IWC Schaffhausen – Portugieser Sidérale Scafusia – The Millennium Watch Book

The Portugieser Sidérale Scafusia is a masterful example of IWC’s technical prowess, combining an impressive constant force tourbillon with a range 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 manufacturer. Requiring no less than ten years of research and development, the model is regulated by a constant force vortex. In addition, it has the particularity of combining a variety of astronomical displays, including two different time indications.

Portugieser Sidérale Scafusia © IWC Schaffhausen

Solar and sidereal time

The first time display is solar time. Traditionally indicated in the center of the dial, this is based on the average solar time, corresponding to the 24-hour day around which our lives revolve, and itself based on the average time taken by the sun to cross two times a given meridian. The second, displayed on an auxiliary dial at 12 o’clock, is sidereal time, from the Latin sidus, which means “star”, “constellation” or “celestial body”. Sidereal time, determined by observing the night sky, is based on the time taken by the Earth to rotate on its own axis relative to the stars, regardless of its rotation around the Sun. Because the Earth revolves around the Sun at the same time as it spins on itself, the solar day is on average four minutes longer than a sidereal day. The gear train design developed by IWC is so precise that the maximum deviation of this indication is only 11.5 seconds over a full year.

The seconds of solar time are displayed at 9 o’clock by the hand mounted on the tourbillon cage; due to its impressive dimensions, the latter is made of titanium. Mounted on a ball bearing, the tourbillon is equipped with a patented constant force mechanism to ensure the regularity of the oscillations of the balance, whatever the level of energy supplied by the barrel and despite any disturbances of the gear train. The operation of this new mechanism, in which an intermediate spring serves as a constant force device, is an impressive technical feat: it releases energy in a cyclical manner. First, the two parallel barrels provide a minimum power reserve of 48 hours in constant force mode, during which the tourbillon advances in steps of one second. For the rest of the power reserve (which totals four days, or 96 hours), the movement switches from constant force mode to normal mode. The tourbillon then advances to the rhythm of the balance, at a frequency of 2.5 Hz (18,000 vibrations per hour), while the seconds hand performs five increments per second.

Portugieser Sidérale Scafusia

Portugieser Sidérale Scafusia © IWC Schaffhausen

Personalized night sky and perpetual calendar

The rear of the Portugieser Sidérale Scafusia has other surprises in store. The hand-wound caliber 94900 is the conductor of a fascinating celestial ballet featuring the night sky. The horizon display, marked in yellow, can be personalized by each customer. There is also a perpetual calendar, the values ​​of which are visible through openings on the rim. This unusual display counts the number of days in the year, from first to last: January 1 is day 1, December 31 is day 365 – or day 366 for leap years, denoted by the letters LY.

In addition to the mean solar time and sidereal time, the watch also displays the times of sunrise and sunset. Calculated for the geographic coordinates of any reference location chosen by the customer, they are indicated by arrows pointing to a 24-hour graduation around the perimeter of the movement. And as a final touch, a polarizing filter shows the background formed by the starry sky gray during the day and blue at night.

The precision mechanism can be fully customized according to the owner’s whims. The night sky, horizon, sunrise and sunset indications are calculated and displayed according to their specifications. The iconic case of the Portugieser collection measures 46 mm in diameter, 17.6 mm in thickness and is water resistant to 30 m; the material used to make it can also be chosen by the customer. Different types of dials and bracelets are also available. A specially designed winding box ensures that the indications of this superb mechanical masterpiece remain precise, even when not worn by its owner for long periods of time. Finally, the watch comes with a magnifying glass through which to admire all the details of the starry sky, as well as abundant documentation on the theme of astronomy.

* This year, GMT Magazine and WorldTempus have embarked on the ambitious project of summarizing the last 20 years of the Tourbillon in The Millenium Watch Book – Tourbillons, a large, beautifully laid out tabletop book. This article is an excerpt. The Millenium Watch Book – Tourbillons is available on, in French and in English.