IWC emerged from last year’s summer break with a very complicated watch called the Portuguese Sidérale Scafusia. It was launched successively in the southern hemisphere at the Paranal Observatory in Chile and at Schaffhausen in Switzerland, the birthplace of this unusual watch. This model represents a milestone in the journey of this brand, for it joins the small number of astronomical wristwatches that display a map of the heavens. Although the watch features such exotic complications as sidereal time, the hours of sunrise and sunset as well as a constant-force tourbillion, the real eye-catcher is the starry sky.

The universe might be expanding at a tremendous rate ever since the Big Bang, but from our point of view on Earth, the familiar constellations haven’t changed in generations. The stars might be a few light years away or a few thousand, but to us they appear dotted on a giant dome rotating above the Earth. Although we now know that it is the Earth that rotates, the illusion serves as a convenient model. Since the axis of the Earth’s rotation goes through the poles, a patient observer at the North Pole will see the stars orbiting anti-clockwise around a point directly above his head, where the North Star appears immobile. The orbits of the stars increase with their distance from the Pole Star.
This phenomenon can be observed at all latitudes, except that the Pole Star is no longer directly overhead. Its angle above the horizon decreases the further south you go, but it can be seen from any point in the northern hemisphere and always in the same place every night.

Flattening the sky
It is possible to lay out a somewhat distorted version of the heavenly dome on a flat disc, thereby contriving a rotating map of the firmament, which is a great help in identifying the stars when you don’t have an iPhone at hand. The projection of a hemisphere on a flat surface, called a planisphere, is an old skill. In the 16th century, Arab scientists were famous for their brass astrolabes, which were nothing more than rotatable star charts that determined the time at night from the position of the stars.
When you go out to observe the night sky, you stand in the middle of a circle formed by the horizon where the stars rise and set. On a planisphere, the horizon is represented as an ellipse inscribed on a transparent disc above the rotating disc of stars. As the star chart rotates, the stars rise across the horizon in the east and disappear in the west.

James Ward Packard paid 12,815 Swiss francs for this 1927 Patek Philippe pocket-watch with star chart.

Time from an infinite point in space
Sidereal time is the time scale that navigators use when they take the altitude of a star. It’s a much more constant scale of time than solar time because it is based on the actual time of the Earth’s 360° rotation against an infinite point in space—the vernal equinox. The sidereal day is about four minutes shorter than the mean solar day. Sidereal time, like solar time, is local; the sidereal noon is when the vernal equinox is directly above the meridian of the locality.
The introduction of the astrolabe to Europe inspired efforts to drive the mechanism by clockwork to get the positions of the heavenly bodies in real time. The best-known machine of this kind was the 14th-century astrarium by Giovanni Dondi. The clock was lost but replicas—one of them in the International Horology Museum in La Chaux-de-Fonds—have been made from the original notes and drawings. In addition to the astrolabe, the clock has five other dials showing the planetary orbits from the geocentric point of view of the time. Astrolabes also featured in tower and church clocks: the one on Prague’s town hall, the Zytglogge in Bern and the Strasbourg cathedral clock are notable examples.

Stars for rival collectors
Watches with celestial charts appeared in the 20th century to gratify wealthy Americans vying to own the most complicated Swiss watches. The industrialist James Ward Packard had Patek Philippe provide a pocket-watch with a chart of the sky as it appeared over his home in Ohio. The best-known example of this rivalry is the 1933 Graves watch with 24 complications, delivered to the New York financier Henry Graves Jr. It was superseded in 1989 when Patek Philippe built the Calibre 89 to mark its 150th anniversary. Among its 33 complications is a realistic depiction of the night sky in the northern hemisphere. The Geneva company went on in 2000 to produce the 21-complication Star Calibre pocket-watch, which has a revolving planisphere that also shows the position and phases of the moon. These indications appeared in Patek Philippe’s most complicated wristwatch, the Sky Moon Tourbillon, and lately in the Celestial model, which is devoted entirely to representing the night sky.
This brings us to today’s watches that bring sky to wrist, starting with the earliest astrolabe-inspired models:

1. Ulysse Nardin, Astrolabe Galileo Galilei This remarkable wristwatch, part of a trilogy produced in 1985, was the first to reduce the indications of a large tower-clock like the Zytglogge to the space of a watch dial. The mechanism designed by Ludwig Oechslin gives you more than a pretty sky and is said to be most accurate. Both sidereal and mean time are indicated, but only the high-magnitude stars appear. Oechslin chose the traditional astrolabe format with the firmament and the sun hand going around clockwise. The sun hand shows the time of day as well as the apparent position of the sun on the ecliptic. The moon hand shows the position and phase of the moon. Finally the dragon hand, which shows the lunar nodes and crosses the ecliptic every 18.6 years, foretells the eclipses of the sun and the moon.

