Patek Philippe Watches

Sky Moon Tourbillon by Patek Philippe 5002

The rarest complications united in a wristwatch, Patek Philippe unveiled the Star Calibre 2000, one of the most complicated pocket watches of all times.

Barely six months later, the Geneva workshops introduced another Grand Complication - in a wristwatch format.

The "Sky Moon Tourbillon" Ref. 5002 is the first double-faced wristwatch that features a complete presentation of the nocturnal sky on its reverse side.

The masterpiece displays the apparent movements of the stars, the orbit of the moon, the moon phases, as well as the hours and minutes in sidereal time.

As was already the case with the Star Caliber 2000, Patek Philippe's goal in the development of the "Sky Moon Tourbillon" was not so much the number of complications per se. The real objective was to accommodate the most fascinating complications in the small case of a wristwatch: a perpetual calendar with a retrograde date display, a minute repeater, a tourbillon, the display of sidereal time, and a depiction of the nocturnal sky with the motion of the stars, the orbit of the moon, and the moon phases. This turned out to be a formidable challenge because a chart of the heavenly canopy (as seen in the northern or southern hemisphere) requires a certain minimum space to demonstrate the movements of the stars in a sensible and clearly legible manner. The solution adopted by Patek Philippe was borrowed from the astronomical pocket watches made by the company: a moving sky chart on the reverse side of the timepiece. The mechanical module developed by Patek Philippe for the "Star Calibre 2000" was redesigned especially for the "Sky Moon Tourbillon" and was granted Swiss patent CH 688 171 B5.

The acoustic indication of hours, quarter-hours, and minutes is and undoubtedly remains one of the most spectacular functions that a wristwatch can possess. When the slide on the left of the case is activated, the repeater first strikes the number of hours on a low-tone gong, followed by the quarter-hours with double strikes on the low-tone and a higher-tone gong, and finally by the number of minutes which have elapsed since the last quarter-hour on the higher-tone gong. In the "Sky Moon Tourbillon", this chime is implemented in a rare and fascinating manner. In the comparatively small volume of a wristwatch, it is extremely difficult to generate a clear and rich-sounding tone. A few years ago, after intensive collaboration with metallurgists of the Swiss Federal Institute of Technology in Lausanne, Patek Philippe succeeded in developing an alloy for gongs that create a particularly resonant sound. In the meantime, this alloy has been refined, making it possible to produce a special gong that can be more than one case circumference long. It is called "cathedral gong" because it renders the hour strike with a rich, full-bodied tone that like the bells in a cathedral reverberates for a particularly long period of time.

Every mechanical watch with a balance wheel has a so-called positional error when held vertically, because the centre of gravity of the balance spring is then outside of its axis of rotation. This error can be compensated with a tourbillon, a rotating carriage which incorporates the escapement, balance wheel, and balance spring, and as a rule turns about its own axis once per minute. Due to this rotary motion, the centre of gravity of the balance spring also rotates about the balance axis once per minute and thus automatically compensates the positional error. In the Ref. 5002, the tourbillon, the balance wheel, and the fourth wheel are located on one axis. This configuration is particularly difficult to implement, but it best exploits the principle of the tourbillon. The tourbillon in the Ref. 5002 is quite amazing: Crafted from steel, the tiny mechanism is composed of no less than 69 individual parts, yet it weighs a scant 0.3 grams - this suggests how microscopically small and filigreed the individual components must be.

The perpetual calendars of Patek Philippe are legendary. This also applies to the "Sky Moon Tourbillon", whose perpetual calendar has been refined with a date display that has an automatic flyback hand (retrograde date). It displays the date on a 270° arc on the dial. Every day, the date hand moves forward by one position and after the end of the month (the 28th, 29th, 30th, or 31st day) automatically jumps back to the beginning of the month scale. As opposed to conventional solutions with cams, this retrograde date is based on a patented ratchet wheel mechanism that features an extremely high degree of hand positioning accuracy. At the moment when the date hand leaps from the end to the beginning of the scale, the mechanism prevents it from inadvertently rebounding to the second or third day marker. Thus, when a new monthly cycle begins, the hand is instantly stopped and reliably retained at the position of the first day.

The remaining displays of the perpetual calendar are indicated with four subsidiary dials: the days of the week at 9 o'clock, the leap year cycle at 12 o'clock, the month at 3 o'clock, and the moon age at 6 o'clock. All displays are perfectly harmonized and need not be corrected until the year 2100 provided the watch is regularly wound.

A very unusual feature for a wristwatch can be seen on the reverse side of the "Sky Moon Tourbillon". The canopy of the northern (or on request of the southern) hemisphere rotates counter clockwise under the sapphire crystal case back, showing the motions of the stars and the moon, the meridian passages of Sirius and the moon as well as the waxing and waning moon phases. Additionally, two hands from the centre indicate sidereal time on a 24-hour scale. An elliptical contour indicates the portion of the nocturnal sky which can be viewed from a specific location. The breathtaking presentation conceals a module with a patented gear train never before built in this manner.

With its celestial mechanism for the "Sky Moon Tourbillon," Patek Philippe has succeeded in achieving an unprecedented degree of precision in the display of astronomical indications.

