The Moon plays a vital role in everyday life, even if we don’t know it. The two biggest things the 4.53 billion-year-old rock dictates on Earth are the tide and time. The different phases of the moon have outlined what we termed as months, including the number of days in a month for a Lunar Calendar.
The Lunar Calendar for example is specifically a calendar that is based on the cycles of the moon. It’s estimated that the average time between two full moons is 29.5 days, therefore the months on the Lunar Calendar were made to be either 29 or 30 days.
A moon phase complication is one of the world’s oldest watch complications. The complication dates back to Ancient Greek times, where the earliest moon phase complication was found on the Antikythera Mechanism. This mechanism was developed by the Ancient Greeks to predict astronomical events, and it would show moon phases and eclipses.
In the modern world, when a moon phase complication is mentioned, one immediately thinks of wristwatches. The first moon phase complication on a wristwatch was developed by Patek Phillipe in 1925, which was shortly followed by Rolex with their own interpretation of the moon phase with Reference 8171 in 1949. In the 21st century, the moon phase complication can be found on timepieces ranging from entry-level to high-level luxury pieces. Most brands follow the same design of displaying the moon phase on the dial, however, a few brands have dared to step outside the norm and create incredible pieces that show this complication in a new light.
How does the moon phase complication work?
The most common way a moon phase is designed on a timepiece is by having two moons on a disc that rotates, shown through a half-circle aperture on the dial. This aperture would only show the correct phase of the moon depending on its current position on the Lunar cycle.
As mentioned earlier, the Lunar Cycle is approximately 29.5 days per month. To be accurate for a watch-making complication, however, the Lunar Cycle is taken as 29 days, 12 hours, 44 minutes, and 2.8 seconds. The hour wheel on the gear train plays a vital role in the operation of the moon phase. As the hour wheel makes one full rotation around the dial every 12 hours (on standard 12-hour timepieces), the hour wheel drives the moon wheel at a ratio of 1:2 (24hrs/2).
The standard moon disc will come with a one-tooth gear that is mounted co-axially. Hidden underneath the dial, the moon disc has 59 peripheral teeth, which results in the advancement of the moon disc once per day. The reason there are 59 teeth is because of the Lunar cycle. 59 divided by 2 is 29.5, which means one moon phase will end after 29.5 days. Having two moons on the moon disc means that when the cycle for the first one is finished, the disc automatically will display the Lunar cycle for the second disc, thereby having a continuous rotational moon phase.
A Moon phase can be displayed in many different ways on the dial. The most common is the half-circle aperture on the dial where the moon phase, the starry night sky along with clouds are shown. Below is a list of some of the different variations of moon phases shown on watch dials.