Ref. 5102 Patek Philippe introduced the ?Celestial? at the Basel Fair in 2002, a noteworthy addition to the firm?s impressive complications catalogue and a worthy heir to other remarkable watches. 2 years earlier the ?Star Caliber 2000? had made headlines as one of the most complicated pocket watches ever produced, featuring exact configurations of the nocturnal skies with its star movements, moon position and progressing phases within the lunar cycle. This watch followed in the giant footsteps of earlier and famous super- complicated Patek Philippe watches like the ?Duke of Regla? (1909), the ?Graves Super-Complication? (1933) and the ?Calibre 89? (1989). But the company had even more ambitious plans and in April 2001 it unveiled the ?Sky Moon Tourbillon?, a ?Grand Complication? wristwatch capable of displaying celestial motions with an incredible accuracy. Benefiting from Patek?s great experience in astronomy, mathematics and micro-engineering, the new watch incorporated further miniaturized astronomical functions, combining them with a tourbillon regulator and a minute repeater with long circumference ?cathedral? gongs, which utilized a special steel alloy to surpass any other chime in a wristwatch. It was their most complicated wristwatch ever made and firmly demonstrated Patek?s absolute leadership in the world of horology. The ?Celestial? wristwatch displays the night sky of the Northern Hemisphere by utilizing 3 superimposed sapphire disks, which rotate at different speeds in order to show the angular motions of the stars and the moon as well as the moon phase progressions. The disks are covered by the front sapphire crystal with its inner ellipse, framing the visible portion of the skies above Geneva and other cities sharing the same latitude. The rotation speed of each disk required a certain transmission ratio in order to assure the greatest accuracy of all astronomical indications. The firm calculated the incredible amount of 25 trillion ratio combinations until the highest accuracy degree was found. As a result the lunar day, sidereal day and the phases of the moon are ultra-precise indications and the disks move as follows: A counter-clockwise turn of the sky disc equals 23 hours, 56 minutes and 4,09892 seconds (sidereal day) and indicates the meridian passage of Sirius and the star positions. A counter-clockwise revolution of the moon disc equals 24 hours, 50 minutes and 28,328 seconds (lunar day) and indicates the meridian passage and angular motion of the moon. A clockwise half-turn of the moon phase equals 29 days, 12 hours, 44 minutes and 2,82 seconds, and indicates one full lunation. The ?Celestial? movement consists of 301 handfinished parts; all steel parts were beveled and polished. Production tolerances are extremely small, the teeth of every gear wheel were manually polished with a rotating hardwood disc. This time-consuming treatment not only creates a beautiful, perfectly smooth surface, it also reduces friction at critical points of contac, thus assuring precise indications and extended longevity of the watch. It takes several months to assemble a ?Celestial? wristwatch and quantities are very limited due to the timeconsuming processes of assemblage and regulation. It is an utmost pleasure to wear this divinely complicated and precise miniature cosmos on the wrist.