The Patek Philippe 5102 Sky Moon Celestial

There are a lot of superlatives thrown around rather loosely in the world of luxury watchmaking, but every now and then a watch comes along which deserves every superlative you can throw at it, and then some. A disproportionate number of such watches come from Patek Philippe. It is perhaps an obvious thing to say (as well as something which has been repeated both ad infinitum and ad nauseam) but there is hardly a genre of watchmaking at which Patek has not excelled (okay, they haven’t done all that much with dive watches over the last hundred and eighty or so years but you can’t have everything).
While Patek has made watches of generally very high quality for its entire history — that is, in fact, what the company’s reputation is based on — it is especially famous for its complicated watchmaking and complicated watches. Patek has made, and still makes, superlative versions of every major horological complication. Minute repeaters, perpetual calendars, rattrapante (split seconds) chronographs, as well as astronomical complications, and watches which combine various complications, have all been produced by Patek and today, the “Grand Complication” section of the catalog still lists over 30 watches. Most of these are, as you might expect, exceedingly complex, and which represent the accumulated knowledge of nearly two centuries of complicated watchmaking.
One of the most beautiful of all watches in Patek’s catalog (albeit beauty is in the eye of the beholder) is the reference 5102 Sky Moon Celestial. The watch is immediately seductive even before you find out anything in particular about how it works, or the information shown on display. It’s one of the largest watches Patek has made, at 44mm in diameter (although it’s also quite thin, especially given its complexity — about 10mm, which is not that much thicker than the 8.08mm x 39mm 6119 Calatrava). The dial real estate is put to good use — most of the front of the watch is taken up by a very detailed map of the night sky, which shows the visible stars, the arc of the Milky Way galaxy (which many of us city dwellers have forgotten that you can see on a clear, dark night) as well as a moonphase complication.
<img decoding="async" loading="lazy" class="aligncenter wp-image-1166335 size-full" src="https://watchbox-blog.imgix.net/wp-content/uploads/2023/02/2023_01_17_AWAW_Patek-Grand-Complications-Sky-Moon_5102G-001_05.jpg?max-w=1024&q=50&auto=format,compress&usm=10&usmrad=3" alt="Patek Philippe 5102 Sky Moon Celestial" width="1400" height="908" />
The dial is actually three dials — more specifically, three rotating sapphire disks. The middle disk is deep blue and has an aperture in it for the smaller, lower disk for the moonphase. The uppermost disk is transparent, and is decorated with the star chart as well as the depiction of the Milky Way, and each disk is only 0.2mm thick.
The ellipse engraved on the underside of the dial represents the horizon, and the stars which at any time are visible above the horizon are enclosed by it. The disk carrying the stars rotates once per day — or rather, once <a href="https://en.wikipedia.org/wiki/Sidereal_time">per sidereal day</a>. A day is defined as the amount of time it takes for a given astronomical body to return to the same point in the sky — a solar day, for instance, is the amount of time it takes for the Sun to return to its zenith, or highest point in the sky (on average, 24 hours, although the length of any actual day varies over the course of a year; our standard 24 hour day is a “mean” solar day). The sidereal day, on the other hand, is the amount of time it takes a star to return to its zenith, and a sidereal day is slightly shorter than a solar day — 23 hours, 56 minutes, and 4.0905 seconds, versus 24 hours for a mean solar day.
The reason for this is that the Earth moves along its orbit as it rotates, so it has to rotate slightly more than one full rotation for the Sun to return to its zenith position as seen from Earth. The stars, on the other hand, are so far away (many light years, rather than eight light minutes for the Sun) that this parallax effect does not occur.
Understanding sidereal time is the key to understanding how the other astronomical indications on the 5102 work. The stars, of course, rise above and set below the horizon line on sidereal time. If you draw a line from 12:00 to 6:00 on the dial, you have a line that represents the meridian overhead at the observer’s location. As the moonphase aperture rotates, you can observe the point where it passes the meridian. There is also a small yellow pointer on the dial, next to the large star that represents Sirius — the brightest star in the night sky (Sirius is both intrinsically bright, and quite close to the Earth, at just 8.6 light years away). The pointer will let you observe the meridian passage of Sirius.
<img decoding="async" loading="lazy" class="aligncenter wp-image-1166336 size-full" src="https://watchbox-blog.imgix.net/wp-content/uploads/2023/02/2023_01_17_AWAW_Patek-Grand-Complications-Sky-Moon_5102G-001_03.jpg?max-w=1024&q=50&auto=format,compress&usm=10&usmrad=3" alt="Patek Philippe 5102 Sky Moon Celestial" width="1400" height="908" />
