The sun is located in the center of the solar system, but the moon plays a significant role on Earth. In addition to providing sunlight at night, the moon is the only natural satellite and is the fifth largest satellite in the Solar System.
The moon's gravitational influence causes the tides in the oceans and the lengthening of the Earth's days. Thus, the moonphase watch emerged, which tracks the lunar cycle with advanced techniques. By definition, the moon phase is a watch feature that relays the moon's 29.5-day cycle by showing the current moonphase on the dial. Also known as a moonphase complication, meaning any part on the watch that is not directly related to the time display.
The earliest moonphase displays existed long before the invention of the mechanical clock. Since then, this lunar phase complication has come a long way, from appearing in tower clocks. One example is Antikythera, an ancient Greek' analog computer' that tracks astronomical patterns dating back to 87 BC.
Brief Introduction of The Moonphase
Illustration of moon phase process. Image from Time and Date
Each lunar cycle takes about 29.53 days. Moon phase, as usually depicted on a watch, shows the bright area of the moon as it moves around the Earth during this cycle, from a tiny sliver of a crescent moon to a full moon when fully lit.
The typical moonphase display in a mechanical watch consists of a semicircular window, and below it is a disc with two twin moons. The most common moonphase system is driving a 59-tooth gear, advancing the lunar disk by one notch every 24 hours. Thus each half of the lunar disk will complete its lunar cycle every 29.5 days.
However, since the actual lunar cycle takes about 29.53 days, the accuracy of the basic gearing system will be turned off 0.3 days a month. Therefore, correcting the moonphase display every two and a half years is necessary.
Moonphase in Watches
Most moonphase watches present the moon in its current lunar phase, using an aperture to hide the parts that are not luminous. As one might imagine, the design and visuals on each watch are different.
There are two types of watches with moonphase complications. Typically known as "bosom" moonphase watches, they take a graphical approach to showing the moon in various phases within a crescent-shaped frame. Another popular type of watch is known as a "radial" moonphase watch where it uses the indicator hands to track the moon's cycles and phases.
In one of the earliest mechanisms ever developed, complications of moonphase involved placing the same two Moons on the disk below the dial, which controls the movement of the disc is a drive wheel with 59 teeth, using mechanical fingers to advance the disc once a day.
After the 29.5-day lunar cycle is complete, the second moon appears in the aperture. Naturally, the details of the lunar disk will vary based on the disk design and gap, while the cog remains constant.
Moonphase Watch Mechanism
Miyota Moonphase Movement type caliber 6P20. Image from Miyota
Most moonphase watches are consistently accurate. The 59-tooth cog is also somewhat limited in its precision but accuracy. As a result, standard moon phase watches need to be turned off for a full day every 2 years + 7 months + 15 days.
If you want to keep your moonphase watches accurate, you are recommended to manually adjust your moon phase adjustment to balance your watches with the moon cycle.
Meanwhile, some luxury brands such as Vacheron Constantin moonphase watch are implementing a 135-tooth drive wheel to provide a substantial increase in accuracy without needing to adjust until 122 years.
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