24 February 2016
By definition, the perpetual calendar is a watch capable of recognizing automatically the length of each month, whether it is 30 or 31 days—and, in the case of February, 28 or 29 days. In particular, Audemars Piguet’s Royal Oak Perpetual Calendar shows the date in the subdial at 3 o’clock and the month in the one at 12 o’clock, along with the four-year cycle (in which the letter L stands for leap year). Moreover, it gives the day of the week at 9 o’clock, the number of the week on the inner bezel, indicated with a central arrow-tipped hand, and the phases of the moon at 6 o’clock: highly evocative, the latter presents a perfect reproduction of our satellite on a laser microstructured disc of sapphire glass set against a background of aventurine resembling a starry sky. Despite concentrating so much information, the dial is easy to read and the case very thin (it measures less than a centimeter, 9,5 millimeters to be exact). This is made possible by the exclusive self-winding movement, designed and constructed entirely “in house”: made up of 374 parts, a series of technical expedients have allowed it to be kept to a thickness of just 4.31 millimeters. And it is so accurate that it needs very few manual corrections, if always kept wound: about once every 125 years for the phases of the moon, and in 2100 for the date (which ought to be a leap year, but will instead have a February of 28 days, owing to an exception of the Gregorian calendar). Made of steel or different shades of gold, it is in practice a machine of such great ambition that it looks beyond the individual human lifetime, so that it can be handed down to future generations. And is capable, therefore, of defying the passage of time: in short, the very essence of horology.