Greubel Forsey’s seventh “invention piece,” the QP à Équation, was first unveiled, to wide acclaim, at SIHH 2015. The haute horology brand founded by Robert Greubel and Stephen Forsey takes this technically exceptional timepiece to the next level of luxury in a new Millésimé Edition with a rhodium-colored gold dial.

The Greubel Forsey QP à Équation Millésimé, like its predecessor, is driven by a movement equipped with an ingenious mechanism that Greubel Forsey calls a “Computeur Mécanique,” or mechanical computer. This device, for which the brand has received three patents, is a sub-assembly of 25 parts within the larger 424-part caliber, and partly inspired by the systems used in large astronomical clocks since the end of the 15th century, which managed the information that determined certain key dates in the ecclesiastical calendar in relation to astronomical data.

This is a mechanical computer on your wrist

The mechanical “brain” at the center of this invention consists mainly of rotating, co-axial coded elements in an arrangement complemented by a system of programmed movable sections. Depending on its geometry and speed of rotation, each element generates its own indication in a cyclical, pre-programmed way. In this manner, the mechanical computer is able to automatically display all the indications of the watch, each generated by its own co-axial coded element.

In the case of the QP à Equation, these consist of all the functions of a perpetual calendar, along with the equation of time: seasons, equinoxes, solstices, the equation of time, and the date, day, month and four-digit year. (One of the coded elements, for example, is specifically programmed to take into account and compensate for the differences of a leap year.)

Interested in the difference between civil and solar time?

Despite the immense complexity of its mechanism, the watch is surprisingly simple to operate. All eight of its 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. An array of indications is displayed on the rhodium-colored gold dial, including the hours, minutes, and seconds; the day, month, and large date; the 24-hour day-night indication and the indicator for the movement’s 72-hour power reserve.

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 chose 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.

As is norm, the movement is superbly finished

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 Équation 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.

ALSO READ: Greubel Forsey's First Minute Repeater was 11 Years in the Making.

Like many other Greubel Forsey timepieces, the Millésimé incorporates the brand’s Tourbillon 24 Secondes, its third invention, which uses a fast rotation speed and inclined 25-degree angle to significantly improve the chronometric performance. (It speaks to the complexity and innovation of this watch’s movement that mentioning its inclusion of a tourbillon is almost an afterthought.) The watch’s case is made of white gold and measures 43.5 mm in diameter and 16 mm thick. The Greubel Forsey QP a Equation Millésimé Edition is priced at $750,000.