2. Christiaan van der Klaauw, CK Astrolabium CKAL7766 Van der Klaauw, a member of the AHCI academy of independent watch and clock creators who lives in the Netherlands, makes astronomical indications his speciality. His astrolabe shows the position of the brightest stars, the sun and the moon, and the lunar nodes with a dragon hand. The visible part of the sky is presented on the dial in the manner of an antique astrolabe, behind the hands and the stars.

3. Vacheron Constantin, Tour de l’Ile In this intricate watch, which ranks among the most complicated wristwatches, the sky above Geneva is shown through an elliptical aperture in the dial. Although it does not show sidereal time, it does show the equation of time—the difference between solar time and mean time. Its other complications include a second timezone, age and phase of the moon, minute-repeater, perpetual calendar, tourbillon, the times of sunrise and sunset and a power-reserve indicator.

4. Patek Philippe, ref. 5102PR Celestial The indications derived from the Star Calibre and the Sky Moon Tourbillon show the night sky of the northern hemisphere on the face of this elegant self-winding watch. The fine celestial dome turns anti-clockwise. The ellipse representing the horizon frames the visible portion of the sky above Geneva and elsewhere on the same latitude. Although the moon crosses the sky at a different rate than the stars, its position on the dial always accurately reflects its place in the sky. A hidden mechanism makes the moon wax and wane. Two arrows mark the position of Sirius and the moon and can be adjusted separately to set the lunar and stellar indications.

5. Van Cleef & Arpels, Midnight in Paris This urbane watch, which has the same name as a fragrance marketed by the brand, displays a glittering firmament in aventurine that takes up the whole face. An elliptical frame reveals the position of the stars above Paris. The star disc goes round in one year so the stars are in their true position only once every 24 hours at midnight.

6. Officine Panerai, L’Astronomo Luminor 1950
Panerai launched its most complicated wristwatch in 2009, the International Year of Astronomy. The celestial vault on the back turns anti-clockwise. The clear elliptical patch in the frosted glass reveals the sky visible at a given latitude, which can be set to the owner’s choice.

7. Jaeger-LeCoultre, Master Grande Tradition
This complex masterpiece from Le Sentier is in the same vein as the Sky Moon Tourbillon: the astronomical indications are exactly calculated but the appearance comes first, with a dial-side tourbillon taking up some of the sky. It is placed on the vernal equinox, turns with the stars and shows sidereal time anti-clockwise on a 24-hour scale. The sun is integrated in an ingenious way. It goes around on a transparent disc and shares the 24-hour scale with the sidereal time. It moves anti-clockwise in relation to the Pole Star just as it appears to do so in the sky, but slower than the stars by one degree a day. It is thus caught up by the scale of the zodiacal calendar on which it shows the date. The sun is presumably driven by a circumference wheel, otherwise it would clash with the tourbillon.
The other complications are the flying tourbillon with a silicon escapement and a minute-repeater.

8. IWC, Portuguese Sidérale Scafusia
As its name suggests, this major complication with a tourbillon is dedicated to sidereal time, which is shown in hours and minutes on a small 24-hour dial on the front of the watch. The focus of attention however is on the back: a planisphere of some 500 stars with lines joining them into constellations rotates once a sidereal day around the polar axis. The special feature is that polarising filters change the colour of the sky from day to night through dawn and dusk. IWC takes into account its customer’s location by providing a planisphere for the southern hemisphere. The astrophysicist, Ben Moore, has written a program enabling planispheres to be calculated to individual requirements so that even customers living near the equator get the best possible representation of their night sky at home.
As in the Master Grande Tradition model, sidereal time is shown by a pointer on the planisphere against a 24-hour scale turning in the opposite direction. It’s hard to understand why the sun hand turns clockwise, thereby requiring its own hours scale. If it went in the same direction as the stars, like in the Jaeger-LeCoultre watch, it could indicate the date, and the ecliptic that Moore specified for the star chart would at least make sense. Despite this, the IWC engineers have incorporated another useful astronomical complication—the times of sunrise and sunset—that also exists in different guises in watches by Patek Philippe, Vacheron Constantin and Audemars Piguet. These two times are shown by two small red arrows against the outside scale. The cams for the indications have to be cut and fitted individually for each client.
The other complications: constant-force tourbillon, sunrise and sunset, and a perpetual calendar that shows the number of days since the start of the year rather than the dates.

This article was originally published in issue n°12 of the magazine Watch Around

Reflections on Time, read more:

• Introduction: Suspended Time
• The Mastery of Time
• Hartmut Rosa: The acceleration of time
• The yurt and the equation
• Aphorisms on time
• The Clock: watch of the year
• Carte blanche: Eric Giroud
• Chronometry at the speed of light
• A meeting with Ottavio Di Blasi
• Carte blanche: The White Group
• Carte blanche: Alexis Guillier

Source: Europa Star February - March 2012 Magazine Issue