Moon phase watches
"Symbolising the ancestral ties between Fine Watchmaking and astronomy, Vacheron Constantin moon-phase watches are developed and manufactured in the finest watchmaking traditions.
Moon-phase watches are timepieces indicating the evolution of the lunar cycle (new moon, first quarter, full moon, last quarter). These watches are distinguished by an additional wheel fitted next to their pinion and serving to mark off a cycle lasting 29 days, 12 hours and 45 minutes. They thus generate a discrepancy of just 57 seconds per lunar cycle, meaning less than 2 seconds a day. Manual adjustment of Vacheron Constantin moon-phase watches is thus required only once in every 122 years for high-precision moon-phase displays, and once every three years for simple moon-phase versions."