Audemars Piguet Royal Oak Chronograph Automatic Silver Dial Rose Gold Men's Watch 25960OR.00.1185OR.02

The Equation of Time is a watch that does not immediately reveal all its secrets, instead preferring to cultivate its aura of mystery and fascinating complexity. Nonetheless, its beauty is self-evident. For the very first time, it is interpreted not within a classical line, but in the Royal Oak collection. Since its creation in 1972, the legendary octagon has embodied the most daring innovations in the field of fine watchmaking. The 423 parts serving to drive the equation of time, sunrise and sunset, perpetual calendar and astronomical moon-phase displays are now housed within the famous Royal Oak case. The Royal Oak Equation of Time thereby represents the perfect blend of traditional and modern watchmaking elements.
Inimitable octagon
Its elegant case, available in a choice of pink gold or steel, is distinguished by an octagonal bezel punctuated by polished white gold hexagonal screws. The crown also picks up this hexagonal motif and proudly bears the AP monogram. The signature features include the distinctively shaped studs along with the alternating satin-brushed and polished finishes on the case. By way of example, the side of the bezel is polished while the upper surface is satin-brushed and matt. The silvered dial remains faithful to the identity codes of the Royal Oak collection by picking up the "Grande Tapisserie" motif which highlights the counters and enables pleasant and easy read-off of the various indications: date, day, leap year, moon phase, as well as sunrise and sunset times. The equation of time pointer and the counter hands are in blued steel. The Royal Oak Equation of Time is teamed with a leather strap in brown alligator leather for the pink gold version and in black for the steel model.

The equation of time
The equation of time is the difference between true solar time and legal time. The solar day, meaning the lapse of time between two consecutive passages of the sun through the meridian in a given place varies throughout the year. It lasts exactly 24 hours only four times a year: on April 15th, June 13th, September 1st and December 25th. This variation is due to the earth's elliptical path around the sun as well as its tilted rotation axis. For convenience, our societies have calculated an average of all the days of the year and defined the mean solar day as comprising 24 hours, corresponding to so-called legal time.
Nonetheless, the mean solar culmination point depends on the wearer's exact location, on the longitude of a given place on the globe. The equation of time read-off system developed by Audemars Piguet takes account of this specific place. Unlike most other equation of time mechanisms, it is not set to a given time zone - corresponding to 15 degrees longitude. The "world standard time" system divides the world into 24 time zones and takes the Greenwich meridian as the zero point. While the official time is thus the same across the width of each time zone, true solar time varies by four minutes per degree (15 degrees = 1 hour). Therefore, although the clocks on the Bahnhofstrasse in Zurich and on the Rue du Rhône in Geneva indicate the same time, true noon can be observed a few minutes earlier in Zurich than in Geneva.
Not only does the Audemars Piguet mechanism take account of this local variation, but also enables one to read off the solar culmination time at any time of the day. To do so, this exceptional mechanism is personalised and adjusted according to the degree of longitude chosen by its owner. The name of the chosen city and the solar zenith in this place are engraved on the ring (such as 12.36 for Geneva). The ring also bears a graduated scale running from -15 to + 15 minutes. The zero point of the graduation is located exactly at the mean solar culmination time in the chosen place. The equation of time hand continuously displays the difference between the two values. When the hour hand corresponds to the time indicated on the graduated ring and the equation of time and minute hands are superimposed, the owner of the watch knows that the sun is exactly at its zenith. It's exactly as if he had a sundial on his wrist, and there is no need for any mental arithmetic!

