A decade ago, IWC enlisted its engineers and watchmakers to create a constant force tourbillon watch as a statement of the brand’s horological prowess. The Portuguese Sidérale Scafusia, unveiled last summer, embodies that goal and more. With its astronomical complications as well as a perpetual calendar, this approximately $840,000, made-to-order watch is the most complex timepiece IWC has ever built.

“The watch was not planned this way from the beginning,” explains Hannes Pantli, a member of IWC’s board. “The idea of the star chart came up five years ago, when everyone was coming out with tourbillons. With so many tourbillons, we felt we had to combine the constant force tourbillon with something different and unique.” The project then took on an entirely different scope as IWC sought to top its past watchmaking achievements. “Sidérale is important because we have made tremendous marketing efforts over the years to globalize the brand,” adds Pantli. “Some people may say it’s becoming more of a marketing brand and not so much of an horological brand. This is proof that in Schaffhausen, we really do produce very complicated and special watches within our four walls.”

Polarized filters change the background from blue (day) to gray (night).

Though he left the company in 2005, Jean-François Mojon developed the original concept and designed the movement, while Thomas Gäumann was instrumental in implementing the vision. The Sidérale may appear surprisingly restrained at first glance, despite its massive tourbillon, a 24-hour subdial showing sidereal time, and a power reserve. Turn the piece over, however, and you find an accurate map of the cosmos, displays for the horizon and geographical coordinates, solar time, sidereal time, sunrise and sunset times, as well as indicators that show daytime, nighttime and twilight, which are also visually conveyed by layered polarized filters that gradually change the background from gray (day) to blue (night). An integrated perpetual calendar appears behind an indication for the number of the day of the year. All these functions are adjusted with just two pushers and the crown, and the piece is built to be shock resistant and water resistant to 3 ATM, so it is intended for everyday use.

As the only one currently capable of making the Sidérale, Master Watchmaker Stefan Brass undoubtedly has enhanced clout at the brand these days. Mojon trained Brass, who has been fascinated by the cosmos throughout his life, though he found the movement’s astronomical functions to be less vexing than the novel tourbillon design.

Constant Force Tourbillon

“The constant force tourbillon was the most challenging core of Sidérale in terms of innovation,” he explains.“It’s a patented part of the  movement. The key is that the constant force is within the tourbillon, whereas others are not.” The constant force mechanism, which is fitted between the escapement and the balance and spring, makes the seconds hand jump every second, periodically releasing energy to produce a consistent, precise rate. When fully wound, the watch will run reliably over four days, and in constant force mode for a minimum of two days.

Brass will devote three weeks to assembling each watch, which then undergoes weeks of testing before it’s ready to deliver. Once you place an order, you will have to wait a full year before you can hold your unique piece. In addition to a personal star chart for your designated location, you can also choose case metal, dial color, strap details, and more. While you wait, IWC provides constant progress updates and an iPad app to keep you engaged in the process. When the piece is ready, you are invited to Schaffhausen for a presentation ceremony and the opportunity to meet some of the players involved in creating your personalized Sidérale.

An IWC watchmaker

About a year ago, IWC enlisted astrophysicist Ben Moore, director of the Institute for Theoretical Physics at the University of Zurich, to create the custom star charts and to advise and write materials to explain the piece’s astronomical functions. With his dry British wit, he recalls showing up at the manufacture wearing his 1mm-thick Swatch and knowing nothing about mechanical watchmaking. “You might think the earth spins around once in exactly 24 hours, but it doesn’t,” says Moore explaining sidereal time (which comes from the Latin word sidus for star). “The earth turns 365 degrees in 23 hours, 56 minutes, and 4.1 seconds—that is the sidereal day, it is the time it takes for stars to appear again in the same positions.” So, while we base our solar time on a 24-hour day, a sidereal day is about four minutes shorter. Both times are clearly shown on the dial of the watch with incredible accuracy—the sidereal time will deviate by a maximum of 11.5 seconds per year.

The perpetual calendar.

“I take the GPS coordinates for the client’s chosen location and I prepare a vector diagram with dots and lines, which IWC has printed,” explains Moore, who wrote the computer program that selects about 1,000 stars at a given location from a catalog of 100 million. He then manipulates the sizes and makes selections to enrich the aesthetic of the star map, which is accurate to a meter of the owner’s location.

While he admits astronomers, who require precision to the nearest millionth of a second, need to use a computer when aiming their telescopes, he is fascinated by the micromechanics involved in producing the Sidérale Scafusia. “What interests me is the ability to get something so sophisticated into a small watch,” he says, noting his love of gadgets. “And getting a mechanical thing to perform accurately to 10 seconds per day is pretty tough to do.” As for his star chart, he wryly notes, “I guarantee its accuracy for 10,000 years—by that time, the stars will move a little and you will need a new one.”