The Equation of Time is a watch for the true collector: 423 parts simultaneously indicate the equation of time, sunrise and sunset, the perpetual calendar and astronomical moon-phases. 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

The 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.

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.

The unique nature of the 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.

Material: Rose Gold
Model: 26603OR.OO.D092CR.01
Condition: Store Fresh
Details: This watch is Store-Fresh. It has never been carried/worn and includes all of the original packaging and materials. It is crafted in a rose gold case measuring 42mm. It features automatic movement, ivory dial and a rose gold bracelet. This particular one is specific to Mexico, but we are able to source from any country.
Includes: Box, Original Store Receipt, Authenticity Certificate

Signs best suited for: Aries, Leo

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