One other key point is that the horizon ellipse is specific to a particular latitude — the Celestial, for instance, is constructed so as to show the night sky as it appears above Geneva, which is at 42.6 degrees North latitude (New York for comparison, is at about 40 degrees North; the North Pole is at 90 degrees North). However, the star chart will, if set correctly, be accurate for any location around the world at the same latitude as Geneva and will be a close approximation for any location close to that latitude as well, although the accuracy obviously falls off the further away you get from 46.2 North.
Mean solar time is shown conventionally, with an hour and minute hand.
<img decoding="async" loading="lazy" class="aligncenter wp-image-1166343 size-full" src="https://watchbox-blog.imgix.net/wp-content/uploads/2023/02/Graves-Supercomp-1400-Wide.jpeg?max-w=1024&q=50&auto=format,compress&usm=10&usmrad=3" alt="Patek Philippe" width="1200" height="800" />
The star chart complication is technically known as a planisphere and it’s one that Patek has used before — the planispheric star chart was used in two of Patek’s most famous complicated pocket watches. Industrialist and automotive pioneer James Ward Packard had Patek build him a highly complex pocket watch with a star chart for his hometown of Warren, Ohio, in 1922 and of course, the Henry Graves supercomplication, delivered in 1933 — for many years the most complicated watch in the world — had a star chart for New York City (the charts for the Packard watch and the Graves watch would, I imagine, be pretty close — Warren is at 41.2376°North latitude, and New York is at 40.7128°North.) The Caliber 89 Patek pocket watches, launched in 1989 to celebrate the company’s 150th anniversary, also had (among other things) a planispheric star chart, for the night sky over Geneva.
<img decoding="async" loading="lazy" class="aligncenter wp-image-1166339 size-full" src="https://watchbox-blog.imgix.net/wp-content/uploads/2023/02/6_1800@2x.jpeg?max-w=1024&q=50&auto=format,compress&usm=10&usmrad=3" alt="Patek Philippe" width="1400" height="1400" />
The planispheric star chart itself is older than Patek Philippe — much older. In fact, it’s older than mechanical horology. The ancestor of the star chart in watches, is the astrolabe — basically, a planispheric star chart operated by hand, rather than by clockwork. The first astrolabes were invented in Ancient Greece and they were brought to a very high level of sophistication in the Medieval Islamic world. Astrolabes, like planispheric star charts, are usually designed for a particular location but universal astrolabes were designed and constructed as well. Such star charts, and similar astronomical indications, are present in the earliest known European mechanical clocks including the abbot Richard of Wallingford’s astronomical clock, <a href="https://en.wikipedia.org/wiki/Richard_of_Wallingford#Studies_in_astronomy_and_mathematics">which was completed in the mid-1300s</a>. The clock showed mean time in equal and <a href="https://en.wikipedia.org/wiki/Unequal_hours">unequal hours</a>, as well as the true solar time. It also displayed the phases of the moon and showed the positions of the lunar nodes and the height of tide at London Bridge — all this at a time when European civilization was being ravaged by social unrest and the Black Death, which just goes to show you how much you can get done if you’re not goofing around on the Internet.
Every mechanical watch is in a sense, a time machine — mechanical timepieces powered by spiral springs and with mechanical escapements have been around for something like five centuries and clocks, for even longer, so there’s a direct connection to a very long history behind any mechanical watch. But the Sky Moon Celestial is a very deep dive into not only the history of watches, or of mechanical horology. When you look at the dial, you see the universe in miniature on your wrist — a representation of the human perspective on something infinitely larger than our own small world. But you are also seeing back in time, very far back, through the history of astronomy and the instruments created out of both practical necessity, and a fascination with the harmonious and apparently unvarying movement of the glowing points of light in the night sky. The Sky Moon Celestial is, in other words, and in contrast to much of what gets attention nowadays in luxury watchmaking, a serious watch — one which will be owned by very few, thanks to its cost and complexity, but one which will provide intellectual, emotional and even spiritual rewards few other watches can match.
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Patek Philippe

Written By

Jack Forster

WatchBox Global Editorial Director Jack Forster has been fascinated by every aspect of watches, clocks, and watchmaking for his entire life. He began systematically studying and learning about watches over 20 years ago, and during the first explosion of watch enthusiast forums, he became a moderator on thepurists.com. Jack became a full-time watch writer in 2007, when he joined Revolution Magazine, first as group technical and features editor, and then as editor in chief of Revolution Magazine USA. He joined the enthusiast website Hodinkee in 2015 as managing editor and then as Editor in Chief. In 2022, Jack joined WatchBox as the Global Editorial Director. He is the author of Cartier Time Art: Mechanics of Passion, and has written about watches, and other subjects, for many other venues including CNN, American Express Departures, and Vanity Fair.

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