A lunar day is defined as the period of time which elapses between two consecutive passages of the moon across a certain meridian. On average, it is 24 hours, 50 minutes, and 28.328 seconds long. A lunation (the period between two consecutive full moons) lasts an average of 29 days, 12 hours, 44 minutes, and 2.82 seconds. A sidereal day is defined as the period of time between two consecutive passages of a fixed star (Sirius, for example) across a certain meridian. Its duration averages 23 hours, 56 minutes, and 4.09892 seconds in units of mean solar time. The geographical longitude of a specific location can be calculated with sidereal time.

For the respective displays, these deviations from mean solar time require appropriate transmission ratios in the gear train that delivers the force from the barrel to the individual astronomical displays. Some of these ratios favour the accuracy of the moon phase display versus the sidereal day, others the accuracy of the sidereal day versus the motion of the moon, etc. Overall, more than 25 trillion possibilities of varying the reciprocal transmission ratios in the gear train were computed. From this staggering number, Patek Philippe has found the optimum variation that achieves the smallest possible error for all displays. The result of the theoretical calculations has surpassed all expectations:

-For a lunar day, the error is 0.05 seconds per day, 18.385 seconds per year, or 30 minutes and 38.5 seconds per century.

-For a sidereal day, the deviation is 0.088 seconds per sidereal day, 32.139 seconds per year or 53 minutes and 33.9 seconds per century.

-For the moon phases, the error is 6.51 seconds per lunation.

The manually wound movement is masterpiece of precision engineering composed of a total of 694 individual parts, each meticulously finished by hand. The edges of all steel parts are bevelled and each individual tooth of every single gear is hand polished in an additional process using a rotating hardwood disk. This degree of perfection is not an end in itself. Polishing reduces friction at the tooth contact surfaces, thus contributing significantly to the timekeeping precision and longevity for which Patek Philippe watches have a legendary reputation. The plate and the module with the perpetual calendar are finished on both sides with an engraving technique known as "perlage" or circular graining. The bridges are decorate with an engraving technique known as "vagues de Genève" or Geneva striping. Because of the painstaking processes and work-intensive steps needed to test timekeeping accuracy, it takes many months to complete one movement. The result is a flawless marvel of precision engineering worthy of the prestigious Geneva Seal embossed on its bridge, the highest official hallmark of quality awarded to mechanical timepieces. Each individual "Sky Moon Tourbillon" Ref. 5002 watch is delivered with a COSC chronometer certificate, as are all Patek Philippe tourbillon timepieces.

The case in 18K yellow gold or solid platinum affords both aesthetic and effective protection of the "Sky Moon Tourbillon" movement. It looks gracious and elegant despite its sizable dimensions: 42.8 millimetres in diameter and 16.25 millimetres in height. The comparatively wide case is decorated with an engraving of halved Calatrava crosses. The crown at 4 o'clock is used to wind the movement and set the hands on the front side. The crown at 2 o'clock corrects the position of the sky chart and the hands that indicate sidereal time; turned in the other direction, it corrects the position of the moon and the moon phase display. The side of the case also accommodates a number of correction buttons which are activated with a setting stylus. The first button, between 11 and 12 o'clock, simultaneously corrects the date and the day of the week. The second one, between 3 and 4 o'clock, corrects the month. The third button, between 5 and 6 o'clock, corrects the moon age display, and the fourth one, between 6 and 7 o'clock, corrects the day of the week only. The slide which starts the minute repeater is recessed in the case on the left-hand side. The precious timepiece is worn with a hand-stitched strap made of crocodile leather; the prong buckle is made of 18K yellow gold or platinum to match the case.

The design of the dials on the front and reverse sides of the timepiece is a challenge in itself. A multitude of indications must be accommodated on comparatively small surfaces without compromising legibility and the harmony of the individual displays. The front dial is crafted from gold with a silvery opaline coating; at its periphery, the hours are represented by Roman numerals from I to XII. The retrograde date display is on an inner ring between 2 and 10 o'clock with a feuille hand made of blued steel. The minutes are indicated on a railway scale with a gold poire hand that matches the colour of the case. The day of the week and the months are displayed with blued-steel poire hands on subsidiary dials at 9 and 3 o'clock, respectively. The leap-year cycle and the moon age are indicated with gold feuille hands on subsidiary dials at 12 and 6 o'clock. The moon symbols on the moon age display are made of yellow or white gold to match the case. The new moon symbol is black. The middle of the dial is decorated with a pattern composed of small Calatrava crosses.

The rear dial showcases the astronomical functions. It has a 24-hour scale with Arabic numerals at its outermost periphery, used to indicate sidereal time with white, balanced poire hands. A blue sapphire crystal disc with 279 teeth rotates to track the angular motion of the moon and in a small round aperture shows the moon phases. The sky chart is on a separate, transparent sapphire crystal disc with 356 teeth. Both discs are protected by the sapphire crystal case back which on its inside bears the 24-hour sidereal dial and a golden ellipse framing the portion of the sky that is visible from a given location.

The "Sky Moon Tourbillon" Ref. 5002 is the most complicated wristwatch in regular production by Patek Philippe. Because of its complexity and the great amount of time that must be dedicated to the many highly sophisticated production, assembly, and regulating steps, annual production is limited to only two watches.

Further Reading

Patek Philippe

Complicated Wrist Watches

Giorgio Gregato, Paolo De Vecchi

Full details at: Greenwich Book

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