Wristwatch with simple complete calendar
Moon phase watches follow the progression of the moon through its different phases of new moon, first quarter, full moon and last quarter. The most sophisticated, with "astronomical" moon phases, need only be corrected by one day every 122 years.
A quasi-obligatory complement to a Perpetual Calendar, moon phases are nonetheless a complication in their own right and can stand alone on a pocket watch or wristwatch dial. The phases are shown on a disc visible through an aperture, or more rarely by a hand. In an ordinary moon phase watch this disc, portraying two moons, is driven by a wheel with 59 teeth. A finger advances the wheel by one notch every 24 hours. The lunation displayed by an ordinary moon phase watch corresponds to 29.5 days when in reality a lunation lasts 29 days, 12 hours, 44 minutes and 2.8 seconds (29.53 days). Consequently, the system is one day out every 2 years, 7 months and approximately 20 days.
Superior watches incorporate a far more complex and precise system, sometimes referred to as “astronomical moon phases”. Its moon disc is driven by a wheel with 135 teeth. The precision of this mechanism is a lunation of 29 days, 12 hours and 45 minutes, reducing the difference between the measured and actual lunar cycle to just one day every 122 years. Some watches also indicate the “age of the moon”, which is the number of days since the last new moon.