What is a Moonphase Complication

A moonphase complication displays the current phase of the moon as seen from Earth. It is one of the oldest complications in watchmaking, predating the wristwatch by centuries. The display typically appears as a disc visible through an aperture in the dial, showing a depiction of the moon against a night sky background. As the disc rotates, the aperture reveals different portions of the moon, simulating the waxing and waning of the lunar cycle.

The Lunar Cycle

The moon completes one full cycle of phases (new moon to new moon) in approximately 29.53059 days. This is called the synodic period. A moonphase complication must track this cycle mechanically.

The cycle consists of eight principal phases: new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, last quarter, and waning crescent. A moonphase display does not show discrete phases. It shows the continuous progression from one phase to the next through the gradual movement of the disc behind the aperture.

The 59-Tooth Mechanism

The standard moonphase mechanism uses a disc with two identical moon images positioned on opposite sides. This disc is driven by a 59-tooth gear. A finger attached to a wheel in the movement (typically driven by the hour wheel through an intermediate reduction) advances the 59-tooth gear by one tooth every 24 hours.

Why 59 teeth? A full lunar cycle is approximately 29.5 days. With two moons on the disc, each moon is displayed for one half of the disc's rotation. 29.5 days per moon times 2 moons equals 59 steps for a full rotation. The 59-tooth gear therefore approximates the lunar cycle at 29.5 days exactly.

The actual synodic period is 29.53059 days, not 29.5. The difference of 0.03059 days per cycle means the standard 59-tooth mechanism accumulates an error of approximately one day every 2 years and 7 months. The wearer must manually advance the moonphase display by one day at that interval to maintain accuracy.

High-Accuracy Moonphase

Some manufacturers use more complex gear trains to reduce drift. A 135-tooth gear, for example, divides the cycle more finely and reduces the error to one day in approximately 122 years. A. Lange and Sohne, IWC, and Jaeger-LeCoultre have produced moonphase mechanisms with this level of accuracy.

The most precise moonphase mechanism in a wristwatch uses a 1,058-tooth gear system and is accurate to one day in 2,000 years. At this level of precision, the mechanical drift is smaller than the irregularity of the lunar cycle itself.

The Bosom Aperture

The aperture through which the moonphase disc is visible is traditionally shaped as two overlapping circles, creating an opening that reveals a crescent-shaped portion of the disc as it moves. This shape is sometimes called the bosom aperture because of its double-arched profile.

The geometry of the aperture determines how accurately the displayed phase matches the actual phase. A well-designed aperture creates a visual representation that closely tracks the illumination pattern of the real moon at each point in the cycle. A poorly designed aperture may show a full moon for several days before and after the actual full moon, reducing the complication's usefulness.

Setting the Moonphase

Setting a moonphase complication requires knowing the current phase of the moon. The procedure is:

1. Determine the date of the last full moon. This information is available from any lunar calendar or from the LoupeLab Moon Phase Visualizer. 2. Advance the moonphase display to show a full moon. On most watches, this is done by pressing a recessed pusher on the side of the case (usually at 10 o'clock) repeatedly until the full moon is centered in the aperture. 3. Press the pusher once for each day that has elapsed since the last full moon. Each press advances the display by one day.

The moonphase display should only be advanced, never reversed. The mechanism is designed to move in one direction. Forcing it backward can damage the teeth on the driving gear.

Some watches with a quickset date function also allow rapid advancement of the moonphase via the crown, but the procedure varies by movement. Always consult the manufacturer's instructions for the specific caliber.

Moonphase and the Date Mechanism

In most implementations, the moonphase is driven by the same wheel train that drives the date display. Both advance once every 24 hours. This means the moonphase display is coupled to the watch's timekeeping. If the watch stops and is restarted, both the date and the moonphase will be incorrect by the number of days the watch was stopped, and both must be corrected.

In watches with a perpetual calendar, the moonphase is typically integrated into the calendar mechanism and benefits from the same corrections the calendar makes for month length and leap years. The moonphase in a perpetual calendar is often of the high-accuracy type (135 teeth or more) because the target buyer expects that level of precision.

Drift and Correction

The LoupeLab drift calculator can compute the accumulated error for any moonphase watch based on its gear mechanism. For a standard 59-tooth system, the drift is predictable:

• After 1 year: approximately 0.37 days of error
• After 2 years: approximately 0.74 days
• After 2 years and 7 months: approximately 1 full day of error

At the one-day mark, the wearer presses the moonphase corrector once to re-synchronize. For a high-accuracy mechanism, the correction interval extends to decades or centuries, making it effectively set-and-forget for the owner's lifetime.

Aesthetic and Practical Value

The moonphase is among the most visually distinctive complications. The painted moon disc, the shaped aperture, and the slow daily progression create a display that changes perceptibly from day to day. Unlike a chronograph (which is static until activated) or a date window (which changes once a day in a single step), the moonphase provides a continuous, evolving visual element.

Practically, the moonphase has limited utility for most wearers. Knowing the current moon phase was historically important for agriculture, navigation, and tidal prediction. Today it is primarily of interest to astronomers, photographers planning for specific lighting conditions, and watch enthusiasts who appreciate the mechanical achievement.

The complication adds minimal thickness to a movement. The moonphase disc and its driving gear occupy a thin layer above or beside the dial, and the mechanism has very few components. It is one of the simpler complications to service and one of the least likely to fail.