Disc

In watchmaking, the disc is a flat rotating component that replaces, integrates, or duplicates the function of a conventional hand. It is not merely a decorative element: it is a constructive choice with direct consequences on readability, mechanical footprint, and energy consumption.

The most common form is the date disc, positioned just below the dial and visible through a window. It rotates one thirty-sixth of a turn every 24 hours, driven by a star wheel. Glashütte Original made famous the double-disc variant, the so-called Panorama Date, where two discs work in synchronization to display tens and units on separate apertures, drastically improving legibility. On the opposite front, the pointer date uses a numbered disc paired with a central index that completes one full rotation in a month: no window, no dial cutout, reading by pointing like on a regulator watch.

The hour disc — employed in jump hour complications — accumulates the energy of the entire movement for sixty minutes and releases it in an instant, causing the numeral to snap to the next position. The result is immediate readability, but the mechanism imposes a more generous power reserve: the Gerald Charles Maestro caliber mounts a disc twenty-five times heavier than an equivalent hand, which gives an idea of the forces at play. The jumping disc, an intermediate solution, gradually slides between two numerals showing the transition: more honest from a readability standpoint, less spectacular.

In world timers, the 24-hour disc — a system formalized by Louis Cottier in the 1930s and adopted by Patek Philippe — completes one full rotation per day and is read in conjunction with a city ring. The Baltic Heures du Monde uses a transparent disc overlaid on the dial, independently adjustable, allowing one to set the second time zone without stopping the main display. In so-called flyer GMT watches, such as the Miyota 9075, the second time zone is indicated precisely by a 24-section disc with a colored marker, not by an additional hand.

Moonphases are perhaps the oldest application: a disc with two moons painted or engraved completes one revolution every 29.53 days. The Patek Philippe 5840P adds a stationary perforated disc shaped like a moon silhouette that masks the shadowed portion, revealing on the wheel beneath only the crescent or full disc visible from reality. The effect is precise and the construction is entirely mechanical.

In wandering hours systems, discs called satellites rotate through a 120-degree arc, each with four numerals engraved, pointing the minutes on a fixed track. Zirconia bearings, as in the Kudoke 5, serve to reduce friction on large-diameter discs: they eliminate the need for additional lubrication and stabilize rotation over time. It is a technical detail often unseen but felt at the moment of service.