The Portugieser Sidérale Scafusia is a masterful example of IWC’s technical prowess, combining an impressive constant-force tourbillon with an array of astronomical displays.
Unveiled in style in the Atacama Desert, Chile, in 2011, the IWC Portugieser Sidérale Scafusia is one of the most complex watches ever designed by the Schaffhausen-based Manufacture. Requiring no fewer than ten years of research and development, the model is regulated by a constant-force tourbillon. In addition, it has the distinction of combining an array of astronomical displays, including two different time indications.
Solar and sidereal time
The first time display is solar time. Indicated traditionally in the dial centre, this is based on solar mean time, corresponding to the 24-hour day around which our lives are based, and itself based on the average time it takes the sun to cross a given meridian twice in succession. The second, shown on a subdial at 12 o’clock, is sidereal time, from the Latin sidus, meaning ‘star’, ‘constellation’, or ‘heavenly body’. Sidereal time, determined by observation of the night sky, is based on the length of time taken by the Earth to complete a rotation on its own axis with reference to the stars, irrespective of its rotation around the Sun. Due to the fact that the Earth orbits the Sun at the same time as rotating on its own axis, the solar day is on average four minutes longer than a sidereal day. The design of the gear train developed by IWC is so precise that the maximum deviation of this indication is a mere 11.5 seconds over the course of a full year.
The seconds for solar time are displayed at 9 o’clock by the hand fitted to the tourbillon cage; owing to its impressive dimensions, the latter is made of titanium. Assembled on a ball bearing, the tourbillon comes with a patented constant-force mechanism to ensure the balance wheel oscillations remain regular, irrespective of the level of energy supplied by the barrel and despite any disturbances from the gear train. The workings of this new mechanism, in which an intermediate spring serves as the constant-force device, are an impressive technical achievement: it releases energy cyclically. Initially, the two parallel barrels provide a minimum power reserve of 48 hours in constant-force mode, during which the tourbillon advances in one-second increments. For the rest of the power reserve (which totals four days, i.e. 96 hours), the movement switches over from constant force to normal mode. The tourbillon then moves forward at the pace of the balance, at a rate of 2.5Hz (18,000 vibrations per hour), while the seconds hand completes five increments per second.
Customised night sky and perpetual calendar
The back of the Portugieser Sidérale Scafusia holds other surprises. The manual-winding 94900 calibre acts as the conductor of a fascinating celestial ballet depicting the sky at night. The display of the horizon, marked in yellow, can be customised by each client. There is also a perpetual calendar, its values visible through openings on the rim. This unusual display counts the number of days of the year, from the first to the last: 1 January corresponds to Day 1, 31 December to Day 365 – or Day 366 for leap years, denoted by the letters LY.
In addition to solar mean time and sidereal time, the watch also displays sunrise and sunset times. Calculated for the geographical coordinates of any place of reference chosen by the customer, they are indicated by arrows that point to a 24-hour graduation around the edge of the movement. And in a final touch, a polarising filter shows the background formed by the starry sky as grey by day and blue by night.
The precision mechanism can be fully customised in line with the owner’s whims. The night sky, horizon, and sunrise and sunset indications are calculated and displayed according to their specifications. The iconic Portugieser collection case is 46mm in diameter, 17.6mm thick, and water-resistant to 30m; the material used to make it can also be chosen by the customer. Different types of dials and straps are available, too. A specially-designed winding box ensures the indications of this superb mechanical masterpiece remain accurate, even when not worn by its owner for prolonged periods. Last but not least, the watch comes with a magnifying glass through which to admire all the details of the starry sky, as well as extensive documentation on the subject of astronomy.
*This year GMT Magazine and WorldTempus have embarked on the ambitious project of summarising the last 20 years of the Tourbillon in The Millennium Watch Book - Tourbillons, a big, beautifully laid out coffee table book. This article is an extract. The Millennium Watch Book - Tourbillons is available on www.the-watch-book.com, in French and English.
