February 29: leap years and watchmaking. Perpetual Calendars, the eternal complication
Decoding perpetual calendars. Every four years, February has 29 day. Hence, today February 29 we thought it was time to go over the particularities and the complexity of including the European calendar in watches.
Time is infinite and is defined according to the perception of men, women and, all things considered, every living organism. It evolves according to the level of conscience of nations, beliefs, peoples and the precision of measurements attained thanks to scientific discoveries. In a way, our life is controlled by many crossed temporalities and somehow, no one seems to be aware of this.
Sidereal time vs civil time
In order understand that different time standards may coexist, we need firstly to grasp the fact that the Earth does not always revolve at the same speed. Further, the Earth’s orbit is elliptical, which means that, when approaching the sun, our planet speeds up. That is why days are perfectly quantifiable variable values.
It took men considerable scientific progress to realize the existence of this vacuity. Sidereal time has long been used as the basis for calculation, yet significant variations were detected thanks to advances in watchmaking. On average, a sidereal day lasts 23 hours 56 minutes and 4 seconds, that is, 3 minutes and 56 seconds less than a day in civil time.
We use mean civil time in our daily life and in watches. It gives days a value of exactly 24 hours all year round. This value is obtained by calculating the annual variation average of mean solar time, which is the legal time in a certain territory according to a defined time zone.
The fact that the Earth’s orbit around the sun is elliptical and that its axial is tilted means that the difference between the shortest and the longest day of every year amounts to exactly 30 minutes and 45 seconds.
Ever since the West has been using reliable time instruments, the duration of days has been calculated based on an average solar – 86,400 seconds in a day, that is, our legally accepted 24 hours.
In one year, there are only four days in the calendar when solar time and mean time coincide. Said days are: April 16, June 14, September 1st and December 25. Whilst a common year has 365 days, a leap year counts 366. So much for our little rundown.
What does calendar mean?
It is a set of conventions adopted to make the civil year and the tropical year coincide with the aim of establishing subdivisions such as months, weeks and days. Calendars have been modified for centuries to find a more suitable approach to reflect the real duration of a year. The latter is exactly 365.2422 days (Gregorian year) – or, in “common year” terms: 365 days, 5 hours, 48 minutes and 45 seconds. The solar calendar is most commonly used in Western Europe, with the exception of Jewish and Muslim calendars, which are lunar.
Ours has been called the Gregorian calendar since 1582, when Pope Gregory XIII ordered to correct the many errors cumulated in the Julian calendar, which Julius Caesar himself had adopted to make it more precise. The Gregorian calendar, hence, added a leap year every hundred years except in centennial years, which are the ones that are divisible by 400. That means that 2100 will not be a leap year and hence timepieces equipped with Perpetual Calendars will have to be corrected manually. Only Secular Calendars, that is, ultra-complicated watches, follow this variable by proposing a time measuring operation that occurs every 400 years. Yes, that’s right, it is possible.
Developing the perpetual calendar
The perpetual calendar is a mechanical complication with gear trains that work as counters. Starting from the hour wheel, the gear trains count the amount of dial rotations the hand makes and transcribe them into days and dates. The day-indicator disk moves a notch every 24 hours and can be manually stopped to be synchronized with the date display. The 31-day calendar mechanism (the one that deals with the date) engages with the disc of the month indicator.
In order for the date indication to be considered “perpetual”, it must take into account, without exterior help, the 30-day and 31-day months, the repetitive odd months – July and August – and, more importantly, the additional day that is added to February every leap year.
To memorize and transmit the information, the month wheel is linked to a rotating mechanism with notches of different depths on the rim. At the end of each month, a probe delves into one of the holes whose depth will determine the date to be displayed. It should be noted that this complex system often comes with a moon phase, which is absolutely optional, as this function has no direct link to the perpetual calendar and can work autonomously.
Front picture: MB&F Legacy Machine Perpetual