam and then connects to the date wheel via a ‘finger’ or pawl that can engage with the teeth on the date wheel. The position of the lever (dictated by the ridge or gap on the month cam) will determine when the finger engages with the date wheel and hence when it forces the wheel to move to the 1st. Normally, the date is changed by another finger that engages at midnight (or thereabouts) and moves the date forward one day. When the finger from the lever attached to the month cam engages it does so at the 31st and overrides the finger that usually moved the date, thereby moving the date to the 1st.

For February, the additional rotating piece that sits under the date wheel is essentially a way to adjust the depth of the dent. It has four different settings and rotates once per year (off of the month mechanism). For three years out of four the setting is deep enough for the finger to engage on the 28th, on the fourth it makes the dent slightly shallower and triggers the finger to engage on the 29th.

Many perpetual watches also include a season indication, and that also works off of the month wheel, although it generally rotates each month to indicate not just the season, but where in the season we are (1st month, 2nd month, 3rd month). Seasons don’t tend to be driven off of the day or date because that would require allowance for the leap year as well to avoid slowly getting out of synch. For the same reason, week indicators (1 through 52) tend to be a feature on semi perpetuals rather than true perpetuals.

For a moon phase, the challenge is that the lunar cycle is actually 29.5 days, so a simple once a day movement doesn’t work too well. Watch manufacturers generally ‘cheat’ by treating the lunar cycle as 59 days and showing two full moons in that period. The wheel therefore has 59 indentations for a full rotation with the full moon painted on both sides of the dial and a window in just one half of the dial. The mechanism is triggered in a similar way to the date change movement (though generally not at midnight).

This is hugely oversimplified – a perpetual calendar adds at least 100 parts to a watch, but hopefully it gives some idea!