Indication of sunrise and sunset times
The Royal Oak Equation of Time is one of the rare watches to indicate the sunrise and sunset times. These indications depend on three parameters: the date, as well as the longitude and latitude of a given place. The counter at 9 o'clock displays the sunrise time, while the sunset time is read off on the 3 o'clock counters. Their hands are controlled by two cams performing one full rotation per year. The shape of the cams is determined by the latitude of each location, while their position along the driving wheel depends on longitude. These cams must be machined with extreme precision, since a mere hair's breadth corresponds to a discrepancy of eight minutes! The cams are pre-calculated and available for 250 cities. On request, it is also possible to obtain cams calibrated for any other location, provided it lies between latitudes of 55 degrees north and 55 degrees south. For places closer to the poles, it becomes impossible to maintain a sufficiently wide diameter for the cam.
The perpetual calendar
The perpetual calendar mechanism is a marvel of miniaturisation, a highly sophisticated mechanical reproduction of our calendar. How can a timepiece, merely by the meshing of its toothed wheels, levers and clicks, successfully indicate the day, the date, the month, the moon phases and the leap-year cycle?
The heart of the perpetual calendar system lies in the cam on which the various lengths of the month are defined. The protruding sections around its circumference represent 31-day months, and the notches those of 30 days. On the same axis, a wheel bearing a cam indicating standard months of February and a leap-year month of February performs one turn every four years and is thus able to keep accurate track of the leap-year cycle. The perpetual calendar thus requires no adjustments before March 1st 2100, when simply activating the correctors recessed into the side of the case will be enough to readjust the mechanism for another century. The perpetual calendar also drives the other complications (apart from the moon phase). This additional technical feat means that if the watch has stopped, adjusting the date automatically resets the other functions, thus ensuring that the displays remain synchronised.
The astronomical moon
As the 'daughter' or astronomy, watchmaking has constantly attempted to reproduce on dials the appearance of the moon at various stages in its cycle. The most commonplace display device consists of a disc carrying two moons driven by a 59-tooth wheel. A fingerpiece drives the wheel one notch forward every 24 hours and the moon phase corresponding to that particular day appears through a dial aperture. Within this system, a lunar cycle thus corresponds to 59:2 days, meaning 29 days and 12 hours. Yet a true lunar cycle lasts 29 days, 11 hours, 44 minutes, 2 seconds and 80 hundredths of a second. This difference thus leads to a discrepancy. To solve this problem, the Audemars Piguet master-watchmakers have developed a more accurate moon-phase display: an "astronomical" moon. The wheel driving the moon comprises 135 teeth instead of the usual 59. The larger number makes it possible to measure a lunar cycle of 29 days, 12 hours and 45 minutes, which is just 57 seconds and 20 hundredths of a second more than a true lunar cycle. This system thus requires a one-day correction just once every 122 years and 44 days!

Base Calibre 2120
The impressive complexity of the above-described mechanisms - equation of time, sunrise and sunset indication, perpetual calendar and astronomical - puts the Royal Oak Equation of Time firmly at the pinnacle of the complicated watches that are the pride of Haute Horlogerie. These brilliant mechanisms are teamed here with a base calibre that is equally favoured by specialists: Calibre 2120. Measuring a mere 2.45 mm thick, it is quite simply the world's thinnest mechanical selfwinding movement with a central rotor: a paragon of fineness, reliability and performance entirely developed, produced and exquisitely finished within the Audemars Piguet workshops in Le Brassus.
The automatic winding is handled by a central rotor with a 21-carat gold segment. In addition to the sheer nobility of this metal, its weight also optimises bidirectional winding and thus the energy transmitted to the barrel - which in turn guarantees the power reserve. This effective winding system combined with the substantial power reserve ensures constant efficiency over a longer time period, and the quality of the winding also favourably influences the accuracy of the watch.
This same concern for performance led Audemars Piguet to choose a central rotor rather than an integrated mini-rotor. This particular technical solution means that the oscillating weight runs on another level than the barrel and the balance, thereby providing the latter two organs with more space in which to operate. The larger the barrel diameter, the greater the power reserve - which in this case amounts to 40 hours. Moreover, the larger the balance diameter, the greater the precision of the watch.
The impeccable finishing of the mirror-polished steel parts of the gear train and the use of 41 exclusively top-quality jewels guarantee optimal transmission of the barrel force to the escapement. The variable inertia balance enables particularly fine adjustment, thereby ensuring peerless precision over a longer period of time.
This outstandingly distinguished base movement is thus enriched with the equation of time, sunrise and sunset, perpetual calendar and astronomical moon mechanisms. The skill and mastery acquired by Audemars Piguet's watchmakers in the field of ultra-thin movements has enabled them to house the 423 parts of the Royal Oak Equation of Time within a tiny space just 5.35 mm thick and 28 mm in diameter (12 ½ lignes).
The finishing and decoration of this movement are entirely on a par with this wealth of technical ingenuity. All the bridges are chamfered and polished by hand. Connoisseurs will take particular delight in the work on the internal and exterior angles. They will also be pleased to know that even the parts hidden from their view are the object of meticulous care. By way of example, not only is the upper part of the bridges adorned with a Côtes de Geneve motif, but the invisible lower part is also finely circular-grained. The mainplate features five different circular-graining diameters that create an exquisitely refined effect, while even the inside of the case-back is also circular-grained, as a natural extension of the overriding principle of fine craftsmanship.
The unique nature of the Royal Oak Equation of Time lies in its capacity for customisation in accordance with the owner's wishes. To ensure correct read-off of the solar culmination times, the equation of time must refer to a given longitude. The sunrise and sunset times depend on both longitude and latitude. These two parameters imply choosing a precise spot on the planet, the owner's favourite place, the place of his choice. The indication of this place is then engraved on the graduated ring, along with the mean solar culmination time for that same spot. The lucky owner may also decide to personalise the oscillating weight of his timepiece by having it skeleton-worked to form his initials, his zodiac sign or any other motif of his choice.