Patek Philippe’s Sky Moon Tourbillon is the most complicated watch in the 174 year old Genevan firm’s repertoire. First introduced as the ref. 5002, its 686 components add up to a unique combination of high complications. It starts with a tourbillon, adjusted to run within a very close rate of -2/+1 seconds maxiumum variation per day. In addition, the Sky Moon Tourbillon is a minute repeater and perpetual calendar, with, on the reverse, displays showing a sky map of the northern hemisphere; a horizon ellipse for a particular location, indications for the meridian passage of Sirius (chosen because it is the brightest star in the night sky, at magnitude -1.46 –nearly twice as bright as Canopus, the next brightest star) and the Moon. Finally, there’s an indication for sidereal time as well.
Originally, the Sky Moon Tourbillon ref. 5002 –launched in 2001, in the wake of the 2000 debut the Star Caliber 2000 –of was an exercise in extremely classical design –its obvious spiritual heirs were watches like the Graves Supercomplication and, in more recent years, the 150th Anniversary Calibre 89 and Star Calibre 2000. Of especial interest are its unusual combination of celestial complications.
Though the Sky Moon Tourbillon incorporates a perpetual calendar, minute repeater, and tourbillon, it’s the displays on the back of the watch that really put it in a class by itself. The reverse of the watch shows the movement of the stars over the course of a sidereal day, as well as having an hour and minute hand showing sidereal time, in contrast to the mean solar time shown on the front of the watch. Sidereal, or “star” time, is based on the amount of time it takes for a given star to return to a given point in the sky; this is in turn is determined by the amount of time it takes for the Earth to make one full rotation on its axis.
A mean solar day is defined, on the other hand, by the amount of time it takes for the Sun to return to a given point in the sky. You might think that this would be the same amount of time as for a sidereal day, but in fact the Earth has to make slightly more than one full rotation on its axis for the Sun to return to any given point. The reason for this is that while the Earth is rotating, it is also moving along its orbit. This means that a parallax shift with respect to the Sun occurs as the Earth moves, which is why slightly more than 360 degrees of rotation are necessary for a mean solar day. (This, of course, ignores the Equation of Time, which is the variation in length of a true solar day over the course of a year caused by a combination of the eccentricity of the Earth’s orbit and the inclination of the Earth’s axis with respect to the plane of the ecliptic.)
The stars, however, are so distant that parallax effects are minute and can be effectively ignored. Since a mean solar day is defined as 24 hours, a sidereal day is slightly shorter –about 23 hours, 56 minutes, and 4.098892 seconds.
The transparent crystal disk on the back of the Sky Moon Tourbillon rotates once per sidereal day, and this is also the amount of time that’s the basis for the sidereal time display. The meridian passage of Sirius is shown as well, and the disk is driven by means of teeth on its circumference, which are hidden from view by the bezel (rather like the system used to move the hands of mystery clocks.)
Exploded view of the celestial display disks
Below the transparent disk carrying the star map, another disk rotates as well; this disk carries an aperture showing the phase of the moon. (The phase of the moon and the age is also shown on the front of the watch, in a more traditional moon phase display.) This disk rotates once per lunar day and as it does so, it indicates the meridian passage of the moon –one lunar day, in contrast to a sidereal day, is slightly longer than a mean solar day, averaging approximately 24 hours, 50 minutes, and 28.328 seconds. The gear trains for the celestial displays have been calculated to a high degree of accuracy –the error for the sidereal day is only 33.9 seconds per century; for the lunar day only 38.5 seconds per century, and for the phase of the moon, only -6.51 seconds per lunation (lunar month.)
Demonstration of engraving at the Ref. 6002G launch event
Until now, the Sky Moon Tourbillon had been offered in only one reference: the 5002, in which the calendar indications are shown by hands in sub-dials. The ref. 6002G uses essentially the same movement, but with the day, leap year, and month shown in windows in the dial; the date is shown by a retrograde date hand.
While technically the ref. 5002 and 6002G are virtually identical, it’s in the execution of decorative details that they most noticeably differ. Ref. 6002G features a deep relief-engraved case in white gold, while the dial and moonphase disk are decorated using grand feu enameling (a combination of cloisonné and champlevé techniques.) At the launch event, Patek Philippe Creative Director Ms. Sandrine Stern explained to us that the design of the watch was done with hand-drawn and painted renderings –the decoration of the watch is extremely labor intensive, with the engraving alone requiring over 100 hours of work.
Of course, some of the most exquisite decorative work in the ref. 6002G will never be seen by the owner: the elaborate finish lavished on the movement. It is, in addition to its many fascinating technical features, a veritable museum of the decorative vocabulary of haute de gamme watchmaking –Geneva stripes, elaborate chamfering of wheel spokes and mirror polishing of gear teeth and pinion leaves, exquisitely exact anglage on plates and bridges, with the sharp inner corners that are the signature of hand-applied beveling, and black-polished steel components, such as the tourbillon bridge and repeater hammers, are all present.
Hand finishing a minute repeater hammer at Patek Philippe
As is the case with ref. 5002, ref. 6002G is an “application only” piece –every potential client must be approved by Patek Philippe CEO Thierry Stern. Sky Moon Tourbillons very rarely appear at auction and in the few cases that they do, they invariably hammer for well in excess of $1 million (a Sky Moon Tourbillon ref. 5002 was recently auctioned by Antiquorum in Geneva for $1.2 million.) Of course, the fantastic cost of these references place them out of reach of ownership of all but a small number of connoisseurs and there is the additional hurdle of obtaining approval from Patek Philippe for the sale, but those who do will have the pleasure of owning something of not only great rarity, but great horological sophistication as well.
Movement: RTO 27 WR SID LU CL, manually wound with minute repeater, tourbillon, perpetual calendar with aperture displays and retrograde date, moon phase and age, sidereal time, meridian transits of Sirius and the Moon, display of the night sky and horizon. 38×12.61 mm, 686 parts running in 55 jewels.
Tourbillon: 1 minute, 69 parts, steel cage, 21,600 vph, Breguet overcoil.
Repeater: hour, quarter and minute strikes on 2 cathedral gongs.
18k white gold case, deep relief engraving with arabesque and “Calatrava Cross” motifs; dial and moonphase disk decorated with grand feu champlevé and cloisonneé enameling. Overall diameter 42.80 mm, case thickness 16.25mm.
Price: not announced at the time of its launch but don’t expect change back from $1 million, we’d guess.
Press release images courtesy Patek Philippe; all other event and workshop photos by Jack Forster for Revolution Press Ltd; all rights reserved.