Moonphase
A display of the current lunar phase
Two painted moons on a 59-tooth disc rotate behind an arched aperture, advancing one tooth each day.
What it is
The moonphase complication displays the current phase of the moon through an arched aperture in the dial. A disc carrying two painted moon images rotates behind the aperture, one image always visible, growing from new to full and back over 29.5 days. The standard disc has 59 teeth (two lunar cycles) and deviates from the true 29.53059-day cycle by approximately one day every 2.5 years. Precision moonphase mechanisms use a 135-tooth disc and achieve one day's deviation per 122 years.
History
Moonphase complications appeared in pocket watches from the 17th century, when they were practically useful for navigation and agriculture. In wristwatches the complication is primarily aesthetic; Patek Philippe's reference 5050 (1992) and later 5396 restored collector prestige to it. The Patek reference 240 LU in ultra-thin automatic format and the A. Lange & Söhne Langematik Perpetual both use precision moonphase mechanisms with dramatically reduced deviation rates. The standard 59-tooth disc requires correction roughly in April each year if you want the display to remain accurate.
How it works
A 24-hour cam advances the moonphase disc by one tooth each day via a driving lever. The disc carries two identical moon images positioned exactly 180 degrees apart, so one is always visible through the aperture as the disc rotates. The aperture is shaped like a crescent arch to frame the circular moon image. Setting a moonphase after a long stop or battery change requires advancing the disc to the current lunar phase using the moonphase corrector pusher.
Parts required
Moonphase disc (59 teeth on standard; 135 teeth on high-precision variants), 24-hour driving cam, moonphase driving lever
In the catalog
Related
- Perpetual calendar: Programs every month length, including leap years, through 2100
- Equation of time: The difference between solar time and mean time, displayed on the dial
