Photograph by Tom Schierlitz
This complication, which arose from astronomical clocks popular in the 19th century, found its way into wristwatches in the 20th. The traditional moon-phase mechanism consists of a disk with two identical, illustrated moons. A 59-notch gear, controlled by a mechanical finger, drives the disk, advancing it one notch every 24 hours. Thus, the disk makes a complete revolution every 59 days, or roughly two lunar cycles, with the appropriate moon phase revealed through an aperture. (The so-called astronomical moon phase has a gear that’s even more in sync with the true lunar cycle of 29.53 days.) Blancpain helped revive interest in the moon phase in 1983, and the company continues to lead with models such as the Quantième Complet 8 Jours.