Running equation of time

In the history of fine watchmaking, perhaps

the most mystical and precious complication

of all has been the equation of time.

An equation of time display ties the watch to the cycle of the sun. For convenience, humans have defined the day to be exactly 24 hours in length. Faithful to that definition, watches from the most humble inexpensive quartz watch to the most prized mechanical marvel measure time according to that defined standard. In reality, however, the defined 24-hour day is a convenience, an average, that serves most purposes well but does not correspond exactly to the actual length of a solar day. Because the earth’s orbit is not exactly round and because the earth’s axis of rotation is inclined by 23 degrees, the actual solar day may be several minutes longer or shorter, depending on the time of year, than 24 hours. The difference between the length of the actual solar day, termed “solar time”, and the 24-hour day, termed “civil time”, is called the equation of time. The accumulated differences between civil time and solar time can be as much as +14 minutes and –16 minutes; on four days per year the errors catch up and the solar time and civil time correspond exactly.

Two centuries ago, fascination with the sun and this phenomenon of a day which varies in length inspired watch and clockmakers to record this time difference, the equation of time, on the face of a timepiece. Since then the equation complication has been reserved for only the most important watches and clocks.

In the early development of equation of time movements, two methods of recording the time difference were conceived. The more simple of the two is a display of the difference between solar and civil time on a plus/minus scale. Far more complicated was the second, an équation marchante movement. With the équation marchante or “running equation” movement, a second minutes-hand is added indicating solar time. This offers the advantage that the solar time can be directly read from the face of the watch. The difference can also be discerned from the difference between the solar minutes-hand and the conventional civil minutes-hand.

Equation of time displays in wristwatches have always been extraordinarily rare and, following the tradition developed over two centuries with pocket watches and clocks, they have been incorporated in only the most refined of timepieces. However, working in the small dimensions of a wristwatch, the equation complication has, until now, only been of the more simplified plus/minus scale variety.

In 2004, Blancpain debuted a revolutionary equation of time watch, the Equation du Temps Marchante. Blancpain’s watchmakers undertook to bring to wristwatches, for the first time, the rare and difficult running equation complication that had existed only in large clocks and pocket watches. To do this they had to design an innovative gear train, with an ingenious differential that combines the running of the equation gear train controlled by a complex-shaped cam and the running train of the watch’s civil minutes-hand, to drive the running equation hand. In addition, Calibre 3863 provides a plus/minus scale equation display. As befits this world’s first wristwatch, Blancpain combined these two separate equation displays with an innovative retrograde moon phase indication and a complete perpetual calendar. The decoration of the movement celebrates this achievement with a special hand carving of the bridges and a meticulously hand-fashioned winding rotor bearing an artist’s image of the sun.

Key models Running equation of time

6638-3431-55B6638-3631-55B