icture this: the lunar day, which is the time between two consecutive trips by the Moon across the meridian of a given location, lasts 24 hours, 50 minutes and 28.328 seconds, on average. One lunar month – the time between two full moons – measures on average 29 days, 12 hours, 44 minutes and 2.282 seconds. The duration of a sidereal day, which is the time it takes for the Earth to complete one full rotation on its axis relative to the background stars (Sirius, to be precise), is 23 hours, 56 minutes and 4.09892 seconds. On average.

Reconciling all these different periods mechanically – all of them are different from the average solar day, which is what we base our conventional 24-hour clock on – requires a phenomenal number of calculations to combine the different transmission ratios in the watch’s gear train with the various functions displayed on the dial of the Patek Philippe Sky Moon Ref. 5102. These include standard hours and minutes, a sky chart, the time that Sirius crosses the meridian, the time the Moon crosses the meridian, the angular motion of the Moon and moon phases, plus a peripheral date in later versions – the Ref. 6104 launched in 2010 and the Ref. 6102 launched in 2011.

The Ref. 5102 Sky Moon by Patek Philippe, released in 2002, is a follow-up to the Star Calibre 2000, one of the most complex pocket watches of its time. For the very first time on a watch, its 21 complications included the configuration of the night sky, including the apparent movement of the stars, the position of the Moon, the lunar cycle and moon phases. Six months later, the Sky Moon Tourbillon was released, in wristwatch format, equipped with the same miniaturised astronomical module, placed on the back of the watch. With the Ref. 5102, the sky takes over the entire dial. This magnificent depiction of the sky displays standard hours and minutes (mean solar time), a sky chart, the time Sirius crosses the meridian, the time the Moon crosses the meridian, angular motion of the Moon and phases of the Moon. This miniature cosmos is as precise as it is poetic, and comes in a reduced diameter of 43.1 mm with a height of 9.78 mm. The first Ref. 5102 in a hand-engraved rose gold case with platinum bezel. The Ref. 6104 from 2010, a gem-set version with the addition of the date on the periphery of the dial, indicated by a hand with a crescent Moon. The Ref. 6102 of 2011, unset version with date.

In order to determine the optimal mechanical configuration to ensure the smallest possible deviation of all these interdependent forces, Patek Philippe had to calculate 25 trillion possible variations in the gear train transmission ratios! The result of these calculations is fascinating: for the lunar day, the error is 0.05 second per day, equivalent to 18.385 seconds per year or 30 minutes and 38.5 seconds per century. For the sidereal day (Sirius), the error is 0.088 second per day, 32.139 seconds per year or 53 minutes and 33.9 seconds per century. For the moon phases, the deviation is 6.51 seconds every lunar month.

But the great cosmic clock whose motion defines the celestial vault over our heads is also a stunning sight, with infinite visual depth. To reproduce this effect as faithfully as possible, the dial of the Sky Moon is made from a system of superimposed sapphire discs that rotate at different speeds. At the bottom, a blue sapphire disc represents the sky. Integrated into this first disc, a wheel with 279 teeth governs the angular motion of the orbiting Moon on the second disc, and its different phases, which appear in a round window. The third disc, made from transparent sapphire, governs the sky chart on its upper surface, and the Milky Way on its interior surface. And under the inside of the sapphire crystal that protects this micro-cosmos, a gold ellipse indicates the portion of the sky visible from Geneva.

The 301 components of this self-winding micro-rotor movement and its Sky Moon module can be seen from the back of the watch, through a sapphire display back. The meticulous polishing and chamfering of each component (it takes several months to complete a single movement, and only a small handful leave Patek Philippe’s workshops each year) is not merely cosmetic. Polishing reduces friction between the wheels, which enhances the watch’s precision and longevity. For a watch that attempts, in its own way, to capture some of the precision of the eternal astronomical clock, these qualities are the very least we can expect. The outward visual beauty and poetry of this piece conceals the fascinating mechanical virtuosity that brings it to life.