Vacheron Constantin's Les Cabinotiers Celestia Astronomical Grand Complication 3600 has walked away with the Mechanical Exception Watch Prize from the Grand Prix d'Horlogerie de Genève – one of the world's most prestigious watch competitions. First unveiled in January 2017, the top-end wristwatch is a one-of-a-kind piece valued at US$1 million and is the result of five years of blank sheet development to create an astronomical timepiece with 23 complications and three drive trains in a movement only 8.7 mm thick.

Inspired by Geneva's 18th century "cabinotiers," – which were workers employed in a small workshop in eighteenth and nineteenth-century Geneva located on the top floor of a house where there was the most natural light – Les Cabinotiers Celestia Astronomical Grand Complication 3600 is a follow-on from the company's Reference 57260 – the world's most complicated watch equipped with 57 complications.

However, the haute horlogerie 3600 does not directly share any design elements with its predecessor. It took a single master watchmaker five years of development, including two years of design, to create the literally unique timepiece and, though it isn't anywhere near as complicated as the 57260, it is much more compact.

Les Cabinotiers Celestia Astronomical Grand Complication 3600 (Reference 9720C/000G-B281) is a Hallmark of Geneva certified timepiece. It's powered by the 3600 calibre developed and manufactured by Vacheron Constantin and is a mechanical, manual-winding movement 36 mm (1.4 in) in diameter and only 8.7 mm (0.34 in) thick. The 2.5 Hz movement has 514 components, including 64 transparent jewels for optimal readability.

A manual winding watch may seem a bit of a bother these days, but the 3600 has a three-week power reserve thanks to its six barrels made from sturdy and ductile Bioflex alloy combined with an involute gears mesh. This not only allows for significant power to be stored, but also ensures that the calibre is as thin as possible.

Through its 23 astronomical complications, the Les Cabinotiers Celestia Astronomical Grand Complication 3600 has slate-colored opaline twin dials front and reverse, with the 15 complications on the front laid out in a simple, readable style without undue ornamentation – though the makers did include 18K gold applied hour-markers and 18K gold hands.

The Les Cabinotiers Celestia Astronomical Grand Complication 3600 tells time in not one or two, but three ways. First, civil time is told in the traditional manner on the front dial using a pair of white gold open-tipped hands. Civil time is the time shown on conventional watches, which is the ideal 24 hour day in a 365 day year.

The second readout is for solar time, or the actual time it takes for the sun to make a single revolution through the sky. This varies from day to day by +14 to -16 minutes depending on the time of year because the Earth's orbit is an ellipse, not a circle. Solar and civil time only coincide four times a year at the solstices and the equinoxes. It's read by the sun hand that precedes or follows the civil minute hand. This also displays true solar noon and the equinoxes and solstices, as well as the running equation of time, which is by how many minutes civil and solar time diverge.

The final time readout is of sidereal time, which is based on the day as marked by a single revolution of the stars. This is an actual rotation of the Earth, which is 23 hours, 56 minutes, and four seconds of civil time. This is due to the movement of the sun in the sky each day as the Earth revolves around it accounts for the difference. This and the solar time are calculated by a separate Tropical drive train.

The sidereal time is displayed on the reverse of the watch using two superimposed sapphire discs. The lower disc carries the celestial dome, the celestial time minute-track and the four cardinal points of the compass. The upper disc is etched with the constellations of the Northern Hemisphere, a red circle representing the ecliptic, and a white circle for the equator. This dial also displays zodiacal signs and the seasons and is ringed by an anthracite inner bezel ring that shows the months of the year, as well as the gauge-type power reserve indicator. In addition, at six o'clock is a glimpse of the tourbillon.

Back on the front, there is a perpetual calendar that needs no correction for 400 years and displays the day of the week, the month and the leap-year cycle through windows and the date using a serpentine hand. There's also a discreet day/night indicator and a precision moon phase indicator with a laser-engraved true image of the full moon that shows the phases of the moon and progression of the lunar month and will only be off by one day every 122 years. Nearby is the sunrise and sunset time that also shows the day and night length using two graduated scales and a special gauge at six o'clock.

Rounding out the front is the mareoscope – a tide level indicator that includes a 3D depiction of the Earth-Moon-Sun alignment as well as indications of the tides' present state as well as how long until the next high or low tide.

All of this is sealed in an 18K white gold case 5 mm diameter and 13.6 mm thick with a transparent sapphire crystal caseback that's water resistant to three ATM ( 30 m, (100 ft) and is engraved with "Pièce unique" and "Les Cabinotiers" on the reverse. It's secured with a black Mississippiensis alligator leather with alligator inner shell, It's hand-stitched, saddle-finished with large square scales and a polished 18K white gold pin buckle that is half Maltese cross-shaped. It comes with a corrector pen and a magnifying glass and is delivered in a presentation box adorned with wood marquetry.

"It is truly an honor for a Maison of connoisseurs such as Vacheron Constantin to win this prize for excellence, awarded by a jury also composed of recognized experts and connoisseurs," said Vacheron Constantin CEO Louis Ferla when accepting the award during the GPHG prize-giving ceremony last week. "The Celestia is a model perfectly reflecting our DNA: watchmaking expertise and a profoundly human adventure perpetuated since 1755, along with a spirit of innovation and a consistently forward-driven approach that enables us to continually push technical boundaries."

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