Despite the immense complexity of its mechanism, Greubel Forsey’s seventh invention is surprisingly simple to operate. All eight of the watch’s indications are controlled by a single selector set into the movement’s winding crown; the wearer can simply turn the crown in either direction to make adjustments without any risk of damage to the mechanism.
Greubel Forsey is a watch manufacturer that is not known for doing things simply — its first invention, after all, was an inclined double-tourbillon movement — and true to form, for the first application of its mechanical computer, it has chosen to focus on one of the rarest (and, frankly, probably least understood) astronomical complications, the Equation of Time — which is directly linked to the perpetual calendar.
Put simply, the equation of time measures the difference between “true” solar time and the more conventional “civil” or mean time, which is fixed at 24 hours per day, throughout the year. The length of a day in solar time varies according to the position of the earth on its elliptical orbit around the sun. For example, a solar day is about 16 minutes longer than a civil day in November, but about 14 minutes shorter in February. The unique display of the equation of time on the dial of the Greubel Forsey QP à Equation uses two sapphire crystal disks, driven by the mechanical computer, rotating independently to show the time difference as it changes throughout the year. The equinoxes and solstices, along with the four seasons, are also displayed in the equation of time indicator, located on the back side of the movement.