Timekeeping was born out of man’s desire to make sense of the movement of the sun, the moon and the constellations. When the early horologists graduated from the sundial to gears and hands, one of the important events they tracked was the lunar cycle. This is because although the sun’s arc dictates days and seasons, the moon was the first method of telling the passing of months. From new moon to full moon and back again is 29 and a half days, providing a regular cycle that has been the life cycle for any number of things over the centuries, from pagan celebrations to paychecks.
A moonphase watch shows the current phase of the moon as you see it in the sky. As our orbiting pet rock passes across the sky, so too does its miniature version in the watch travel around, displayed in an aperture on the dial. This type of watch is perhaps the most elemental in its graphical representation of the passage of time, as opposed to the more precise and sterile hands and numbers that other complications use. It is a more whimsical and artistic complication, one that is fun to own, linking us with our sky-watching ancestors and a lineage of watchmakers and artisans all at once.
The moon has always played a major role in man’s movements and daily existence, from the ebb and flow of tides to religious ceremonies, crop harvests and maritime navigation, which is why it has always been historically important to keep track of its orbit. Though the phases of the Moon were tracked thousands of years ago in crude fashion, the moonphase was commonly found in the more complicated pocket watches in the late 19th and early 20th centuries. When the pocket watch made way for the wristwatch, so too did the moonphase migrate to the wrist. The moonphase really represents the watchmaker’s connection to time itself; it is a sort of miniature version of the gear train that is our universe.
Behind the dial of a typical moonphase watch is a disc with two identical moons on it. This disc rotates one complete cycle every twenty-nine and a half days, with the waxing and waning face of the moon accounted for by the curved edges of the dial aperture. The moon disc is driven by a 59-tooth gear that is advanced one notch by a mechanical finger every 24 hours, thus corresponding to one full rotation for the entire lunar cycle — almost.
In reality, the moon isn’t this cooperative. The true lunar cycle lasts 29 days, 12 hours and 44 minutes, or 29.53 days. While this accuracy is good enough for the average person, watchmakers are a precise lot and not content with rounding up or down. Being off by .03 days per month means that the entire moonphase cycle will be off by one full day every two years, seven and a half months. To overcome the discrepancy between the measured lunar cycle and the actual moon phases, a more sophisticated mechanism was developed that incorporates a 135-tooth gear to drive the moon disc. This improvement increases the accuracy of the movement so that the moonphase complication will only be off by a day once every 122 years, meaning that your grandson might need to do some adjustment, hopefully remembering your generosity when he does.
Our lives may no longer be tied as closely to the moon’s orbit, just as the wristwatch has largely become an anachronism in this age of smart phones and smart watches. Which makes it all the more fitting to own a moonphase watch: its timeless charm and whimsical artistry remind us that we’re all just spinning through space on this pale blue dot, orbited by a glowing rock, mere cogs in a bigger clockwork. Here are five of our favorite reminders of that fact.
Read the rest of the article at https://gearpatrol.com/2015/04/14/complications-the-moonphase/ !