After hacking our way with blunt machetes through the information-dense breakdown of the Celestia’s triple-time display, we now come to the most interesting part by far of the Celestia — solar time as represented by a running equation of time hand adjusted to the tropical year. (Standard YMMV disclaimer: my list of Most Interesting Things has been known to cure chronic insomnia. Proceed with caution and Red Bull.)

For all those reasons previously mentioned, solar time isn’t exactly the most consistent thing on the planet. Sometimes it’s late, sometimes it’s early — it’s only ever on time four times a year. But that’s okay, we all got friends like that, and we love them anyway.

Now these deviations aren’t really that big a deal when taken on their own — the longest solar day is about 30 seconds longer than a mean solar day, the shortest solar day is about 21 seconds shorter than a mean solar day.

However, let’s imagine you get a whole run of solar days, each of them just a few seconds longer than a mean solar day. That difference of a few seconds starts to stack up real quick. One day you realise that, although your beautiful and accurate Swiss watch tells you it’s noon, the sun in the sky essentially DGAF and is like a whole quarter of an hour away from reaching its celestial zenith. Shock and horror!

That said, if your beautiful and accurate Swiss watch happens to be the Vacheron Constantin Celestia (please don’t make me type out its full name), there will be no shock and there will be no horror. You have a truly excellent polished pink-gold hand tipped with a splendid roseate sun, co-axial with the hours and minutes hands, indicating exactly how far the solar day is running ahead or behind civil time. Yay, science!

So this daily discrepancy, this information about the gain/lag between solar time and civil time, is called the equation of time. A bunch of watches have it. Blancpain make equation-of-time watches. So do Chopard. Those guys over at Audemars Piguet, they’ve got a few of them. Patek Philippe, obviously. Greubel Forsey, because they’re crazy overachieving geniuses like that.

I’m not gonna give a full list of watches with equation of time here, because it’s too much work to look up all that stuff. Just take my word for it when I say that the majority of equation-of-time watches display this function on a separate counter and you gotta do a bunch of mental arithmetic to figure out what the solar time is.

The running equation of time (équation du temps marchante), which you see on the Celestia, is far rarer. Off the top of my head, I can only think of one other company that does it — a Vallée de Joux outfit, as you’d expect.

At this point I can speak only for the Celestia, since that’s the watch whose movement diagrams I’m staring at right now. In this instance, the running equation of time is achieved by a hand that is linked to a gear train that translates the incremental gains and losses of each solar day into an intuitive reading relative to civil time.

If you want a thorough explanation of how this tropical gear train (named for its adherence to the units of the tropical year, or true solar year) gets it done, the “how”s and the “what”s and the “where”s and the “when”s, I’m gonna have to let you down here. I’m not a master watchmaker who’s worked on this mechanism for five years. I’m a watch writer who’s been struck down by her four-year-old niece’s cold (kid germs are stronger than Valyrian steel; it is known). There’s an eccentric cam, there’s a differential; I’ll leave the quod erat demonstrandum to someone else.

Disappointed? Don’t be! That someone else is ready and waiting — in fact, there’s three of them. They all go by the same name, so that might be kinda weird at first, but they look totally different, so bottom line is, we’re good. They are the Vacheron Constantin Metiers d’Art Copernicus Celestial Spheres 2460 RT and they will make you feel fine.

The Copernicus zeroed in on the most technically stomach-churning complication of the Celestia — the expression of the tropical year — and gave it the love that lifts it up where it belongs. On the dial of each of the three Copernicus watches, a miniature Earth travels along an elliptical path around a gold sun.

It doesn’t just move at any speed it likes; one rotation of the Earth around the Sun on the dial of the Copernicus is exactly one tropical year, and the Earth also rotates once per day on its own axis. The margin of error in this mechanism is so small that it amounts to one correction needed in 8,000 years. That’s a lot of years. Presumably the Earth won’t have fallen into the Sun by then, otherwise you’d save your descendants the trouble of making that correction.

Take a look at the movement and you’ll immediately see that this mechanism takes a ridiculous amount of energy (massive Earth disc with two axes of rotation, massive gold counterweights and inertia blocks, peripheral rotating differential). That’s the tropical gear train writ large, with an expanded field of expression. If you think about it, 36 hours of power reserve is really pretty good, especially when it comes with automatic winding.

It definitely doesn’t hurt that the Copernicus is exceptionally easy on the eyes. But we’ll get to that a little later.

Related Notes + Observations

– There’s something we have to get straight here. There’s the difference between a true solar day and a mean solar day. The mean solar day is always 24 hours. A true solar day varies from the mean solar day by about a minute (it can be shorter by roughly 20 seconds, or longer by about 30 seconds, or somewhere in between). Then, there’s the equation of time, which is the accumulated lag or gain that builds up because of these little variations. For instance, the solar day on 11 February is exactly 24 hours long, congruent with the mean solar day, but it lags by 14 minutes and 15 seconds — as reflected by the equation of time. The equation of time doesn’t tell you whether the solar day is longer or shorter than a mean solar day, it tells you if the solar day is ahead or behind the mean solar day.

– Another two things that are easy to get mixed up are the length of the solar day and the amount of daylight, and it doesn’t exactly help that we use the same words in both contexts. “Day” means the unit of time between two consecutive zeniths of the sun (as in “days, weeks and months”), but it also means the period of time in which the sun hangs around above the horizon (as in “day and night”). You can have a long solar day with relatively few hours of daylight. The longest day of the year is technically not in the summer, but in winter. I’ll tell you something else: despite what you might think, this information is not useful as a conversation starter at parties.

– Next up: Getting busy with the métiers d’art component of the Copernicus watches.

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